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The Digital Equipment Corporation PDP-8

Models and Options

Part of the PDP-8 Collection
by Douglas W. Jones
THE UNIVERSITY OF IOWA Department of Computer Science

Contents

Introduction

The purpose of this document is to supplement the material in the primary Frequently Asked Questions about the PDP-8 file with more detailed information about the hardware and options of the different models of the PDP-8 sold by DEC.

Although this document is something of a history of the DEC PDP-8 family, the primary purpose of this document is as a guide and general outline to the PDP-8 models and options likely to be encountered by those involved in collecting and restoring such systems.

Clickable links in the following have been verified to be useful as of mid 2013. URLs that don't respond to clicking are either dead or unchecked. Maintenance of this web page is on a sporadic basis, at best.

The PDP-5

Date of introduction:  Aug 11, 1963, unveiled at WESCON.
Date of withdrawal:    early 1967.
Total production run:  116.
Price: $27,000
Technology:
The PDP-5 was built with DEC System Modules, the original line of transistorized logic modules sold by DEC. The supply voltages were +10 and -15 volts, with logic levels of -3 (logic 1) and 0 (logic 0). Logic was packaged on boards that were about 4.75 inches high with each card mounted in a metal frame with a 22 pin edge connector.

Input output devices were connected to the daisy-chained I/O bus using military-style armored cables and connectors. Use of toggle switches (as opposed to slide switches) on the front panel was another vestige of military-style design.

Reason for introduction:
This machine was inspired by the success of the CDC-160, Seymour Cray's 12 bit minicomputer, and by the success of the LINC, a machine that was built by DEC customers out of System modules. These demonstrated that there was a market for a small inexpensive computer, and from the start, DEC's advertisements were aimed at this market. "Now you can own the PDP-5 computer for what a core memory alone used to cost: $27,000", ran one 1964 ad.

Ken Olson has stated that the PDP-5 was not originally meant to be a computer; it was designed for a company that wanted an automatic controller for some industrial work. He told them they could make a small programmable controller instead of the hardwired machine they were asking for, and since they weren't entirely certain of the control equations they wanted to run, they accepted the idea. The result was the PDP-5.

Reason for withdrawal:
The PDP-8 outperformed the PDP-5, and did so for a lower price.

Compatability:
The core of the PDP-8 instruction set is present, but memory location zero is the program counter, and interrupts are handled differently. The Group 1 OPR rotate instructions cannot be combined with IAC or CMA; this limits the ability of the PDP-5 to support code from later models. The machine does not support 3 cycle data-break (DMA transfers using memory to hold buffer address and word-count information), so many later PDP-8 peripherals cannot be used on the PDP-5. In addition, DMA transfers are not allowed outside the program's current 4K data field, severely limiting software compatability on systems with over 4K of memory where either interrupts or software initiated changes to the data field during a transfer would cause chaos.

Standard configuration:
CPU with 1K or 4K of memory (2K and 3K versions were not available).

Peripherals:
An extended arithmetic element (EAE) was available; this was an I/O device, using IOT instructions to evoke EAE operations. As a result, it was not compatable with the later PDP-8 EAEs. In addition, machines with the EAE option had a different front panel from those without.

The type 552 DECtape control and type 555 dual DECtape transports were originally developed for the PDP-5 and contemporaneous DEC systems such as the PDP-6.

After the PDP-8 was introduced, DEC offered a bus converter that allowed the PDP-5 to support standard PDP-8 negibus ueripherals, so long as they avoided using 3-cycle data break transfers. The standard 804 PDP-8 expander box was frequently sold as an upgrade to PDP-5 systems.

Survival:
A small number of PDP-5 systems survive:

The PDP-8

Date of introduction:  1965 (Unveiled March 22, in New York).
Date of withdrawal:    1968.
Total production run:  1450.
Also known as:
        Classic PDP-8 (to point out lack of a model suffix)
        Straight-8 (Again, points out the lack of a model suffix)
        PCP-88, an OEM label, used by Foxboro Corporation.
        AN/GYK-6, (Army-Navy Ground-based (Y)data-processing Komputer 6)
Price: $18,000
Technology:
Mostly standard DEC R-series logic modules; these were originally discrete component transistor logic, but around the time the PDP-8 was introduced, DEC introduced the Flip Chip, a hybrid diode/resistor "integrated circuit" on a ceramic substrate. These could directly replace some of the discrete components on some logic modules, and DEC quickly began to refer to all R-series modules as flip-chip modules; they even advertised the PDP-8 as an integrated circuit computer. A typical flip-chip module, the R111, had three 2-input nand gates and cost $14, with no price change from 1965 to 1970. Some special dual height R-series modules were designed specifically for the PDP-8.

S and B-series logic modules were also used; these are similar to their R-series cousins, but with different speed/fanout tradeoffs in their design. Some logic modules have trimmers that must be tuned to the context, making replacement of such modules more complex than simply swapping boards.

As with the system modules used in the PDP-5, the supply voltages were +10 and -15 volts and the logic levels were -3 (logic 1) and 0 (logic 0). Logic was packaged on boards that were 2.5 inches wide by 5 inches long. The card edge connector had 18 contacts on 1/8 inch centers. Some double height cards were used; these had two card edge connectors and were 5 1/8 inches high. Machine wrapped wire-wrap technology was used on the backplane using 24-gauge wire.

The "negibus" or negative logic I/O bus used -3 and 0 volt logic levels in 92 ohm coaxial cable, with 9 coaxial cables bundled per connector card and 6 bundles making up the basic bus. 5 (later 4) more bundles were required to support data-break (DMA) transfers. The total bus length was limited to 50 feet, and bus termination was generally kluged in with 100 ohm resistors clipped or wrapped into the backplane, although a bus terminator card was sometimes used. Some time after the first year of production, flat ribbon cable made of multiple coaxial cables was used, and later still, shielded flat stripline cable was used (but this cut the allowed bus length by a factor of two).

Core memory was used, originally made by FERROXCUBE, with a 1.5 microsecond cycle time, giving the machine an add time of 3 microseconds. 4K of core occupied an aluminum box 6 inches on a side and needed numerous auxiliary flip-chips and for support, as well as an array of boards from the core vendor. It is worth noting that the PDP-8 was about as fast as was practical with the logic technology used; only by using tricks like memory interleaving or pipelining could the machine have been made much faster.

