The University of Iowa's DEC PDP-8

Restoration Log

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



This is a chronological log of the progress restoring the University of Iowa's PDP-8 computer. Entries are added at the end as work progresses. Click on any thumbnail image to see full-sized image.

Apr. 4, 2014, TTY Work, Power Supply, Fans

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TTY print mechanism pieces
Bug 25: The Teletype print head would not move sideways on its carriage, so Harrison Pickett and Doug Jones removed the platten and then extracted the print head and carriage from the print mechanism so that the frozen ball bearings could be removed. All of them were stiff with congealed grease, and several would not rotate at all. Repeated soakings in WD40 removed all of the congealed grease so that the bearings spun freely. This was very messy work.

With the print head removed, it is easy to see the cluster of vertical code bars that extend side-to-side under the carriage. These rise and fall to convey the ASCII code of the letter to be printed. The bottom sized of these bars have a system of notches and tabs that make up the "stunt box" that decodes control characters such as CR, LF, and BEL, while the heavy horizontal bar across the front of the machine actually powers the carriage, causing it to raise and rotate the print wheel to position the right character for printing, and then driving the hammer against the print wheel.

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Mounting the new C17
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All of the capacitors
Bug 20: The replacement for C17 arrived (See log entry for Mar. 27, 2014). The new capacitor was significantly smaller than the old one, so we had to re-size the capacitor clamp (a simple matter of bending some U-bends into the sides of the clamp and tightening the curves at the ends), and then Nick Becker drilled a new hole in the power supply frame to hold the shortened clamp.

With the new C17 in place, all of the capacitors in the supply have been replaced or reformed. The only job remaining to be completed on the power supply is that of fixing the broken connectors and, of course, testing the result.

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Fan cleaning
Nick Becker and Jacob Accord removed the fan assemblies from the bottoms of the two relay racks and set to work cleaning the fans and air filters. As is usually the case with fans, they were dirty, and the air filters were filthy. The stickers on the air filters that gave the cleaning instructions fell off immediately when the filters were removed, but their instructins were clear enough. In the photo, Nick Becker is cleaning gummy deposits off of the fan with Goo Gone.

The filters are E Z Kleen filters made by Reserch Products Corporation. This company is still around, doing business as Aprilaire. The filters are designed to be coated with a product (still available) called RP Super Filter Coat. U.S. Patent 2,865,466 explains this coating as a tacky self-emulsifying petroleum product. The idea is that it acts as an adhesive when the filter is in use, grabbing dust particles, but it emulsifies in warm water, releasing the accumulated dirt and rinsing off of the filter.

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An unsafe plug
Bug 16: On close inspection, we found electrical tape around one fan's power leads, and on removal, we found a poorly done splice. We cut that lead and used a crimp splice to attach a replacement wire. Both fans were tested and work; as fans go, they are reasonably quiet. We also concluded that the plug on the PDP-8 line cord was unsafe both because it is almost impossible to safely terminate fine stranded 12/gauge wire on the screw terminals in the plug and because the cardboard insulation over the termination can easily allow individual escaped wire strands to escape the connector shell.

Apr. 14, 2014, Misc

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Tally Reader Disassembly
Bug 28: The Tally paper-tape reader has been removed from the rack and partially disassembled for cleaning. The photos, by Jacob Accord, show the arrangement of the wires before disassembly. We have no evidence concerning the relay socket to the left in the image of the main reader-cable connector. Note, also, the splicing of the blue wires to the power switch. Evidently, the power switch in the bottom center of the front panel is a local addition to the Tally reader.

Bug 18: We got a box of 100 screws and lock washers, indistinguishable from DEC's original screws. The correct specification is 10-32 Stainless Steel Truss Head Screws with #10 External Tooth Stainless Steel Lock Washers. All missing screws have now been replaced, and we have many spares.

Bug 25: We researched the KS7470 that the Teletype manual calls for, and concluded that it was equivalent to Starrett 1620 Tool and Instrument Oil, which we then ordered.

Apr. 20, 2014, TTY and Tally Reader Cleaning

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The Teletype punch
and print unit base
Bug 25: The code bars under the Teletype print mechanism are caked with grease residue. To get access to these for cleaning, we had to remove the paper-tape punch and platten. With these removed, the base of the Teletype printing unit looks rather naked.

