The University of Iowa's DEC PDP-8A Tour/Inventory of the System
Part of
the UI-8 pages
|
The University of Iowa's DEC PDP-8A Tour/Inventory of the System
Part of
the UI-8 pages
|
|
| The system. |
|---|
What follows is an inventory that can also serve as a guided tour through the parts of the PDP-8 computer originally purchased by the University of Iowa psychology department in 1965, serial number 85. Click on any of the thumbnail photos for a larger view. Where a part of the system is labeled, we have made an effort to transcribe that lablel here; as such, all of the manufacturing quality control dates and serial numbers are here. All significant changes we have made to the system since receipt are briefly introduced here; we do not count as significant the replacement of defective parts or temporary removal of components for repair; those count as normal repair and maintenance.
See also the corresponding section of the Bug List
|
| The computer rack. |
|---|
The CAB 8-A rack stands just over 69 inches high and is 42 inches wide and just over 27 inches deep, with the side skins in place and the front and rear doors closed. If the doors, side skins, electronics and all removable fittings are removed, what remains remains of the rack is just 4 vertical posts joining the top of the rack to the bottom. The posts are made of 1 by 2 inch rectangular steel tube, welded to the base and top assemblies, which are themselves made of similar tubing and also 1 by 1 inch square steel tube. The clear space between the posts at the front and back of the rack is 17 3/4 inches wide, while the clear space on the sides is 19 3/8 inches wide. It appears that DEC built their own racks even before the introduction of the H960 cabinet that they used when they moved to TTL logic with the PDP-8/I and later machines.
The front rails of the rack are drilled and fitted with 10-32 threaded inserts (probably Rivnuts) on alternating 3 inch and 2.25 inch centers. These spacings are compatible with standard 19-inch relay racks, allowing mounting equipment that is a multiple 5.25 inches in height. 5.25 inches is 3 standard rack units, a unit of measurement dating back to the 1930s.
The side skins of the rack are easily removed. They are held on by gravity
and friction. To remove them, just lift up (hard) and pull away from the rack.
At the bottom of the rack is a Caravel intake fan sitting atop an air filter assembly. Comair Rotron still makes the Caravel line of fans. The circular black-on-aluminum label on the fan hub would look something like this if it were unrolled:
|
BALL BEARINGS LUBRICATED FOR LIFE 115 VAC 50/60 CPS 53 WATTS MODEL CL2L |
ROTRON MANUFACTURING CO. WOODSTOCK · NEW YORK · MADE IN USA |
The filter itself had the following glossy paper labels on the side, but these fell off as soon as the filter was removed from the assembly for cleaning:
|
| |||||||||
Research Products Corporation is still around, doing business as Aprilaire and they still sell RP Super Filter Coat Adhesive.
Since receiving the PDP-8, we have made the following changes:
See also the corresponding section of the Bug List
|
| The computer on slides. |
|---|
The two halves of the backplane of the computer are mounted on hinges on the top half of the slide-out framework, while the front panel hangs below. The framework is made of 1 by 1 inch square and 1 by 2 rectangular welded steel tube, the same type of tubing used in the relay racks. Under the center rear of the frame is a difficult-to-read aluminum label carrying the machine's serial number:
At the bottom left of the rosewood veneer front of the computer just above the front panel there are two yellowed pieces of paper taped to the front. They contain typed text, octal memory dumps of a modified version of the RIM loader and a tape copy routine:
d i g i t a l EQUIPMENT PDP-8 CORPORATION MAYNARD MASSACHUSSETTS 85
HIGH SPEED RIM LOADER MOD 7756- 6014 6011 5357 6016 7762- 7106 7006 7510 5374 7766- 7006 6011 5367 6016 7772- 7420 3776 3376 5356 7776- 0000 5306
TAPE COPY ROUTINE: 200 7200 6016 6026 6011 5203 6021 5205 5200
The computer came with the following hardware documentation:
The User's Handbook lists the combined weight of the computer and power supply as cabinet weight as 250 pounds.
|
|
| Front panel | |
|---|---|
The actual artwork on the front panel is reverse-painted (or rather, silk-screen printed) on glass, with fiber composition board serving as structural support and as a spacer between the circuit board and the glass panel. The lights are all incandescent bulbs, not LEDs, as would be common in later computers.