Reason for introduction:
This machine was inspired by the success of the PDP-5 and by the realization that, with their new Flip-Chip technology, DEC could make a table-top computer that could be powered by a single standard wall outlet; of course, adding any peripherals quickly increased the power requirement!

Reason for withdrawal:
The PDP-8/I was less expensive, and after initial production difficulties, it equalled the performance of the PDP-8.

Compatability:
This machine defines the core of the PDP-8 instruction set, but with restrictions that were lifted on later machines. The Group 1 OPR instruction IAC cannot be combined with any of the rotate instructions. If RAR and RAL or RTR and RTL are combined, the results are unpredictable (simultaneous set and reset of bits of AC results in metastable behavior). The IOT 0 instruction was used for the internal type 189 ADC, and not for the later CAF (clear all flags) instruction. As a result, if the ADC option was not present, IOT 6004 (or microcoded variants) would hang the machine.

The SWP instruction (exchange AC and MQ) never works, even if the extended arithmetic element is present. This works on later models when the EAE is present, although it was only documented with the introduction of the PDP-8/E. Finally, the EAE lacks the SCL (shift count load) instruction that is present on later models.

On machines with 8K or more, an attempt to change the data field to a non-existant field caused a bizarre double-indirect and skip instruction execution that must be accounted for in memory diagnostics.

Standard configuration:
The PDP-8 was sold as a CPU with 4K of memory, a 110 baud current loop teletype interface and an ASR 33 Teletype. In addition, the standard in-cabinet logic includes support for the full negibus interface, including data-break (DMA) transfers.

Both a rack-mount model with rosewood trim and an elegant plexiglass enclosed table-top configuration were standard. Under the skin, the basic machine occupies a volume 33 inches high by 19 inches wide by 22 inches deep. The two halves of the backplane are mounted vertically, like the covers of a book, with the spine in back and circuit modules inserted from the two sides. Sliding the CPU out of the relay rack or removing the plexiglass covers allows the backplane to swung open to access the wires-wrap.

Expandability:
In-cabinet options for which there were pre-wired backplane slots included:

With the Type 183 memory expansion controller, the memory can be expanded to a total of 32K, but expansion beyond 4K of memory requires rack mounting space. The rack-mount CPU occupies the top of one rack, allowing room for a single memory expansion module below the CPU; generally, a second rack was needed for added peripherals or memory (at $690 per CAB-8 rack). Given rack space, you could install:

At the end of the production run, some PDP-8 systems were sold with PDP-8/I memory, allowing room for an additional 4K without need for an expansion chassis. These nonstandard machines were very difficult to maintain!

Peripherals:
At the time of introduction, the following negibus peripherals were offered. By 1966, the following peripherals had been added to the line: By 1967, the following peripherals had been added to the line: By 1968, the following new peripheral had been added: Finally, as DEC abandoned the negibus, they introduced the DW08B negibus to posibus converter so newer posibus peripherals could be used on older negibus machines, and the DW08A posibus to negibus converter to allow use of old peripherals on new machines.

Survival:
Many classic PDP-8 systems survive to this day in working or restorable condition. Among them, the following are noteworthy:

The LINC-8

Date of introduction:  1966 (during or before March).
Date of withdrawal:    1969
Total production run:  142.
Price: $38,500
Technology:
DEC Flip Chip modules, as in the PDP-8, with a LINC CPU partially reimplemented in Flip Chips and partially emulated with PDP-8 instructions. (The original LINC was built from the same System Modules used in the PDP-5.)

Reason for withdrawal:
The PDP-12 accomplished the same goals at a lower cost.

Compatability:
The PDP-8 part of the machine was identical to the PDP-8.

Standard configuration:
The combined PDP-8/LINC CPU, plus 4K of memory was central to the system. The set of peripherals bundled with the machine was impressive:

The X and Y axis control for the scope came from DACs attached to the LINC's AC and MB registers, respectively. The entire package was mounted in a nonstandard relay rack, with a section of standard 19-inch rack on the left holding the scope and LINCtape drives, and a half-width section to the right holding the analog interface panel. The console spanned the full width of the rack front, exposing the entire internal state of both the LINC and PDP-8 processors.

Expandability:
In addition to standard PDP-8 peripherals, up to 3 additional pairs of LINCtap drives could be added, for a total of 8 drives. The design of the type 555 dual DECtape transport was based on that of the LINCtape drive.

Up to 2 additional ranks of 8 ADC channels could be added.

Remote oscilliscope could be added.

Survival:
A few LINC-8 systems survive, most notably:

The PDP-8/S

Date of introduction:  1966 (Unveiled, Aug 23, WESCON, Los Angeles).
Date of withdrawal:    1970.
Total production run:  1024, or over 1500
        The first figure is from Computers and Automation, based on figures
        released by the manufacturer.  The second figure is based on memory
        of the first-year production run.  We need to look at the serial
        numbers on surviving machines to pin this down!
Price: $10,000
Technology:
DEC Flip Chip modules and core memory, as in the PDP-8. Unlike the PDP-8, the PDP-8/S memory module was mounted between a pair of quad-height single-width boards that plugged into the standard flip-chip sockets (this was sold separately as the H201 core memory unit, at $2000 for 4K by 13 bits). It is noteworthy that the prototype machine was built using Digital Logic Laboratory H901 plugboards and patchcords, based entirely on off-the-shelf modules. Another new feature of the PDP-8/S was its use of a single internal bus within the machine for all register transfers. This was, of course, bit serial, but the idea formed the basis for the DEC UNIBUS and OMNIBUS and essentially all later bus-oriented CPU designs.

Reason for introduction:
This machine was developed as a successful exercise in minimizing the cost of the machine, in response to a complaint by Ken Olson that the company hadn't gotten the price of the PDP-8 down any further, and the vision that someday, people ought to be able to buy a desktop PDP-8 for under $10,000. The result was the least expensive general purpose computer ever made with second generation (discrete transistor) technology, and it was one of the smallest such machines to be mass produced (a number of smaller machines were made for aerospace applications). It was also incredibly slow, with a 36 microsecond add time, and some instructions taking as much as 78 microseconds, even though the internal clock ran faster than that of the original PDP-8! By 1967, DEC took the then unusual step of offering this machine for off the shelf delivery, with one machine stocked in each field office available for retail sale.

Reason for withdrawal:
The PDP-8/L vastly outperformed the PDP-8/S, and and it did so at a lower price.