In the photo of the Teletype print unit base, the code bars run vertically. These carry the ASCII code of the character to be printed, being "read" by the carriage from above. The horizontal bars below the center portion of the code bars are the "stunt box" that decodes the control characters to actuate carriage return, line feed, bell and other non-printing functions. The shaft extending vertically between the motor and the code bars is the main cam shaft, while the selector magnet is at the bottom center. The latter actuates the code bars, with a cluster of cams on the camshaft used to determine which code bar is being addressed at each instant.

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Tally 424 capstan and take-up reel drive
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Motor Run Capacitor
Bug 28: The Tally 424 paper tape reader mechanism was rather grimy, so we decided to take it apart and clean it. This allowed some nice photos of the many differentials in the mechanism.

Bug 29: In disassembling the reader, we realized that, as with the PDP-8 power supply, this reader includes a motor-run capacitor that is probably PCB-filled and devoid of internal protection. As these are prone to explosion and constitute an environmental hazard, we will replace it. The engraved markings on the capacitor read:

62-34 60CY

That is, this capacitor was made by General Electric in the 34th week of 1962, and it has a capacity of 3 microfarads at 370 volts.

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An RS-232 converter
We scrounged up an ancient Dataprobe CL/232 converter box, never used, still in its original wrapping. This box converts from RS-232 voltage levels to the 20mA current loop signalling conventions required by the Teletype. This will allow us to test the Teletype without relying on the PDP-8, and it will allow us (with different internal jumpering) to substitute a modern computer for the Teletype as a console device on the PDP-8.

The CL/232 converter was made in the 1980s and never used until this year, so its power supply capacitors were in as much need of reforming as those in the PDP-8. Its power supplies are all simple linerar supplies, so we hooked it to a Variac and slowly raised the line voltage over a period of a week in order to reform the power supply capacitors. We also built a cable for it to connect to the serial port of a Raspberry PI computer. We still need to construct the cables needed to connect it to the PDP-8.

Apr. 25, 2014, TTY reader-run relay, Tally motor

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Reader-run relay
Bug 14: Having obtained a working salvaged reader-run relay from Wayne Durkee, we decided to install it, following the instructions in DEC's Teletypewriters LT33 drawings, with supplementary explanations given by Harris Application Note HM005. The documentation was straightforward, but DEC's material did not provide the explanations that Harris provided. In sum, our Teletype had already been modified to operate on a 20mA current loop -- half of the DEC modification had to do with that. What remained to do was install the reader-run relay board and wire the cable appropriately.

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Motor rebuild
Bug 29 and Bug 30: While starting work at replacingt the Tally paper-tape reader's motor run capacitor, we discovered that the insulation on the motor leads was in bad shape. While the wiring in other parts of the reader seems to have modern plastic insulation, the motor wires appear to have been insulated with natural rubber, and like much old rubber insulation, it tends to crack under stress. Almost an inch of insulation was gone from one motor lead, and several cracks were evident. Therefore, we set out to replace the motor leads with modern wire. The photo shows motor rebuild in progress, with one of the old wires set across the end to display the condition of the old insulation.

The most time-consuming part of the motor rebuild process was removing the old lacings on the end of the motor winding and re-lacing the windings. This was necessary because the crimp connectors connecting the motor leads to the windings were held in by the lacings, and any strain on the leads was taken by the lacing. We used new waxed lacing tape and did our best to duplicate the origina lacing pattern. Note that the crimp connectors we replaced were more compact than the new ones, but tight lacing holds the new bigger connectors down so they just fit inside the motor end caps.

Apr. 29, 2014, Tally motor, TTY, PDP-8 backplane

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Motor testing
Bug 29 and Bug 30: The motor rebuild was completed, oil was added to the bearings, and the motor, and the new motor run capacitor were installed on the tape-reader chassis shelf, wired up and tested. While disassembling the motor, all leads were tagged as to where they had been connected. The wires were replaced one at a time and the tag transferred to the new wire. This allowed us to rewire things with some assurance that the wires went to the right places.