Large parts of Section C of the User Handbook explain how to use the front panel.
Status: As of April 13, 2015
the front panel is fully repaired functional.
|
|
|
| Memory half backplane | ||
|---|---|---|
There is a paper label just to the left of top center when seen from the wire-wrap side of the memory half backplane. The label is a pre-printed form with typed in identifying numbers, and it is starting to peel off. The label reads:
DIGITAL EQUIPMENT CORPORATION CORE MEMORY 08-291-1000 Type 184A
The back (hinge-side) vertical rail of the frame has an aluminum sticker on its upper inside face -- almost entirely obscured by the pins of the wire-wrapped backplane. This reads:
d i g i t a l EQUIPMENT M8 100 CORPORATION MAYNARD MASSACHUSSETTS
The black box filling the center of the upper third of the memory half backplane holds the actual core memory of the machine, 4K words of 12 bits each. The memory box has two labels on its top side, a larger aluminum label giving the serial number and a smaller paper quality-control label:
F
F FERROXCUBE
F MEMORY EQUIPMENT MODEL: SERIAL: 51-238-71
ACCEPTED BY DATE 10/25/5
The most likely interpretation of the handwritten date on the quality control label is October 25, 1965, but the writing is cramped enough that there is some doubt about the day.
The theory and high level operation of the core memory is explained in Chapter 4 of the Maintenance Manual:
|
| Map of memory half backplane |
|---|
The memory half of the backplane came prewired for three options that could be installed by merely plugging in the appropriate boards.
Max Dietrich, who worked on this machine between 1966 and 1968, recalls being asked, at the end of each fiscal year, what to spend the surplus grant funds on. He always asked for memory. Unfortunately, the cost to add the first 4K additional words was $13,000, which was far more than the grant surplus.
The Memory Utilization Module List in chapter 10 of the Maintenance Manual gives the detailed map of the memory half backplane, identifying what board plugs into what slot and (on a coarse scale) what data flows through that board. This is a giant fold-out page just under 2 by 3 feet.
Status: As of Dec. 16, 2024
we have found and begun to fix noise problems on the memory address bus.
|
|
| CPU half backplane | |
|---|---|
There is a paper label just to the right of top center when seen from the wire-wrap side of the CPU half backplane. The label is a pre-printed form with typed in identifying numbers, and it is starting to peel off. The label reads:
DIGITAL EQUIPMENT CORPORATION CENTRAL PROCESSOR 08-291-0100 Type PDP8-85
Note that (unless this is a coincidence) the serial number of the machine is embedded in this label.
The back (hinge-side) vertical rail of the frame has an aluminum sticker on its upper inside face -- almost entirely obscured by the pins of the wire-wrapped backplane. This reads:
d i g i t a l EQUIPMENT 8P-103 CORPORATION MAYNARD MASSACHUSSETTS
|
| Map of CPU half backplane |
|---|
The boards are mostly DEC R-Series flip-chips. Most of these are documented in the 1967 Logic Handbook section on R-Series Logic, pages 26-82. The exceptions are special purpose R-Series boards made specifically for the PDP-8, which are documented in the Maintenance Manual, Chapter 10.
The CPU half of the backplane came prewired for two options that could be installed by merely plugging in the appropriate boards in the bottom two rows.
The Processor Utilization Module List (UML-E-8P-0-19) gives the detailed map of the CPU half backplane, identifying what board plugs into what slot and (on a coarse scale) what data flows through that board. This is a giant fold-out page just under 2 by 3 feet in size included in the Maintenance Manual.
Status: As of Nov. 17, 2017 the CPU registers appear to be good. However, until we can get some kind of response from memory, we cannot be sure of anything.