Compatability:
The core of the PDP-8 instruction set is present, but there are a sufficient number of incompatabilities that, as with the PDP-5, many otherwise portable "family of 8" programs will not run on the PDP-8/S. Perhaps the worst incompatability is that the Group 1 OPR instruction CMA cannot be combined with any of the rotate instructions; as with the original PDP-8, IAC also cannot be combined with rotate.

Standard configuration:
CPU with 4K of memory, plus PT08 110 baud current loop teletype interface and teletype. The basic machine was 10.25" high by 19" wide by 27.5" deep, and was sold with either a table-top case or in a rack-mount configuration with mounting slides. In the rack-mount version, the heavy ferroresonant (self regulating) power supply transformer was moved out of the basic machine to the base of the rack, but aside from that and the sheet metal enclosure, the rack-mount and table-top models were identical.

Expandability:
The following options were available:

The MC8S alone (with the original 4K core module relocated from the PDP-8/S chassis into the OMD8S chassis) allows an 8K system configuration. Add 3 ME8S plus 6 MM8S modules, and you have a total of 32K words of core memory.

The CPU supported the standard PDP-8 negibus, but I/O bandwidth was 1/5 that of the PDP-8. Thus, most, but not all PDP-8 peripherals could be used. A few DEC peripherals such as the DF32 came with special options such as interleaving to slow them down for compatability with the PDP-8/S with a DB8S. Faster peripherals such as DECtape were never supported on the 8/S.

Survival:
Several PDP-8/S machines survive:

The PDP-8/I

Date of introduction:  1968 (Announced before December '67)
Date of withdrawal:    1971.
Total production run:  3698.
Price: $12,800
Technology:
DEC M-series logic modules, called M-series flip-chips as the term flip-chip was applied to the module format instead of to DEC's hybrid integrated circuits. M-series modules used TTL chips, with a +5 volt supply, packaged on the same board format used with the original flip-chips, but with double-sided card-edge connectors (36 contacts instead of 18). Modules were limited to typically 4 SSI ICs each. The M113, a typical M-series module, had 10 2-input nand gates and cost $23 in 1967 (the price fell to $18 in 1970). Wire-wrapped backplanes used 30-gauge wire (as opposed to the 24-gauge wire used with the older single-sided flip-chip modules).

The PDP-8/I, as originally sold, supported the then-standard PDP-8 negibus. 4K words of core were packaged in a 1 inch thick module made of 5 rigidly connected 5 by 5 inch two-sided printed circuit boards. Connectors and support electronics occupied an additional 32 backplane slots.

Nominally, the core memory (which, curiously, used a negative logic interface!) was supposed to run at a 1.5 microsecond cycle time, but many early PDP-8/I systems were delivered running at a slower rate because of memory quality problems. DEC went through many vendors in the search for good memory! The memory interface was asynchronous, allowing the CPU to delay for slow memory. DEC continued to make the classic PDP-8 until the problems with memory speed were solved.

Reason for introduction:
This machine was developed in response to the introduction of DIP component packaging of TTL integrated circuits. This allowed a machine of about the same performance as the original PDP-8 to fit in about half the volume and sell for a lower price.

Reason for withdrawal:
The PDP-8/E made slight performance improvements with vastly simplified support for peripheral interfaces while undercutting the price of the PDP-8/I.

Compatability:
The core of the PDP-8 instruction set is present, and unlike the original PDP-8, IAC can be combined with rotate in a single microcoded Group 1 OPR instruction. Combined RAR and RAL or RTR and RTL produce the logical and of the expected results from each of the combined shifts.

If the extended arithmetic element is present, the SWP (exchange AC and MQ) instruction works, but this was not documented.

On large memory configurations, memory fetches from a nonexistant memory field take about 30 microseconds (waiting for a bus timeout) and then they return either 0000 or 7777 depending on the memory configuration and the field that was addressed.

A front panel bug prevented continue after load-address without first clearing the machine.

Standard configuration:
CPU with 4K of memory, plus 110 baud current loop teletype interface. Pedestal, table-top and rack-mount versions were made. The pedestal mounted version was futuristic looking; the table-top version split the pedistal version, with the CPU on the table and the power supply (the base of the pedistal) on the floor beside the table. The standard rack-mounted version had the power supply bolted to the right side of the rack while the CPU, mounted on slides, slid out of the left side of the rack.

Expandability:
4K of memory could be added internally, and additional memory could be added externally using a rack-mounted MM8I memory expansion module for each 4K or 8K addition over 8K.

The backplane of the PDP-8/I was prewired to hold a Calcomp plotter interface, with the adjacent backplane slot reserved for the cable connection to the plotter. There may be other built-in options.

Initially, the CPU was sold with bus drivers for the PDP-8 negibus, allowing this machine to support all older DEC peripherals, but later machines were sold with posibus interfaces, and many older machines were converted in the field.

A posibus to negibus converter, the DW08A, allowed use of all older PDP-8 peripherals, with small modifications. The change from negibus to posibus during the period of PDP-8/I production leads to confusion because surviving CPUs and peripherals may have any of three I/O bus configurations: Negibus, early posibus, or final posibus. The early posibus used the same connectors and cables as the negibus, with only 9 conductors per connector, while the final posibus used both sides of the connector paddles for 18 bus lines per connector. Y-shaped cables for converting from one physical bus layout to the other were available. To add to this confusion, some negibus PDP-8/I systems were rewired to use 18 conductor posibus cables with negative logic!

Eventually, an add-on box was sold that allowed PDP-8/E (OMNIBUS) memory to be added to a PDP-8/I. Additionally, Fabritek sold a 24K memory box for the 8/I and PDP-12.

Survival:
A fair number of rack-mounted PDP-8/I systems survive. Are there any surviving table-top or pedistal mounted machines? Among the survivors are:

The PDP-8/L

Date of introduction:  1968 (Announced before August '68)
Date of withdrawal:    1971.
Total production run:  3902.
Price: $8,500
Technology:
DEC M-series flip Chip modules and a wire-wrapped backplane, as in the PDP-8/I, with the same core memory as the 8/I, but with a memory cycle cycle of 1.6 microseconds to avoid the speed problems that plagued early -8/I systems.

The positive I/O bus, or posibus, was a 100 ohm bus clamped between 0 and 3 volts with TTL drivers and receivers. This was packaged with 18 signal lines per 2-sided interconnect cable, using double-sided shielded mylar ribbon cable in most cases. Electrically, coaxial cable could be used, but the slots in the CPU box were too small for this.

