This guide is provides comprehensive information on the OMNIBUS used in the PDP-8/E,F,M and A series of machines.

A 20 slot PDP-8/E backplane

Overview of the bus

The OMNIBUS is a general purpose bus consisting of 144 signals.

OMNIBUS backplanes consist of 1 or more 10 slot backplanes and are normally quad width. The later 8/A is the exception to the rule where some slots, designated for core memory are five wide.

Each slot is general purpose and in theory cards can be positioned in any slot, only limited by timing and noise considerations. Due to the general purpose nature of the bus, inter-board signals are made by way of over the top H851 edge connectors.

The OMNIBUS signals are 5V based with Logic 1 represented by -0.4-0.5V and logic level 0 by 3V-5V.

OMNIBUS Signal Reference

The following signals are presented on the omnibus. (NOTE TO SELF: This map needs to be QA-ed)

OMNIBUS Cards

The following OMNIBUS cards are known to exist

Here is a review of recent known PDP-8 sales. As ever, prices appear to be heading upwards.

PDP-8/M

Ebay US sale, item #260385453584. Sold for £1023.23 / $1500.00

Original Description:

Winning Bidder will receive one (1) Complete 100% Working Digital pdp8M Computer with 8k of Iron core memory, the computer comes with the below listed boards, unit was removed from a 100% operational cnc control cabinet. Listed shipping fee for lower 48 only. (The top cover is included)

M8330 Timing Generator

M8300 Majo Registers

M8310 Major Registers Control

M837 Memory Extension Control

M847 ROM

M848 Power Fail / Auto Restart

G111 Sense Inhibit (1 of 3 8k stack)

H212 Planar Stack (2 of 3 8k stack)

G233 XY Driver (3 of 3 8k stack)

W968 I/O Interface

M849 Shield

M8320 Bus Load Module

Commentary

A very nice looking 8-M. The 8-M is a nice compact machine and this one looks like it should make its new owner very happy. The price seemed to be about spot on for a working 8-M. Interestingly its RF shield appears to be in the wrong location, I would normally expect it to sit between the CPU/options and core memory.

PDP-8/A

Ebay US sale, item #230334635912. Sold for £525.25 / $769.99

Original Description

Dec PDP-8/a computer removed from a working machine ( Wire EDM ) seems clean and in good condition but due to it’s age I cannot guarantee its operation.
Includes boards M8315 M8317 M8316 H219B G649 Power supply and an AXIS Board.

Computer has been in storage for 10+ years – it IS dusty, and needs cleaning – Collectors please note! the computer did run when removed but there is no guarantee to it’s current operation, sold AS IS because of it’s age .

Commentary

A very clean looking machine. Price was in my opinion on the high side given that the machine is the less desirable 8/A and not in known working condition. The machine has (for an 8/A) a fairly modest 8KW of core memory.

To ensure nothing gets missed, I use a checklist of tasks to be done when rebuilding machines. This is the the list for my 8/M rebuild:

Completed

1. Perform a general appraisal of machine
2. Photograph machine in detail
3. Remove console and cards
4. Remove power supply
5. Remove wiring looms and any other remaining items attached to chassis
6. Remove backplane
7. Clean and sand chassis
8. Spray chassis to primer
9. Rub down primer
10. Respray chassis (to matt black)
11. Clean backplane
12. Clean wiring loom and other chassis furniture
13. Take detailed pictures of power supply
14. Dismantle PSU
15. Clean power supply components
16. Remove and reform capacitors
17. Reassemble power supply
18. Bench test power supply and confirm operation
19. Reinstall power supply (but do not reconnect AC or DC at this point)
20. Reinstall backplane
21. Reinstall chassis furniture
22. Power on and confirm fans are operational
23. Reconnect AC to power supply and confirm operation of PSU
24. Reconnect DC from power supply to backplane and confirm voltages on backplane
25. Install minimal card set (console and CPU) and test console operation

To Do

1. Add further earth straps – they appeared to be non existent when I got the machine and I’m far from convinced the chassis is earthed at the moment
2. Tidy up DC wiring loom
3. Configure and fit core memory
4. Debug console to establish why address bit 5 isn’t working
5. Confirm correct operation of the console
6. Enter basic test programs and confirm operation
7. Solve problem of some console keys catching on console plexi
8. Repair 3 console keys that having broken pivot shafts
9. Fit console and confirm operation
10. Fit RX8E and confirm its working

Having reinstalled the power supply into the chassis, the console was put back into slot 1 and carefully reassembled.

The minimum board set was then installed consisting of:

• Slot2: M8330 KK8-E timings
• Slot3: M8300 KK8-E major registers
• Slot4: M8310 KK8-E major register control
• Slot 20: M8320 KK8-E bus loads

The machine was then powered on and LEDs on the console sprung into life. Some basic operation from the console was possible (I was able to set the PC) although bit 5 of the PC appears to be stuck off. I will have to investigate whether this is a fault on the console or not.

I also installed an 8KW core memory set and retested with this in place. With these boards in place, the PC was all bits high and could not be changed.

So still some way to go! The next step will be to establish if the KK8-E is fully working and track down the issue that is causing the bit 4 of the address bus to appear stuck low on the console.

The PDP-8/M uses a H740 switch mode power supply that provides +15V @ 1A, +5V @ 17A, -15V @ 5A together with LTC-L, PWR OK L, PWK OK EN, AC LO L, and DC LO L signals (it should be noted that not all of these signals are used within the PDP-8M). This power supply is also used within the 5.25″ PDP-11/05 and PDP-11/10.

To rebuild, I stripped the supply down and gave it a gentle clean with electrical cleaning fluid and a clean cloth. A paint brush was used to dislodge any dust in hard to reach places. The electrolytic capacitors were then removed and reformed.

The power supply was then reassembled and plugged back into its mains connector in the chassis. Power was then applied for short periods of 1 second, 5 seconds, 10 seconds, 30 seconds, 1 minute, 5 minutes, 10 minutes.

After each application of power, I carefully touched the cases of the capacitors to confirm there was no build up of heat. Care should be taken as these beasts hold a lot of charge for a long time, believe me you really don’t want to find out how much it hurts to have one discharge through you.

Once I reached 1 minute, I also tested the +5V, +15V and -15V DC ouptuts to confirm all was well. Luckily for me it was.

The final step was to hook up an oscilloscope and to check each of the power rails to check that they were stable and to confirm all of the control signals were working correctly. The line time clock (LTC-L) is the only signal of note when checking with an scope as this should give a 50HZ (or 60HZ in US) square wave.

Having passed all checks, the power supply was reinstalled into the chassis and connected back upto the OMNIBUS backplane. The consists of 4 connections, -15V, +5V, +15V and the PWR OK L signal (spade marked DV2)

Useful Links:

• H740 Schematics (PDF)
• Article on reforming capacitors

The chassis had all internal components removed (after carefully photographing everything to aid in reassembly) before being sanded down. Any areas of rust were sanded back to bare metal.

Unfortunately I could not persuade the build label to come off of the chassis cleanly so this had to be sacraficed. Before I removed it I carefully took a scan of it by placing the chassis on a flat bed scanner. This will allow me to replace the label with a replica at the end of the rebuild.

The chassis was resprayed in 3 coats of grey primer and then 4 coats of matt black paint. I’m really happy with the results, although in some areas I should have spent longer sanding to get a really perfect finish.