Before reassembly, all wires were tested statically for continuity through the motor with an ohm-meter, and after reassembly, it was verified that there was no conductive path from the motor windings to ground. Then, as shown in the photo, the motor was powered up with clip leads, using baggies over all clip-lead to wire connections for safety. The drive gear on the left end of the motor is a blur in the photo because it is spinning. The motor ran almost silently, and after running a few minutes, it was warm but not hot. With an AC voltmeter, the motor housing measured about 20 volts, but this was at a very high source impedance, almost certainly the result of capacitive coupling to the motor windings, as DC testing of the motor after installation confirmed again that there is no DC path from motor windings to housing.

Bug 26: Cale Bierman and Matt Adamczyk began reassembling the Teletype after degumming was complete.

Bug 19: Tony Andrys has completed the backplane inventory for the PDP-8. We have confirmed 3 serious discrepancies where it appears that the wrong boards are in the wrong slots, probably the result of "tourists" pulling random boards to show them to people and then putting them back in the wrong places. Specifically:

SlotBoard FoundBoard Expected
PA29 R1115 R603
PB22 R603 R111
PA36 R302 W501

We will need to do some reverse engineering of the wiring for these slots in order to verify whether they are wired for these anomalous boards or wired in conformance with the documentation.

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Installing a rivnut
Bug 31: When the Tally reader was removed from the rack, we found that two of the original rivnuts in the rack had been removed and new holes drilled to accomodate the mounting screws for the Tally reader. Since our plans are to put the reader back in a different rack locaiton, we decided to replace the missing rivnuts with new ones.

Rivnut installation tools are widely available for $20 or more, but an improvised tool can be made from a stack of washers, a hex-head or allen-head screw, a nut and a pair of wrenches. We made such an improvised tool and used it to replace the missing rivnuts.

Apr. 30, 2014, TTY reassembly, 779 power supply

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TTY reassembly
Bug 26: Cale Bierman and Matt Adamczyk continued reassembling the Teletype. The focus was on the selector cam cluster and codebar clutch. Reassembly was straightforward until it came time to install the codebar clutch drum onto the clutch shoe assembly. No matter how we manipulated the pieces, the drum would not fit over the two shoes. Referring to drawings in the Teletype manuals, we found that the clutch shoe lever had to be pushed all the way to the stop lug against the tension of the clutch shoe lever spring in order to bring the clutch shoes into disengaged position before slipping the cutch drum in place.

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Type 709 supply and odd wiring
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Supply wiring details
Bug 10 and Bug 32: The Type 709 power supply was pulled from the ADC rack in preparation for reforming the capacitors there. The first three photos here were taken by Jacob Accord. They document the original wiring of the supply. Note, particularly, the connection of the red wire to the common terminal of the right side of the supply. This wire was simply pushed into the terminal! It was obviously a local addition, apparently the ground wire for the home-made mystery power supply mounted above the 779 supply. If we reconnect that supply, we will have to do a better job of connecting it.

May 1, 2014, TTY cable and reader-run relay

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Completed CCU changes
Bug 8, Bug 14 and Bug 15: The shorter half of the old Teletype cable, terminated with a new 6-pin Jones plug, was wired to the terminal block at the back of the Teletype's call control unit, with two wires continuing forward to the reader-run relay. All of this was done following the instructions in DEC's Teletypewriters LT33 drawings. The cable routing used was based on the very poorly reproduced photo in the center of sheet 1 of drawing 7505038-0-0 Rev. C (the first drawing sheet the document). Because of the low quality of the photo, we had to guess some of the cable route, but the drawing (and the length of the wire in the salvaged reader-run relay) make it clear that the cable had to be routed along the left side of the the call control unit, very close to the moving parts of the Teletype. Our photos are much better than the original DEC photo and should make our cable routing clear. Also note that we added a disconnect point in the wiring to the reader-run relay so that the external portion of the Teletype cable can be removed without the need to redo the wiring all the way to the Reader Run relay.

See the log entry for March 20 for the terminal strip connections in the Teletype and the pinout on the 6-pin Jones plugs in the cable.

May 5, 2014, Rack repair, power connectors

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The doorstop
Bug 31, The top French doors on the front of the ADC rack had no doorstop to prevent their pressing inward against the wire-wrap pins on the backplanes they conceal. Close inspection of the bar that forms the bottom threshold of this pair of doors showed a pair of mounting holes, 3 inches apart, centered in the door opening. These evidently held a doorstop that mated with the magnetic latches on the lower backs of the doors.