Reason for introduction:
This machine was developed as a moderately successful exercise using M-series logic to produce a lower cost but moderately fast machine. The idea was to cut costs by limiting provisions for expansion.

Reason for withdrawal:
The PDP-8/E made performance improvements and vastly simplified input/output device interfacing while slightly undercutting the price of the PDP-8/L.

Compatability:
The core of the PDP-8 instruction set is present, but all Group 3 OPR instructions are no-ops, even the Group 3 version of the CLA instruction. This is because there was no provision made for adding an EAE to this machine. Microcoding RAR and RAL together works as in the PDP-8/I. Finally, a new front panel feature was added, the protect switch. When thrown, this makes the last page of the last field of memory read-only (to protect your bootstrap code).

The instruction to change the data field on an 8/L becomes a no-op when the destination data field is non-existant; on all other machines, attempts to address non-existant fields are possible. One option for expanding the 8/L was to add a box that allowed 8/E memory modules to be added to the 8/L; when this was done, access to nonexistant data fields becomes possible and always returns 0000 on read.

Standard configuration:
CPU with 4K of memory, plus 110 baud current loop teletype interface and teletype. The basic machine was 9" high by 19" wide by 21" deep, and was sold with either a table-top case or in a rack-mount configuration with mounting slides. Aside from differences in the sheet metal enclosure, the rack-mount and table-top models were identical, with the wire-wrapped backplane was on top, and modules plugged in from below.

Expandability:
The CPU supported a new bus standard, the PDP-8 posibus. There is little space for in-box peripherals, but an expander box with the same volume as the CPU was available, the BA08A; this was prewired to hold an additional 4K of memory and to support in-box peripheral interfaces for such devices as a Calcomp plotter interface, a card-reader interface, a 4 line asynch terminal interface, a real-time clock, and more.

DEC eventually offered the BM12L, an 8K expansion box that is essentially the same as the MM8I, but using positive logic and thus incompatable with the -8/I and -12. This allowed a total memory of 12K on a PDP-8/L. This contains precisely the modules needed to upgrade a 4K PDP-8/I or PDP-12 to an 8K machine, or to populate an MM8I box to add 8K of additional memory to an 8/I or PDP-12.

Finally, DEC eventually offered a box allowing PDP-8/E (OMNIBUS) memory to be used with the PDP-8/L. PDP-8/L configurations with over 8K of memory were awkward because the front panel only showed one bit of the extended memory address. As a result, extra lights and switches for the additional bits of the memory address were mounted on the front of the memory expander boxes for the large configurations.

A variety of posibus peripherals were introduced, most of which were built with the option of negibus interface logic (the -P and -N suffixes on these new peripherals indicated which was which). Many early PDP-8/L systems were sold with DW08A bus level converters to run old negibus peripherals.

Posibus peripherals introduced after the PDP-8/L (and also used with posibus versions of the PDP-8/I) included:

Survival:
Numerous machines survive; the following machines are in public museum collections.

The PDP-12

Date of introduction:  1969 (February or earlier).
Date of withdrawal:    1973.
Total production run:  755.
Price: $27,900
Technology:
DEC M-series flip Chip modules, as in the PDP-8/I.

Reason for introduction:
This machine was developed as a follow-up to the LINC-8. Originally it was to be called the LINC-8/I, but somehow it got its own number. In effect, it was a PDP-8/I with added logic to allow it to execute the LINC instruction set.

Reason for withdrawal:
The LAB-8/E and the LAB-11 (a PDP-8/E and a PDP-11/20 with lab peripherals) eventually proved the equal of the PDP-12 in practice, and LINC compatability eventually proved to be of insufficient value to keep the machine alive in the marketplace.

Compatability:
This machine is fully compatable with the PDP-8/I, with additional instructions to flip from PDP-8 mode to LINC mode and back. IOT 0 could enable the API, causing trouble with later PDP-8 code that assumes IOT 0 is "Clear all flags". Also, the DECtape instruction DTLA (6766) becomes part of a stack-oriented extension to the instruction set, PUSHJ, on late model (or field updated) machines with the KF12-B backplane.

The PDP-12 supports trapping of those LINC functions that are emulated by software on the LINC-8. This allows it to run many LINC-8 bootable systems (but not all, due mostly to incompatabilities in LINKtape support), and it allows such things as emulation of LINKtape instructions for reading and writing disk.

The TC12F Linktape controller can, with appropriate software, read or write DECtape. This support is unreliable, and is not software compatable with the TC01 or TC08 DECtape controller.

Standard configuration:
PDP-8/LINC CPU with 4K of memory, plus 110 baud current loop interface, plus output relay registers. In addition, the standard configuration included either two TU55 or one TU56 drive, with a PDP-12 only controller allowing it to handle LINCtape. In addition, digital to analog converters and a 12" scope were always included, along with a connector for an optional second scope. The package was assembled in a custom relay rack that had a standard 19-inch bay on the right for the CPU, scope and DECtape drives, plus a narrow bay on the left for the analog interfaces.

Expandability:
An analog to digital converter and multiplexor was needed to fully support knob-oriented LINC software. Other options included:

Fabritek made a 24K memory box that could be added to a PDP-8/I or PDP-12.

Survival:
A few PDP-12 systems are in operating condition, and some are in public museums.

The PDP-8/E

Date of introduction:  1970 (during or before August).
Date of withdrawal:    1978.
Also known as:
        PDP-8/OEM
        Industrial-8 (with a red color scheme)
        LAB-8/E (with a green color scheme)
Total production run:  Over 10,000?
Price: $6,500
Technology:
SSI and MSI TTL logic were used on the boards making up this machine, and the entire CPU fit on 3 boards. Nominally, these were DEC M-series flip Chip modules, but in a new large format, quad-high (10.5 inch), extended-length (9 inch, including card-edge connector, excluding handles). The terms used for board height and length are based on the original working assumption that all flip-chips were plugged horizontally into a vertially mounted card-edge connector. On the PDP-8/E, the cards were plugged vertically down into a horizontally mounted connector, so many users incorrectly refer to these boards as quad-wide double-high.

Interconnection between boards was through a new bus, the OMNIBUS. This eliminated the need for a wire-wrapped backplane, since all slots in the bus were wired identically and could therefore be connected by a single printed-circuit board. A new line of peripheral interfaces was produced, most being single cards that could be plugged directly into the inside the main enclosure. These included a set of posibus adapters allowing use of older peripherals on the new machine.