See also the corresponding section of the Bug List
|
|
| Power supply |
|---|
The power supply front panel has the total-hours meter for the machine, reading 26879.0 hours when we received the machine. That comes to almost 1120 days or a bit over 3 years. If used for 40 hours per week and 50 weeks per year, this translates to over 13 years of productive use, roughly 1966 to 1979. (The machine was almost certainly used more in its early years and less in its later years.)
This panel is only intended to be accessible when the machine is undergoing maintenance. It has a power outlet where a technician can plug in small equipment, an auxiliary on-off switch, a meter to monitor the supply output, and a knob for the variac that controls the marginal check voltage.
All power supply outputs are somewhat regulated by the use of a constant voltage transformer. Banks of large electrolytic capacitors are used to eliminate ripple. The capacitors were sized to allow continued operation during a power outage lasting 1/20 of a second. The marginal check and memory inhibit outputs of the power supply are more complex, with linear regulators. The supply has the following outputs:
Output Regulated? Capacitance
(Farads)Current
(Amps)Purpose -15 no 0.210 ~24 logic +10 no 0.105 ~12 logic +/- 0-15 variac 0.035 2 marginal check 27-37 linear 0.034 2 memory inhibit 27-37 linear 1.5 memory read/write
In restoring the power supply, we found C15 was bad (the final output filter capacitor on the memory inhibit supply). After the bad capacitor was removed, we found that it had a date code reading 6532. 4-digit date codes are frequently composed of a 2-digit year code followed by a 2-digit week code. If so, this capacitor was manufactured during the 32nd week of 1965 -- some time in early August.
The details of the power supply indicators and controls are given in the Maintenance Manual.
The internal workings are documented here, with a focus on the relationship of the power supply to the core memory:
The schematics for the basic supply and regulators are also given:
David Gesswein has extensively documented his experience repairing the power supply for his PDP-8 computer:
Faston quick-connect connectors: In working with the power supply, we learned that the chassis mount Faston quick-connect connectors on the supply outputs (also used on several other components of the system) can be a bit fragile. These connectors have enough friction that it is tempting to use a screwdriver for leverage unplugging them and to use tools to get extra push when re-plugging them. Several were broken when we got the machine, and we may have broken more working on the machine. We strongly recommend two precautions when working with these connectors:
Since receiving the PDP-8, we have made the following changes:
Status: As of Dec. 16, 2014 the power supply appears to operate within specs.
|
| The plenum door |
|---|
|
| Power Control |
|---|
d i g i t a l POWER 834 CONTROL
The outside face of the plenum door has an on-off switch. Inside, there is a power contactor Behind the plexiglass, there is a relay and several terminal strips.
A descriptive paragraph and schematic for this panel is included in the DECtape Control Unit Type TC01 Instruction Manual.
The most prominent component of the power control panel is a mercury contactor that switches on the power controlled by this panel, closing its contact when the on-off key switch on the front panel of the PDP-8 is turned on. The contactor has the following label:
TYPE COIL TUBE LOAD RATINGS MOTOR EM-1 115 V.
A.C.
60 cy.A.C.
115 V./60 A.
230 V./35 A.D.C.
120 V./12 A.
220 V./ 7 A.A.C. - 2 HP.
D.C. - 1/2 HP.E EBERT ELECTRONICS CORP.
FLORAL PARK, L. I., N. Y.U.L.
Since receiving the PDP-8, we have made the following changes:
Status: As of Feb. 4, 2015 the power control panel and related wiring are safe to use.