The magnetic latches extend to almost an inch above the bottom threshold and are almost six inches apart. When the doors are held so that their front surfaces are coplanar, the latches are about half an inch forward of the line through the two doorstop mounting holes. From this, we inferred that the doorstop had been a piece of 1" angle iron 6" long. We fabricated a new stop and installed it, as shown in the photo. The iron is unfinished, with the front surface polished to a high shine using a wire brush. It might be appropriate to paint it, but the few poor images we have found of similar DEC racks show a shiny metal doorstop.

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Bug 20. Anixter Component Solutions of Dorset, England provided free samples of the DC-202 connector bushings, in black, and Michael Thompson and the Rhode Island Computer Museum provided salvged DC-202 bushings with H-202 connectors in the correct colors. The photo shows both, before unsoldering the salvaged connector clusters so that individual connectors could be reinstalled in the Type 708 power supply. See the log entry for March 20 for additional information about these power connectors.

May 6, 2014, Power supply reassembly

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Repairing the Type 708 supply
Bug 20. After replacing the broken DC-202 connectors facing upward on the back of the PDP-8 power supply, we loosened the sides of the supply and then Cale Bierman, Matt Adamczyk and Doug Jones carefully tilted the back wall of the supply up and screwed it in place. Recall that the back wall of the power supply was tilted down on Feb. 13 to allow reforming of the capacitors mounted on the back wall. This involved drilling out the pop rivets that held the back wall in place. On Feb. 17 we installed swage nuts to allow eventual reassembly. Today, we used those swage nuts. Finally, with the back wall of the supply in place, we replaced the final broken DC-202 connector. The first photo, taken by Matt Adamczyk shows Doug Jones soldering the connections to the final DC-202 connector before pressing it into its place. Zoom in on the back corner of the supply and you can see the screws that replaced the pop rivets visible in the photos from Feb. 13.

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Replaced zip ties
The final step in reassembling the supply was to replace all the zip ties we cut in order to gain access to various capacitors for reforming. All of the original zip ties are white nylon, while our replacements are black. We have changed some wire routings as a result of our approach to swinging the back of the supply down during repair, but most of the cable routings are unchanged.

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Putting it back in the rack
Cale Bierman, Matt Adamczyk and Doug Jones lifted the re-assembled supply into place in the computer rack, and finally, reattached the power cord, with careful attention to the photos taken on Feb. 10. The photo, taken by Matt, shows Cale Bierman and Doug Jones tightening the screws that hold the supply in place. Barring our discovery of errors in our work, this completes the reassembly of this power supply.

Bug 33. The next stage in this project will be to test the reassembled power supply. This must be done before attaching the supply to the PDP-8. Ideally, we will need to attach dummy loads for each of the supply outputs, and then turn on the supplies and measure both the DC output voltage and AC ripple voltage on each output. Only after we confirm that all of these are correct should we risk connecting the power supply outputs to the computer.

May 12, 2014, TTY reassembly

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Typing unit reassembly
Bug 26: Cale Bierman, Matt Adamczyk and Doug Jones reassembled the Teletype's typing unit and re-attached the paper tape punch to it. During the process, we noticed a label we had not previously recorded on the inside of the right carriage support pillar:
    33 TC 
        2,566,031   2,568,249

Assembly took hours. This was despite careful disassembly, with photos, notes, and the various diagrams in the manual, and despite tagging each piece during disassembly. All the small bits were in baggies with labels, and large bits were tagged with string and a label. Despite all these precautions, we kept encountering little bits of Teletype and saying "what's this?" Some parts (such as the right carriage motion pulley mount) could be put on two different ways, and were initially put on the wrong way around. You can see this mistake in Matt Adamczyk's photo of the Teletype label.

Several times, we thought we had the job done, only to pick up the Teletype and find a spring on the table under it, or a screw. Eventually, we ran out of parts and, turning the motor or moving the carriage by hand, everything seemed able to work. We will need to carefully go over all of the adjustments in the manual, and oil or grease all the lubrication points noted in the instructions before we power anything up.