Interboard connectors were needed for some multiboard options, including the CPU and memory subsystems. These used standard 36-pin backplane connectors on the opposite side of the board from the backplane. Some boards, notably memory boards, had a total of 8 connector fingers, 4 for the omnibus and 4 for interboard connectors.

The core memory cycle time was 1.2 or 1.4 microseconds, depending on whether a read-modify-write cycle was involved (a jumper would slow all cycles to 1.4 microseconds). A 4K core plane was packaged on a single quad-wide double-high board, with most of the drive electronics packed onto two adjacent boards. Soon after the machine was introduced, an 8K core plane was released in the same format.

Reason for introduction:
The cost of the PDP-8/I and PDP-8/L was dominated by the cost of the interconnect wiring, and this cost was high as a result of the use of small circuit boards and point-to-point wire-wrap interconnection. By packing a larger number of chips per board, similar function could be attained in a smaller volume because less interboard communication was required. The PDP-8/E exploited this to achieve a new low in cost while attaining a new high in performance.

Reason for withdrawal:
This machine was slowly displaced by the PDP-8/A as the market for large PDP-8 configurations declined in the face of pressure from 16 bit mini and microcomputers.

Compatability:
As with the PDP-8/I and PDP-8/L, there are no limits on the combination of IAC and rotate instructions. Unlike the early machines, basic Group 3 OPR operations for loading and storing the MQ register work even if there is no extended arithmetic element. Finally, a new instruction was added, BSW; this swaps the left and right bytes in AC, and is encoded as a Group 1 OPR instruction using the "double the shift count bit".

An odd quirk of this machine is that the RAL RAR combination ands the AC with the op-code, and the RTR RTL combination does an effective address computation loading the high 5 bits of AC with the current page and the lower bits of AC with the address field of the instruction itself!

The EAE has a new mode, mode B. Previous EAE designs were single-mode. Mode B supports a large set of 24 bit operations and a somewhat more rational set of shift operations than the standard EAE. All prior EAE designs would hang on the microcoded CLA NMI (clear/normalize) instruction applied to a nonzero AC. This instruction is redefined to be a mode changing instruction on the 8/E.

Standard configuration:
A CPU with 4K of memory, plus 110 baud current loop teletype interface. Both a rack-mount table-top versions were sold (both 9" high by 19" wide by 21" deep). The rack mount version was mounted on slides for easy maintenance. The OMNIBUS backplane was on the bottom, with boards inserted from the top.

The PDP-8/OEM had a turn-key front panel, no core, 256 words of ROM and 256 words of RAM, and was priced at $2800 in lots of 100. The target market for this machine was small embedded control systems.

The standard OMNIBUS backplane had 20 slots, with no fixed assignments, but the following conventional uses; certain board sets were jumpered together (shown with brackets) and therefore were required to be adjacent to each other:

Most of the early boards with 3 digit numbers were unable to support long backplanes (with an Omnibus extender cable and second cabinet); redesigned boards with a trailing zero added to their board numbers corrected these problems. Thus, the M833 was generally replaced with an M8330, and the M865 was replaced with the M8650.

Peripherals:
The following are among the OMNIBUS boards that could be added internally:

There were many other internal options. There was room in the basic box for another 20 slot backplane; taking into account the 2 slots occupied by the M935 bridge between the two backplanes, this allowed 38 slots, and a second box could be added to accomodate another 38 slot backplane, bridged to the first box by a pair of BC08H OMNIBUS extension cables.

Given a M837 memory extension control, additional memory could be added in increments of 4K by adding G104, H220, G227 triplets. The suggested arrangement of boards on the OMNIBUS always maintained the M849 shield between memory other options. The one exception was that the M8350 KA8E and M8360 KD8E external posibus interfaces were typically placed at the end of the OMNIBUS right before the terminator.

The following options were introduced later, and there were many options offered by third party suppliers.

Survival:
PDP-8/E systems were common on the surplus market in the 1980s and 1990s, but they are now rare. A very few are still in service doing their orignal jobs, although technical support for these machines is becoming difficult. Several of these machines are now on exhibit in various museums:

The PDP-8/F

Date of introduction:  1972.
Date of withdrawal:    1978.
Total production run:  Over 10,000?
Technology:
an OMNIBUS machine, as with the PDP-8/E. First use of a switching power supply in the PDP-8 family.

Reason for introduction:
The PDP-8/E had a large enough box and a large enough power supply to accomodate a large configuration. By shortening the box and putting in a small switching power supply, a lower cost OMNIBUS machine was possible.

Reason for withdrawal:
The PDP-8/A 800 displaced this machine, providing similar expansion capability at a lower cost.

Compatability:
The PDP-8/F used the PDP-8/E CPU and peripherals.

Standard configuration:
Identical to the PDP-8/E, except that the KC8E front panel was replaced with a KC8M front panel that had LEDs instead of incandescent lights; this front panel could also be installed on PDP-8/E systems, but the PDP-8/E front panel could not be used on a PDP-8/F because of the lack of a +8 supply for the lights. The original PDP-8/F box had a defective power supply, but a revised (slightly larger) box corrected this problem.

With the dintroduction of the M8330, DEC began to require that this board be placed adjacent to the KC8x front panel, although many OMNIBUS PDP-8 systems continued to be configured (by users) with the M8330 elsewhere on the bus. As a result, the suggested order of boards on the omnibus began with:

Expandability:
This machine could be expanded using all PDP-8/E OMNIBUS peripherals, including the external expansion chassis. The relatively small internal power supply and the lack of room for a 20 slot bus expander inside the first box were the only limitations. There were minor compatability problems with some options, for example, the power-fail auto-restart card, as originally sold, was incompatable with the PDP-8/F power supply.

Survival:
As with the PDP-8/E, these machines still occasionally appear on the surplus market. There are many PDP-8/F computers in private collections.

The PDP-8/M

Date of introduction:  1972.
Date of withdrawal:    1978.
Total production run:  Over 10,000?
Technology:
This machine was essentially a re-branded PDP-8/F (with a PDP-8/E CPU).

Reason for introduction:
DEC knew that OEM customers were an important market, so they packaged the PDP-8/F for this market, with no hardware changes behind the front panel.

Reason for withdrawal:
Same as the PDP-8/F.

Compatability:
As with the PDP-8/F, The PDP-8/M used the PDP-8/E CPU and peripherals.