See also the corresponding section of the Bug List
|
| |
| The ADC rack | ||
|---|---|---|
As with the uncommitted areas of the computer rack, the front rails of this rack are drilled and fitted with threaded inserts for rack-mounted equipment that is a multiple of 3 standard rack units in height. The right side of the rack carries an aluminum University of Iowa inventory sticker at the top:
UNIVERSITY OF IOWA
178448
Since receiving the PDP-8, we have made the following changes:
|
|
| The ADC/multiplexor | |
|---|---|
The top front of the ADC rack contains a control panel with a 12-bit display labeled A-D BUFFER and a 6-bit display labeled MULTIPLEX ADDRESS. To the right is a rotary knob labeled WORD LENGTH adjustable from 6 to 12. The legend on the left reads:
A-D CONVERTER 138E MULTIPLEXER 139E
The list price for the Type 138 Analog to Digital Converter was $4,500 and the list price for the Type 139 Analog Multiplexer was $3300. Under the indicator and control panel, behind the French doors on the front of the rack, there are three mounting panels making up a wire-wrapped backplane. The top two panels have identifying stickers on the inside face of the left mounting plate, when seen from the rear. These stickers can only be read with some of the boards removed from the backplane. the topmost mounting panel has the following aluminum sticker:
d i g i t a l EQUIPMENT 138E-14
138E-13
CORPORATION MAYNARD MASSACHUSSETTS
The second mounting panel has two paper labels and an aluminum label sticker, applied one under the other:
DIGITAL EQUIPMENT CORPORATION MULTIPLEXER CTRL 08-291-2600 Type 139
DIGITAL EQUIPMENT CORPORATION AD Converter 08-291-8100 Type 138E
d i g i t a l EQUIPMENT AA03-
10
CORPORATION MAYNARD MASSACHUSSETTS
These stickers confirm that these two panels are backplane of the ADC-multiplexer advertised by the control panel at the top of the rack. The wire harness linking the control panel to these boards is further confirmation. There is documentation for this device in:
Status: On Feb. 21, 2018,
we removed most of the locally installed
wiring between the ADC and the magnetic tape drive controller below it,
see below. A few boards in the lower right side of the
analog multiplexor backplane (seen from the wire side) appear to be part
of the mag tape drive controller. These remain in place, and no other
work has been done on the ADC or multiplexor.
The third panel of the wire-wrapped backplane at the top of the ADC rack has a paper label on the outside of its right mounting plate, when seen from the rear. This reads:
d i g i t a l MOUNTING 1943 PANEL
The Type 1943 mounting panel was a generic backplane segment. The logic handbook gives a base price of $111 for this panel. The mounting panel includes 8 Type H800-W connector blocks, each able to hold 8 single-height or 4 double-height flip-chip circuit boards, using wire-wrapped interconnection with 24-gauge wire.
The wiring on it appears to be hand done with 30-guage wire, the wrong gauge for these blocks. There is a bundle of green wires from the left rear of this panel to the control panel immediately below. From the labels on that panel, we conclude that this is a locally designed and built controller for a long-gone magnetic tape drive.
Max Dietrich, who worked on this machine between 1966 and 1968, recalls
that the lab found a rack-mounted magnetic tape drive somewhere and attempted
to interface it to the PDP-8. At the time he left, the interface was not yet
functional. This is probably the remains of the interface for that drive.
Status: Since we do not have the drive nor any hope of finding an equivalent, nor any documentation, an attempt to restore this interface would be futile. Therefore, on Feb. 21, 2018, we removed most of this interface, along with the control panel, see below.
|
|
| An unknown panel and Tally reader | |
|---|---|
Status:
Since we do not have the tape drive nor any hope of finding an equivalent, nor
any documentation,
on Feb. 21, 2018,
we removed the tape drive controller and this control panel, along with
all of the attached cables to which they were wired.
All that remains from the tape drive is a relay board hanging from the top
rear of the rack.
See also the corresponding section of the Bug List
|
| The Tally reader |
|---|
|
|
| Capstan and take-up reel drive | |
|---|---|
The Tally high-speed paper tape has two labels on the back. The smaller appears to be from the vendor, printed in red ink on aluminum and stuck to the center right top of the panel; the larger label to its right is from the manufacturer, printed in hard-to read negative, blue on aluminum:
SERIAL 9789 TMC TECHNICAL MEASUREMENT CORPORATION
SERIAL NO 351/0811
TALLY 424 TAPE READER MOTOR 60 CYCLE 115 VOLTS .44 AMPS FUSE 6/10 AMP MDL (BUSS) CAPSTAN DRIVE MECHANISM 24 PULSE VOLTS 50 COIL OHMS
This paper-tape reader cost $825 in 1964, according to the A Fourth Survey of Domestic Electronic Digital Computers. An abridged Tape Reader Manual - Model 424 is available on-line. As of 2013, replacement manuals are commercially available for $35.
