[an error occurred while processing this directive] [an error occurred while processing this directive] The U of Iowa's DEC PDP-8 Restoration [an error occurred while processing this directive] [an error occurred while processing this directive] [an error occurred while processing this directive]
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The University of Iowa's DEC PDP-8

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Restoration Log

[an error occurred while processing this directive] Part of [an error occurred while processing this directive] the UI-8 pages
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[an error occurred while processing this directive] [an error occurred while processing this directive] (none) Douglas W. Jones [an error occurred while processing this directive] (none)
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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.

Feb 12, 2018, W034 Cable

Bug 4 and Bug 64: Given that quite a bit of the the CPU seems to be working, we began to investigate the memory. One of the first things we noticed was that the Maintenance Manual (Feb. 1966 Ed., pages 10-70 to 10-76) documents 7 W034 cables between the CPU and memory half backplanes, but there are only 6 present. The missing cable (page 10-76) would connect backplane slots PD02 and MD35. While we have not fully understood the signals that pass through the various cables, this makes it more pressing that we attempt to manufacture replacements for these cables.

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W034D cable connector
We began by taking additional photos of the board that serves as a connector on each end of the cable, supplementing the photo taken on May. 27, 2014. We took key dimensions from the specifications in the Digital Logic Handbook, 1967 Edition, page 225, and measured the others from the photos. The diagrams to the left show our results.

Having done this, we found that Vince Slyngstad has already done much of this. See his CAD Project Files, indexed under ./DEC/Wxxx/W034: Flexprint, 16 connections on "B" side 10 ohms on A2, B2. This includes photos, schematics and board layouts for the W034D, W034H, and W034X. The latter is a modern redesign, while the two former are historical DEC designs. We need to evaluate his modern design to see if it meets our requirements.

Feb 14, 2018, Disconnect mag tape interface

Bug 31: After poking around the ADC rack, we decided to remove the locally built magnetic tape drive interface from the ADC rack. Removal of this device interface is complicated by the fact that it is not daisy-chained into the I/O bus the way off-the-shelf device interfaces were done. Instead, wire-wrap wiring has been used to tap into the I/O bus connectors in the analog to digital converter backplane immediately above it. In addition, spare slots in that backplane have been used for a few additional logic boards and interface plugs needed for the tape drive.

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Using an unwrapping tool
The first step was to remove the wires that connected between the ADC and the tape-drive interface, using the unwrapping end of a standard wire-wrap tool. All of DEC's factory wiring and field-changes were done with 24-gauge wire, almost all of it with yellow insulation. The wires we removed were a mix yellow of 30-gauge wire (with a woefully inadequate number of wraps on each pin) and some red and blue 24-gauge wire. We documented every wire removed in the log book, using DEC's standard coordinate system:

Pin E12B refers to a pin on row E (counting from the top), slot 12 (counting from the left side of the wiring side of the backplane) pin E (as printed in white on the wiring side of the black connector blocks). The tape-drive interface was mostly in rows E and F, with wiring to a few spare slots in row D and one slot (used for a cable connector) in row C. We logged the wire color, source pin and destination pin for each wire we removed.

We also pulled the cables that connected the tape-drive control panel to slots in the backplane segments involved. The wires at one end of this cable were soldered to parts of the control panel, another end of this cable ended in cut-off wires that apparently went to the tape drive, while the third and forth ends of this cable went to card-edge connectors plugged into the backplane. At least one of these was almost certainly plugged into the wrong backplane slot, but we wrote on each card-edge connector the slot number into which it had been plugged.

Feb 21, 2018, Remove mag tape interface, memory sleuthing

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Removing rows E and F
Bug 31: We continued removing wires, documenting the removals in the log book, until we thought no wires remained between rows A though D and rows E and F of the top backplane section of the ADC rack, and then we unscrewed the mounting panel for rows E and F, only to find that one wire remained. It turns out that the power connections to rows E and F was done by wire-wrapped wires from rows C and D instead of using the faston connector tabs on the left side of the mounting panels.

With the final wire removed, we were able to pull the remains of the home-brew tape-drive interface from the rack. We believe that some boards on the lower right end of row D are also part of this home-brew interface. Wiring to those boards from rows E and F has been removed, but the local wiring on row D remains in place.

The home-made tape drive control panel directly below rows E and F had one remaining cable still connecting it to other parts of the rack. In this case, to a block of relays attached to a board cantelevered out from the back of the topmost panel on the back of the rack. We simply cut the wires from the cable to this block of relays. The colors of the cut wires should be sufficient to allow reversing this change, should someone want to do so. We will tag all removed parts to allow this.

Bug 64: We tracked down David Gesswein's photos of a table-top PDP-8 he restored. These photos of his show the cabling between the two half backplanes:

These photos clearly show only 6 cables, all from Px01 to Mx36 (for x from A to F). The photos do not show the 7th cable connecting PD02 to MD35. On Feb. 12, 2018 we had speculated that perhaps this missing cable was the cause of our memory problems. Apparently, this is not the case. It appears that this cable is not needed on machines with only 4K of memory. Therefore, cable replacement moves down on our list of prioritie, but remains there because of the decayed nature of the 6 cables we have.

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Current limiters
The Maintenance Manual (Feb. 1966 Ed., page 9-12) says that the first thing we need to do when checking the memory is to measure and adjust the read/write current and the inhibit current. These currents are to be measured at the wire from the current-determining resistor to the appropriate backplane module. Where are these resistors? The explanation of the memory operation given in Figure 4-4 on page 4-6 of the Maintenance Manual shows an 80&Ohm; "current determining resistor" in the circuit, but it doesn't say where to look for it.

Looking at the detailed drawings in the Maintenance Manual, BS-D-8M-0-12 X Axis Selection, BS-D-8M-0-13 Y Axis Selection and BS-D-AM-0-15 Sense Amps, Inhibit Drivers, and Memory Control all show the detailed locations of the boards involved and document the role of 80&Ohm; current limiting resistors, but they don't show where the resistors are. We found these resistors lurking on an aluminum heat sink behind the balun netowrk boards on the front-side of the core stack. See photo. These are all 80&Ohm; 10W 1% precision resistors. We infer from the fact that two of them have small resistor capacitor networks serving as shunts that those two are the X and Y axis current limiters attached to the G209 modules, while the other 13 resistors each serve one of the G208 inhibit drivers (the 13th resistor is a spare on our system since we don't have the memory parity option.) Note that this little RC network isn't documented in the maintenance manual.