May 27, 2014, Tally reader reassembly, cables

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The spare
Bug 28: We bid on a Tally 424 tape reader we found on eBay. Their buy-it-now asking price was $100, we offered them $15 plus shipping and won. The reader was shipped our way in an extraordinarily battered box held together by more duct tape than I've ever seen on one box before. Nonetheless, it arrived in good shape, which is to say, less than fully operational but still useful as a source of spare parts. What we're most concerned about are all the nylon gears in the differential and the slip clutches on the drive capstan. Matthiew Biger took the photo.

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Reassembly of the Tally paper tape reader went smoothly until the power switch was screwed in place in the front panel. As nearly as we can tell, this switch was added by the U. of Iowa Psychology department, judging by the handwritten label, on adhesive tape, saying "turn off when not in use" and the Dymo OFF label. I removed both in the process of replacing the switch, but added a metal "on-off" label salvaged from an unused switch at the bottom of the ADC rack.

The switch allows users to turn off the motor when the reader is not in use. The switch the Psychology department used has terminals that protrude to the rear, and when we put the switch in place, the motor mount shorted to one of the terminal tabs. The back side of the switch is visible in the exact center of the photo, with the shiny connector lugs protruding back under the motor. A short circuit here could connect 110 volt power wires to the frame of the reader. I reduced the danger by carefully bending the tabs so they would not contact the motor mount, but we should go further and slip insulating sleeves over these tabs.

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Computer Cable
Bug 4: Michael Thompson of the Rhode Island Computer Museum reported that they have a box of black flexprint cables in good condition, and wondered how many cables we need and how long. So, we pulled one of the gummy cables for a more complete description. In the process, we discovered that GooGone™ lives up to its name. Wiping the cables removed the gummyness. A complete degumming would require disassembling the cables, and if we go that far, replacing the gummy wires would be sensible.

There are 6 cable sets connecting the two backplanes of the PDP-8. Each set is terminated, at both ends, with a W034D Flip Chip connector paddle. Stretched out on a work table, the cable sets measure 23 inches from card-edge connector to card-edge connector. There are two flex-print ribbon cables in each assembly, one about 21 inches long, one about 19 inches long. The final 1/8 inch of each cable has had the mylar stripped away so that the foils protrude for soldering.

Each flex-print ribbon has 19 conductors, spaced at 18 conductors per inch, making each cable about 1 3/36 of an inch wide. These are apparently copper plated onto one sheet of mylar, and then etched to make printed-circuit conductors before being coated with an insulating layer to make the finished cable. I speculate that the goo is the result of the depolymerization of the insulating layer.

June 2, 2014, Tally reader reverse engineering

Bug 28: We finished reassembling the Tally reader and checked the electrical continuity of all of the wiring, making reference to the schematics in Figure 9 on page 8 of the Tally manual. Aside from the omission of the end-of-tape switches and capstan commutator options, we noted the following discrepancies. For each, we provide a new drawing; in all drawings, the connector is shown as seen looking at the pins of the rear-facing plug on the back of the Tally reader. This is a Continental Connector Corporation Series 20 34-pin connector.

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Motor Wiring
The power wiring is as indicated in the original Tally schematic, except for two changes: First, the motor has three wires coming out of it, black, red and yellow, instead of two pairs of wires as shown in Tally's schematic. The common connection between the wires (to the yellow lead) is inside the motor, instead of an outside connection, as shown in the schematic. When we rewired the motor, we were careful to retain the original wire colors, so it seems that this was a discrepancy introduced in the Tally factory.

Second, the a power switch was added by cutting the light blue wire between the capacitor and the fuse and then splicing in longer dark blue wire to connect to the switch. In the process of reassembling the tape reader, the cut fragment of light blue wire on the back of the fuse was removed and the dark blue wire was reconnected to the switch.

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Tape Advance Wiring
The wiring to the capstan escapement coils was significantly changed, although the wire colors attached to pins M, H and C of the connector are exactly as indicated in the Tally manual. The Forward coil is the only coil present, and it is wired across the two terminals documented as the reverse coil terminals in the manual. The terminal strip is not shown at all in Tally's wiring schematic, but it is shown in parts diagram for the capstan drive assembly, Item 11, Figure 25 on page 26 of the Tally manual.