According to the The Tally 424 Manual cited above, the Tally 424 came in 3 models: The 424C (console, that is, desktop), the 424PF (fanfold, rack mounted) and the 424PR (reel to reel, rack mounted).) Ours matches the photograph in Figure 4 of the manual showing the 424PR.
The fanfold tape bin below the reader arte nothing like the fanfold tape bins sold by Tally. It is largely held together by pop rivets, and was probably made by the psychology department's machine shop. If we can get accurate dimensions for Tally's original bins, it might be better to try to reproduce them. Figure 3 of the manual shows two of the Tally fandold bins on the 424PF. In addition, a Tally fanfold bin is shown in Figures 1 and 2 of the manual for the Tally Model 420 punch, a device with exactly the same capstan and take-up reel drive mechanisms.
The Tally 424 manual shows several options that are not present in our machine. Some of these may have been stripped out by various users, since the machine shows evidence of at least two generations of use prior to going into storage 30 years ago. There is no end-of-tape sensing switch, and there is no capstan commutator; these appear to be factory options. In addition, there is no coil to trip the reverse escapement. This may also be a factory omission, but there is also evidence of wiring changes that post-date manufacturing. As a result, although the mechanism allows bidirectional tape motion, the electrical system only allows forward operation.
The manual shows a spark-suppressor circuit (snubber) for the escapement coils in section 3.6 based on a resistor-capacitor network. Either Tally changed their design or someone modified this to use a diode snubber (possibly a Zener diode). The terminal strip for the snubber only has 3 terminals plus ground, while Figure 25 in the manual shows a strip with 4-terminals plus ground. If the 3-terminal strip and diode snubber are factory originals, the tape reader must have been sold as a unidirectional machine.
The first generation of field modifications to the tape reader may have been
been made before the Psychology department got hold of the machine.
There is an unused octal
tube socket on the
rear chassis of the tape reader. The socket is not shown in the manual, and
the hole in the aluminum chassis for the socket is not cleanly punched
the way you would expect for a factory installation. The psychology department
appears to have installed a simple on-off switch in the center front of the
reader. As we received this, it came with an ink-on-cloth-adhesive tape label
saying "turn off when not in use".
The lubrication instructions in the reader and punch manuals indicate that the bearings in the motor, capstan and take-up reel drives should be lubricated with SAE 20 oil twice a year. The friction clutches in the capstan drive are to be oiled with silicone oil (DC200, 350CS) rarely; the same silicone oil is to be used on the nylon gears. A quick check shows that this oil is available on eBay in 4oz bottles.
The escapement armature lever fulcrums are to be lubricated with Tally pivot grease A. (See page 19 and 20 in the Tape Perforator Manual.) We have not determined a modern equivalent for this.
Note that the KS7470 oil recommended for the Teletype is close to SAE 20 weight and should work well here. There is no reason to believe that its brother, KS7471 grease or an appropriate substitute, would not be appropriate for the armature lever fulcrums.
Since receiving the PDP-8, we have made the following changes:
Status: On May 27, 2014, we rebuilt the motor and obtained a second and almost identical used tape reader as a source of spare parts. To date, we have not reinstalled the drive in the ADC rack.