Standard configuration:
Identical to the PDP-8/F, except that the KC8M front panel was replaced with a minimal function panel and the color scheme was different. Because of this, one of the following options were required:

Expandability:
All options applying to the PDP-8/F applied. In addition, the KC8M front panel (standard with the PDP-8/F) was available as an option; the only difference between a PDP-8/F and a PDP-8/M with the KC8M front panel was in the artwork on the front panel and on the serial number and configuration stickers on the back of the box.

Survival:
As with the PDP-8/F, but at least one is in a museum collection:

The PDP-8/A

Date of introduction:  1974 (Announced in May for December delivery)
Date of withdrawal:    1984
Also known as:
        KIT-8/A (CPU plus 1K RAM)
        CLASSIC (CLASSroom Interactive Computer)
        DECDataSystem310 (an 8/A 500 sold as a word-processor)
Price: $1,835
Technology:
This machine used the OMNIBUS with a new single-board CPU. The backplane was reoriented so that boards plugged into it from the front, with the board held horizontally. The new omnibus allowed a board format half again as wide as the original (formally, this was called hex height), but the extra 2 groups of contact fingers added to each wide board was largely unused. (the 6th contact group was not connected on most backplane slots; the 5th was unsupported on 8 of the 12 or 20 backplane slots, and was used primarily for additional power and ground distribution).

Reason for introduction:
Using TTL MSI and LSI components, DEC was able to reduce the PDP-8 CPU to a single oversize board (formally, hex height, double width). Similarly, they were able to make an 4K core memory board, and later, an 8K board in this format, and they were able to introduce a static RAM card using semiconductor memory. The minimum system was thus reduced to 3 boards. The relatively expensive lights and toggle switches on the front panel of the PDP-8/E were replaced with an octal membrane keypad and 4-digit 7-segment LED display.

The market for the PDP-8 was dominated by small systems, with fewer and fewer customers needing large-scale expandability. Thus, the 20 slot backplane of the early OMNIBUS machines was too big; with the new single board CPU and memory, a 12 slot backplane was enough, allowing further cost reductions.

Reason for withdrawal:
The market for the PDP-8 family was shrinking in the face of pressure from larger minicomputers and the new monolithic microcomputers. After 1975, many PDP-8 sales were to captive customers who had sufficient software investments that they could not afford to move. Only the word-processing and small business markets remained strong for first-time PDP-8 sales, and in these, the specialized DEC VT-78 and DECmate machines were more cost effective than the open architecture OMNIBUS machines.

Compatability:
The new PDP-8/A CPU was largely compatable with the PDP-8/E CPU, except that the combination of RTR and RTL (Group 1 OPR instructions) loaded the next address. The power-fail auto-restart option included the standard skip on power low instruction, but also a new skip on battery empty instruction to test the battery used for back-up power on the new solid state memory.

The standard parallel port on the M8316 was not software compatable with the earlier line-printer interfaces used with device code 66.

Standard configuration:
The PDP-8/A was sold with a new short OMNIBUS backplane, mounted on its side above a power supply and a battery to back up the solid state memory. The minimum configuration included a limited function control panel and the following components on the bus:

The M8316 board contained a remarkable but useful hodgepodge of commonly used peripherals, including the console terminal interface, a parallel port, the power/fail auto-restart logic, and a 100 Hz real time clock.

The smallest PDP-8/A configuraton marketed was the KIT/8A, either just the KK8A and MS8A 1K boards for $572, or $1350 for a system that appears to have included the M8316 and a 4 slot backplane.

The 8/A 100, was a computer system with a 10 slot backplane and a poor power supply. The 8/A 400 was a better system with a 12 slot backplane, and the 8/A 420 had a 20 slot backplane. The 8/A 600 and 620 were the 8/A 400 and 420 with the KK8E PDP-8/E CPU set allowing added speed and the use of the 8/E EAE.

Expandability:
All PDP-8/E peripherals and options could be used with the PDP-8/A. For those configurations requiring more than 20 backplane slots, A pair of PDP-8/A backplanes could be connected using BC08H cables, and there was a special cable, the BC80C, for connecting a hex wide 8A backplane to a PDP-8/E, -8/F or -8/M backplane.

By February 1975, the PDP-8/A was being sold in a workstation configuration, with the CPU and dual 8" diskette drives in a desk with a video terminal (VT52) and optional letter quality printer on top. For the educational market, this configuration was marketed as the CLASSIC. As an office system, such configurations were marketed as DECDataSystems.

The following additional PDP-8/A (hex) boards were offered:

The PDP-8/A model 800 was the same as the model 600, but with the FPP8A floating point processor included as part of the package.

Note that memory extension to 128K was a new PDP-8/A feature that was necessarily incompatable with the older PDP-8 memory expansion options, although the conventional PDP-8 memory expansion instructions still operate correctly on the first 32K. Access to additional fields involved borrowing IOT instructions that were previously dedicated to other devices.

The MM8A core memory options require the use of a box with a G8018 power supply that provides +20V, while the semicondustor memory options require a G8016 power supply with built-in battery backup. Also, the use of the MSC8 DRAM memory cards require a CPU that supports the memory stall signal; early PDP-8/E CPUs did not.

Survival:
As with the PDP-8/F, these machines still occasionally appear on the surplus market. The following are in public museum collections:

The VT78

Date of introduction:  1978
Date of withdrawal:    1980 (Displaced by the DECmate)
Also known as:
        DECstation
        DECstation 78
Price: $7,995 ($5,436 in lots of 100)
Technology:
Based on the Intersil/Harris 6100 microprocessor, packaged in a VT52 case. The 6100 processor was able to run at 4 MHz, but in the VT78, it was only clocked at 2.2 MHz because of the speed of the DRAM used and the deliberate use of graded out chips.

Reason for introduction:
Using TTL MSI and LSI components, DEC could pack a system into the vacant space in a standard terminal case, allowing PDP-8 systems to compete with personal computers in the small business and office automation market. This was a natural follow-on to the desk-mounted workstation configurations in which the PDP-8/A was already being sold.

Reason for withdrawal:
The DECmate offered similar capabilities at a better price.

Compatability:
The Group I OPR combinations RAL RAR and RTL RTR are no-ops. Unlike all earlier PDP-8 models, autoindex locations 10 to 17 (octal) only work in page zero mode; these operate like all other memory locations when addressed in current page mode from code running on page zero. Other than this, it is fully PDP-8/E compatable, even at the level of I/O instructions for the standard periperals; this was the last PDP-8 to offer this level of compatability.