The Tally manual discusses the use of a snubber or spark suppression circuit on page 6 and page 9, but it does not say where to mount it physically. On our reader, the snubber was mounted on the terminal strip. In fact, given the length of the cable between the escapement coil and the driving circuitry, the snubber inside the Tally reader will not generally suffice. Additonal snubbing must be provided at the point where the drive pulses are actually switched. The Tally manual documents the use of a resistor-capacitor snubber but what we found is either a diode snubber or a more sophisticated varistor-based snubber (we did not look up the device).

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Data Wiring
Finally, we checked the wiring between the connector on the back of the Tally reader and the 8 switches that mechanically sense the presence of lack of holes in each channel of the tape. This wiring corresponds exactly to that shown in the schematic in the Tally manual, but relating that schematic to the hole positions on the tape and to the connector wiring is sufficiently difficult that we have drawn a new diagram here. There are 8 clusters of 3 pins, one cluster for each data channel on the tape. Each cluster is wired identically, only one is shown.

The Tally reader needs lubrication. The lubricaiton instructions are in Figure 12 on page 11 of the Tally Manual, with accompanying text in Section 5.1 on page 10. The 350 centipoise Silicone oil required appears to be equivalent to. Dow Corning DC200, 350CS; A quick check shows that this oil is available on eBay in 4oz bottles. The SAE-20-grade oil requirement should be satisfied by the KS7470 oil recommended for the Teletype. The Tally pivot grease A required for the escapement armature lever fulcrums is more problematic. Perhaps KS7471 grease (or equivalent) will suffice.

Bug 34: The Tally reader interface on the PDP-8 ADC rack needs to be reverse engineered. What we have determined from the reader itself is that this reader has a very simple interface, but unfortunately, that means that the interface logic on the PDP-8 end must be somewhat different from that anticipated by DEC.

June 13, 2014, Tally reader reverse engineering

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Cable Wiring
Bug 12 and Bug 34: Working backward from the Tally reader, I took an ohm meter to the cable that is clearly marked "Tally Reader" and began tracing out the wiring. It immediately became apparent that this is not the cable for this paper tape reader. Comparing the wiring of the plug that connects to the reader with the wiring of the reader itself (see June 2), the wiring for the power connector is correct, as is the wiring to the movable contacts on the data sensing switches, but the wiring for the tape advance escapement coil is wrong, as is the wiring to the fixed contacts on the data sensing switches. Looking at the paper-tape-reader connector, there is solder on some of the pins that are currently unused, making it clear that this cable has been modified from its original wiring.

The engineering of the cable is suspect. The data rate on this cable is 60 pulses per second. At this data rate, there is absolutely no reason to use coaxial cable for each data line. DEC routinely used ribbon cable for such interfaces, and we have a new, unused ribbon cable attached to a W021B connector, still in its factory original packaging from 1968. It has only 2 feet of ribbon cable, but we can get new ribbon cable that exactly matches the original.

A second troublesome feature of the Tally cable involves changes made to the W021 connector at the computer end. As made by DEC, each coaxial cable is supposed to be connected to consecutive tie-points on the connector paddle, with just 9 data conductors and 9 (or 10) grounded shield connections tied together by ground traces on the paddle. As we got it, the ground traces connecting these tie points have been removed (peeled off of the printed circuit board), and instead, the shields of the coaxial cables have been soldered together in a glob at each end.

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Backplane Wiring
On Nov 22, 2013, we determined that the Type 750C High Speed Reader interface was part of the Type 34D Oscilloscope Display package, with the cable to the reader plugging into slot B32 of the display interface backplane. It is also clear that the high-speed reader interface has been modified. Looking at the backplane wiring for slot B32, we found pull-down resistors soldered directly to 8 pins on the connector. As was the case with the paper-tape reader end, none of the backplane wiring can be reconciled with the wiring of the cable itself. The plug on this cable that connects to the Tally reader is almost certainly the original, but this cable has been reworked to serve some other purpose!

Therefore, without doubt, we must build a new cable. The most conservative option, from the point of view of future maintainability of the system, would be to undo what were apparently local modifications to the paper-tape reader and return it to its factory original wiring, and also redo the wiring of the backplane so that the connector can be used in more or less the way it was intended. It should be possible to interface to the Tally reader using just 9 wires, although half-amp tape advance pulses should be isolated from the backplane as much as possible.