|
|
| The scope display interface | |
|---|---|
DIGITAL EQUIPMENT CORPORATION OSCILLOSCOPE DISPLAY 08-291-3400 Type 34D
DIGITAL EQUIPMENT CORPORATION ////////ME OPTION & CLOCK //-/91-2500 Type 151
d i g i t a l EQUIPMENT 34D 18
10
CORPORATION MAYNARD MASSACHUSSETTS
|
| Map of 34D Scope Display |
|---|
The Module Utilization List for the Type 34D Oscilloscope Display documents the display interface as part of a multiple-device backplane wired for the following bundle of devices: The Type 350B Incremental Plotter, the Type 34D Oscilloscope Display, the Type 750C High Speed Reader, the Type 75E High Speed Punch, the CR01C Card Reader and the PC01 (a variant high speed reader-punch). The labels on this device hint that the Type 34D Display was offered bundled with a real-time clock and perhaps other devices, but inspection of this backplane shows that large parts of it conform to the documentation for the above bundle.
From this, we infer that we have the following device interfaces, plus others that seem to have been added locally and may require reverse engineering to work out what they do.
The Oscilloscope Display Type 34D is documented in the following places:
Aside from the I/O bus slots, There appears to be no factory original wiring on the bottom row of this pair of mounting panels. As it came to us, there were loose wires dragging on the floor from this section, and the homebrew wiring here was done with 30-gauge wire, while DEC used 24-gauge wire.
The switches in the blank panel at the very bottom of the rack are further evidence of the extent of local modification: The are both labeled ON and OFF. The left one, in red, says MUST BE ON FOR, with the remainder missing. and OFF. The right one, in green, says MUST BE ON FOR DR RANDALL S PROGRAM . The wiring to these switches appears to be entirely missing.
Status: As of Oct. 2, 2014
we have determined that significant modifications have been made to this
interface. We will need to reverse engineer the interface in order to
figure out what is going on.
|
| ADC back door |
|---|
Since receiving the PDP-8, we have made the following changes:
|
| The interface panels |
|---|
|
| A power supply |
|---|
Status: We suspect this supply provided the power
required by the Tally paper-tape reader. We have not worked on this supply yet.
See also the corresponding section of the Bug List
|
| Type 779 supply |
|---|
d i g i t a l POWER 779 SUPPLY
The type 779 power supply is documented, in passing, in the Oscillosocpe Display Control 34D Instruction Manual. This references additional documentation in the DEC Systems Modules Catalog (C-100). A schematic for this supply is included in the PDP-7 Maintenance Manual. A short manual and schematic for this supply is included in the DECtape Control Unit Type TC01 Instruction Manual.
Status: As of Oct. 9, 2014 this supply has been tested and appears to be working correctly.
See also the corresponding section of the Bug List
|
| The Teletype |
|---|
The Teletype label on the lower back, just to the right of the power cord, reads, in negative, black ink on aluminum:
TELETYPE CORPORATION MODEL 33 CSAM SUPPLY INPUT: 115 V. 60~ A.C. 4 AMP. MAX. SIGNAL INPUT: 50 V. MAX. D. C. 0.1 AMP. MAX.
Under the cover, on the right side of the typing unit chassis, just below the carriage motion belt, there is a label giving the rebuild date of this unit, 10/30/74. It is printed in negative, in bright yellow ink on aluminum (a combination that is not necessarily easy to read):
REMANUFACTURE DATE: 10/30/74 BY CARTERFONE COMM. CORP.
|
| The serial number? |
|---|
The Teletype ASR 33 contains the following subsystems:
|
|
| The TU removed from the TTY | |
|---|---|
|
| TTY Printer |
|---|
|
| Reader & punch |
|---|
|
| Motor & drum |
|---|
The following documents came with this Teletype:
We do not have originals of the following, but they are available on line:
The teletype manuals state that the Teletype should be lubricated with KS7470 oil. This is an old Bell System part number, and it is long gone, but it appears that several white mineral oils are close to exact substitutes for it.
DEC routinely modified Teletypes used with DEC computers. They disabled the "Who are you" answer-back mechanism, and they included hardware flow control. These changes are documented in the Maintenance Manual.
Since receiving the PDP-8, we have made the following changes:
Status: As of Mar. 18, 2017 the teletype mechanism works reliably. The platten needs reconditioning, the back of the stand is missing, and we have not replaced the cover.