It was not possible to continue from a halt without restarting the machine. In addition, none of the peripherals available on this machine needed DMA (data break) transfers.

Standard configuration:
The VT78 was sold with 16k words of DRAM with the keyboard and display of the VT52 terminal. An RX01 dual 8" diskette drive was included, packaged in a teacart pedestal under the terminal. The console (device 03/04) and the serial ports (devices 30/31 and 32/33) are compatible with the M8650 KL8E, with the latter extended to allow software controlled baud rate selection. There are two parallel ports; device 66 (compatible with the M8365 printer controller) and device 47, compatible with the nonstandard port on the M8316 DKC8AA. There is also a 100Hz clock compatible with the clock on the M8316 DKC8AA.

The standard ROM boots the system from the RX01 after setting the baud rates to match that selected by the switches on the bottom of the VT52 case.

Expandability:
This was a closed system, with few options. The base configuration was able to support two RX01 drives (later RX02), for a total of 4 transports. Various boot ROM's were available, including a paper-tape RIM loader ROM for loading diagnostics from tape. Another ROM boots the system from a PDP-11 server in the client/server configuration used by WPS-11.

Survival:
This machine is packaged like several other early personal computers, and as such, it appeals to collectors of such machines. Several are in public museum collections:

The DECmate I

Date of introduction:  1980
Date of withdrawal:    1984
Also known as:  DECmate (prior to the DECmate II, no suffix was used)
                VT278
Technology:
Based on the Harris 6120 microprocessor, packaged in a VT-100 box with keyboard and display.

Reason for introduction:
This machine was aimed primarily at the market originally opened by the VT78, using a new gate-array implementation of the PDP-8 built under contract with DEC by Harris. The Harris 6120 was designed to run at 10 Mhz, and the new packaging was optimized for minimum cost and mass production efficiency.

Reason for withdrawal:
Replaced by the DECmate II.

Compatability:
A new feature was introduced in the 6120 microprocessor: The Group I OPR combination RAL RAR was defined as R3L, or rotate accumulator 3 places left, so that byte swap (BSW) is equivalent to R3L;R3L. RTR RTL remained a no-op, as in the 6100.

Also, the EAE operations not implemented in the basic CPU cause the CPU to hang awaiting completion of the operation by a coprocessor. Unfortunately, no EAE coprocessor was ever offered.

The printer port offered software baud-rate selection compatable with the VT78 baud-rate selection scheme. The dual-port data communications option was flexible but completely incompatable with all previous PDP-8 serial ports.

The console and printer ports are not fully compatable with the earlier PDP-8 serial ports. Specifically, on earlier serial interfaces, it was possible to test flags without resetting them, but on the DECmate machines, testing the keyboard input flag always resets the flag as a side effect. In addition, on the console port, every successful test of the flag must be followed by reading a character or the flag will never be set again.

It was not possible to continue from a halt without restarting the machine.

The large amount of device emulation performed by the CPU in supporting screen updates severely limits the ability of the system to run in real time.

Standard configuration:
The DECmate I was sold with 32k words of memory, with a small control memory added to handle control/status, console device emulation and boot options. The console terminal keyboard and display functions are largely supported by code running in control memory (a less expensive alternative to dedicating hardware for this, as was done in the VT78).

The DECmate I came with an integral printer port, compatable with the VT78 (device 32/33), and it had an RX02 dual 8 inch diskette drive, mounted in the short pedistal under the terminal/CPU box. A 100Hz clock was included, as in the VT78 and PDP-8/A.

Expandability:
This was a closed system, with limited options. Specifically, a second RX02 could be connected (or an RX01, because that had a compatable connector), the DP278A and DP278B communications boards (really the same board, but the DP278B had 2 extra chips), and the RL-278 disk controller, able to accomodate from 1 to 4 RL02 rack mount disk drives.

When the DP278A option is added, additional routines in control memory come alive to handle terminal emulaton and allow diskless operation. The terminal emulator is an extended VT100 subset that is essentially compatable in 80 column mode. The DP278A option could support both asynchronous and synchronous protocols, and the DP278B could handle SDLC and other nasty bit-stuffing protocols.

Various pedestal and desk configurations were sold for housing the RX01 and RX02 drives, most being teacart style designs, but there was also a pedestal version that was essentially a repackaging of the RX02 with either 2 or 4 new 8 inch disk transports (physically incompatable with earlier DEC transports).

Survival:
As with the VT78, there are several in private collections. Are there any in public museum collections?

The DECmate II

Date of introduction:  1982.
Date of withdrawal:    1986.
Also known as:
        PC27X series.
Price: $1,435
Technology:
Based on the same 6120 microprocessor as the DECmate I, this shared the same packaging as DEC's other competitors in the PC market, the Rainbow (8088 based) and the PRO-325 (PDP-11 based).

Reason for introduction:
This machine was introduced in order to allow more flexibility than the DECmate I and to allow more sharing of parts with the VT220 and DEC's other personal computers.

Reason for withdrawal:
Replaced by the DECmate III.

Compatability:
Same as the DECmate I, except it could continue from a halt. There was better hardware for device emulation support, allowing for somewhat better real-time performance. The data communications port was an incompatable improvement on the incompatable DECmate I communications port. No built-in terminal emulation was provided, and the data communications port supported only one line, but aside from this, the data communications port is essentially as powerful as the DP-278B on the DECmate I.

Standard configuration:
The DECmate II was sold with 32K of program memory, plus a second full bank for dedicated control panel function emulation. Code running in the second bank is sometimes referred to as slushware; it looks like hardware to the PDP-8 user, but it is actually device emulation software that is loaded from the boot diskette.

An integral RX50 dual 5 1/4 inch diskette drive with an 8051 controller chip was included, along with a printer port, a 100Hz real-time clock, single data communications port, and interfaces to the monitor and keyboard. The diskette drive can read single-sided 48 track-per-inch diskettes, so it might be possible to read (but not write) IBM PC diskettes on it.

Expandability:
This was the most open of the DECmate systems, with a number of disk options: An additional pair of RX50 drives could be added, and with the RX78 board, it could support a pair of dual 8 inch drives, either RX01 or RX02.

As an alternative to the RX78, there was a controller for an MFM hard drive. The interface to the RX78 board wasn't fully compatable with earlier interfaces to RX01 and RX02, and there was no way to have both an RX78 and an MFM drive. The MFM drive could be up to 64 MB, with 16 sectors per track, 512 bytes each and at most 8 heads and 1024 (or possibly 4096) cylinders. A power supply upgrade was needed to support the MFM drive. DEC sold this machine with 5, 10 and 20 meg hard drives, Seagate ST-506, 412, and 225 respectively.

A graphics board supporting a color monitor could be added in addition to the monochrome console display; two variants of this board were produced during the production run, all slightly incompatable.

A coprocessor board could be added, with communication to and from the coprocessor through device 14. DEC sold three boards, an APU board (Z80 and 64K), and two XPU boards (Z80, 8086 and either 256K or 512K). If these added processors are used, the 6120 processor is usually used as an I/O server for whatever ran on the coprocessor. The XPU boards used a Z80 for I/O support, so 8086 I/O was very indirect, particularly if it involved I/O to a PDP-8 device that was emulated from control memory. Despite this, the DECmate version of MS/DOS is generally faster than MS/DOS on more recent 80286 and 80386 based IBM PCs because of effective use of the coprocessors (but they couldn't run MS/DOS code that bypasses MS/DOS for I/O).

Survival:
As with the DECmate I, and there a few in public museum collections:

The DECmate III

Date of introduction:  1984
Date of withdrawal:    1990
Also known as:
        PC23P-AA/LH with a black and white monitor
        PC23P-AJ/LH with a green monitor
        PC23P-AS/LH with an amber monitor
Price: $1,927 - $2885
	the low price is from an ad on page 2 of
	The Tech,
	May 7, 1985
Technology:
Same as the DECmate II.

Reason for introduction:
Again, DEC discovered that the market for large systems was dominated by other products, and that the PDP-8 based products were rarely expanded to their full potential. Thus, there was no point in paying the price for expandability.

Reason for withdrawal:
Replaced by the DECmate III+

Compatability:
Same as the DECmate II, except that the printer port is fixed at 4800 baud.

Standard configuration:
The DECmate III was sold with 32K of program memory, plus a second full bank for dedicated control panel functions, an integral RX50 dual 5 1/4 inch diskette drive with an 8051 controller chip, a printer port, a 100Hz real-time-clock, a data communications port, and interfaces for the VR-201 monitor and keyboard.

Expandability:
The following options were offered:

The PC23X-CA/LH allowed the standard monochrome monitor to be replaced with a VR-241 color monitor. Dual monitor configurations were not supported.

Survival:
As with the other DECmates, these machines appear in some personal collections. Are there any in public museums?

The DECmate III+

Date of introduction:  1985
Date of withdrawal:    1990
Also known as:
        PC24P-AA/LH with a black and white monitor
        PC24P-AJ/LH with a green monitor
        PC24P-AS/LH with an amber monitor
Price: $5,145
Technology:
Same as the DECmate II.

Reason for introduction:
This machine apparently represents the last gasp of the PDP-8, hunting for the remains of the ever-shrinking market niche that the earlier DECmates had carved out. The market niche was not there, and the production runs for this machine were short enough that UV erasable EPROM technology was used where earlier DECmates had used mask programmed chips.

Reason for withdrawal:
The market demanded word-processors based on PC-compatables.

Compatability:
Same as the DECmate II, but the machine was unable to read 48 track per inch IBM formatted diskettes. Again the printer port was fixed at 4800 baud.

Standard configuration:
The DECmate III+ was sold with 32K of program memory, plus a second bank for dedicated control panel functions, an integral RX33 single 5 1/4 inch diskette drive with an 8751 controller chip, a printer port, a data communications port and interfaces to the monitor and keyboard. A hard disk controller compatable with the optional one on the DECmate II was included, supporting an integral ST-225 20 MB disk; it is likely that it can only handle up to 1024 cylinders, but it is otherwise compatable with the DECmate II.

Expandability:
The same coprocessor option sold with the DECmate III was available, but because of the difficulty of adding a second floppy drive, this was rarely used (the Z80 was most likely to be used to run CP/M, but that system requires two drives to handle the installation procedure; an appropriately configured bootable image created on a DECmate II or III could run on a DECmate III+).

The same graphics board as used on the DECmate III was also available. The circuit traces and connectors for the Scholar modem are present, but this option was never sold on the DECmate III+.

Survival:
As very few were sold, the DECmate III+ is rare. Are there any in personal or museum collections?

Document History

A major revision of this document was undertaken in October 2013, converting it from the old USENET FAQ format to something more appropriate for the modern Internet.

This document had its origins in a USENET FAQ that was posted periodically in the 1990s, ending in 2001. Here is the heading on the last version distributed over USENET: 

Subject: PDP-8 Summary of Models and Options (posted every other month)
Newsgroups: alt.sys.pdp8,alt.answers,news.answers
Distribution: world
From: jones@cs.uiowa.edu (Douglas W. Jones)
Date: 8 Jun 2001 08:08:08 GMT
Followup-To: alt.sys.pdp8
Summary: Descriptions of all models of the DEC PDP-8 computer.
        Those posting to alt.sys.pdp8 should read this.
Expires: 8 Aug 2001 08:08:08 GMT
Organization: Computer Science, University of Iowa, Iowa City, Iowa, USA
Keywords: FAQ DEC PDP 8
Approved: news-answers-request@MIT.Edu

Archive-name: dec-faq/pdp8-models
Last-modified: Oct 1, 2000

Frequently Asked Questions about DEC PDP-8 models and options.

        By Douglas Jones, jones@cs.uiowa.edu
        (with help from many folks)

Sites known to carry reasonably current FTPable copies of this file:

        ftp://rtfm.mit.edu/pub/usenet/alt.sys.pdp8
        ftp://ftp.uu.net/usenet/news.answers/dec-faq
        ftp://src.doc.ic.ac.uk:/pub/usenet/news.answers/alt.sys.pdp8

Reasonably current automatic translations of this document to HTML format
for the World Wide Web are available from:

        http://www.faqs.org/faqs/dec-faq/
        http://www.cs.ruu.nl/wais/html/na-dir/dec-faq/.html

An obsolete version of this file is available on the Walnut Creek USENET
FAQ CDROM; another version will be published as part of the FAQbook by
Pamela Greene et al.

This posting conforms to RFC1153 USENET digest format (with exceptions due
to the fact that it is not really a digest).