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FACT FILE: The OMNIBUS
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)
| Pin | D1 | D2 | C1 | C2 | B1 | B2 | A1 | A2 |
| A | +15V | +5V | +5V | +5V | ||||
| B | -15V | -15V | -15V | -15V | ||||
| C | GND | GND | GND | GND | GND | GND | GND (SP) | GND |
| D | MA8 L | IR0 L | IO PAUSE L | TP1 H | MA4 L | INT STB L | MA0 L | EMA 0 L |
| E | MA9 L | IR1 L | C0 L | TP2 H | MA5 L | BRK PROG L | MA1 L | EMA 1 L |
| F | GND | GND | GND | GND | GND | GND | GND | GND |
| H | MA10 L | IR2 L | C1 L | TP3 H | MA6 L | MA,MS LD L | MA2 L | EMA 2 L |
| J | MA11 L | F L | C2 L | TP4 H | MA7 L | OVERFLOW L | MA3 L | MEM START L |
| K | MD8 L | D L | BUS STB L | TS1 L | MD4 L | BRK DA CT L | MD0 L | MD DIR L |
| L | MD9 L | E L | INT I/O L | TS2 L | MD5 L | BRK CYC L | MD1 L | SOURCE H |
| M | MD10 L | USR MD H | N L XFR L | TS3 L | MD6 L | LA EN L | MD2 L | STROBE H |
| N | GND | GND | GND | GND | GND | GND | GND | GND |
| P | MD11 L | F SET L | INT RQST L | TS4 L | MD7 L | INT PROG H | MD3 L | INHIBIT H |
| R | DATA 8 L | PULSE LA H | INIT H | LNK DATA L | DATA 4 L | RES 1 H | DATA 0 L | RETURN H |
| S | DATA 9 L | STOP L | SKIP L | LNK LOAD L | DATA 5 L | RES 2 H | DATA 1 L | WRITE H |
| T | GND | GND | GND | GND | GND | GND | GND | GND |
| U | DATA 10 L | KEY CTL L | CPMA DIS L | IND 1 L | DATA 6 L | RGN L | DATA 2 L | ROM ADDR L |
| V | DATA 11 L | SW L | M3, IR DIS L | IND 2 L | DATA 7 L | POWER OK H | DATA 3 L | LINK L |
OMNIBUS Cards
The following OMNIBUS cards are known to exist
| Module Number | Option Name | Description |
| 54-9057 | KC8EA | PDP-8/E Programmer’s Console |
| 54-9668 | KC8FL | PDP-8/F Programmer’s Console |
| 54-9668 | KC8ML | PDP-8/M Programmer’s Console |
| . | KC8AA | PDP-8/A Programmer’s Console |
| A231 | AD8EA | Analog to Digital Converter |
| A231 | AD8E | Analog to Digital Converter |
| A232 | AH8EA | Multiplexor for AD8EA |
| A841 | AD8EA | Analog to Digital Converter |
| G111 | MM8EJ | 8K Memory sense/inhibit |
| G111C | MM8EJ | 8K Memory sense/inhibit |
| G104 | MM8E | 4K Memory sense/inhibit |
| G104C | MM8E | 4K Memory sense/inhibit |
| G105 | MP8E | Memory sense/inhibit |
| G227 | MM8E | Memory X/Y drivers |
| G227C | MM8E | Memory X/Y drivers |
| G227 | MP8E | Memory X/Y drivers |
| G233E | MM8EJ | 8K Memory X/Y drivers |
| G619 | MM8E | Memory stack |
| G619A | MM8E | Memory stack (B,M) |
| H649 | MM8AA | PDP8A 8K Memory stack (hex wide) |
| G646C | MM8EJ | 8K Memory stack |
| H650 | MM8AB | PDP8A 16K Memory stack (hex wide) |
| H212 | MM8EJ | 8K Memory stack |
| G219A | MM8AA | PDP8A 8K Memory control (hex wide) |
| G219B | MM8AB | PDP8A 16K Memory control (hex wide) |
| H220 | MM8E | Memory stack |
| H220 | MP8E | Memory stack |
| MS8DJ | PDP8A 32K semiconductor memory module | |
| M1703 | . | 12 bit word 8/e bus interface (input only?) |
| M1703C | . | |
| M1705 | . | dual 12 bit omnibus output interface |
| M1709 | . | Omnibus foundation module (multiword custom I/O) |
| M512 | DK8ED | |
| M518 | DK8EP | Lab Programmable Real Time Clock (requires M860) |
| M518 | DK8EF | Lab Programmable Real Time Clock with lab panel DK8ES |
| M518 | DS8EA | Schmidt Trigger |
| M7104 | RK8E | RK05 Disk Interface |
| M7105 | RK8E | RK05 Disk Interface |
| M7106 | RK8E | RK05 Disk Interface |
| M8300 | KK8E | Major registers |
| M8310 | KK8E | Major register control |
| M8311 | MS8-AA | PDP-8/A Read/Write RAM (1K) |
| M8311 | MS8-AB | PDP-8/A Read/Write RAM (2K) |
| M8311 | MS8-AD | PDP-8/A Read/Write RAM (4K) |
| M8315 | KK8A | PDP-8/A CPU board (hex wide) |
| M8316 | DKC8AA | PDP-8/A I/O serial/parallel/clock (hex wide) |
| M8317 | KM8AA | PDP-8/A bootstrap, powerfail (hex wide) |
| M8317 | KM8AB | PDP-8/A Memory extender (hex wide) |
| M8317YC | KM8AC | PDP-8/A Memory extender, bootstrap, powerfail (hex wide) |
| M8319 | KL8A | PDP-8/A 4 channel serial I/O |
| M8320 | KK8E | Bus loads |
| M8321 | TM8E | Magtape output control module |
| M8322 | TM8E | Magtape control and data break module |
| M8323 | TM8E | Magtape transport status and control module |
| M8326 | DB8EA | Interprocessor buffer |
| M8327 | TM8E | Magtape register module |
| M8329 | LC8E | Parallel interface to DEC-writer I |
| M833 | KK8E | Timing board, good only for 4K TTY systems |
| M8330 | KK8E | Timing board, replaces M833 |
| M8330YA | KK8E | Timing board, replaces M8330, handles MR8E memory stalls |
| M8330YB | KM8F | Timing board, replaces M8330 |
| M8331 | TA8E | DEC Cassette control |
| M8335 | VT8E | |
| M8336 | VT8E | Video Display Clock and Frequency Divider |
| M8337 | VT8E | Video Display Line Buffer Module |
| M8340 | KE8E | Extended arithmetic element |
| M8341 | KE8E | Extended arithmetic element |
| M8342 | LP8E | Centronics printer control (replaces M8329) |
| M835 | . | |
| M8350 | KA8E | External positive I/O bus interface |
| M8357 | RX8E | RX01/RX02 interface |
| M8360 | KD8E | Data Break Interface |
| M8365 | LC8P | LA180 printer control |
| M8366 | LQP8E | LQP01 printer control |
| M837 | MC8E | Extended Memory and Time Share Control |
| M837 | KM8E | Extended Memory and Time Share Control |
| M839 | DP8EA | Synchronous Modem |
| . | DP8EB | Synchronous Modem (Bell series 300 modem) |
| M840 | PC8E | High speed paper tape reader/punch |
| M841 | LE8 | Data Products line printer control |
| M8416 | KT8AA | PDP8A Memory Management (hex wide) |
| M8417 | MSC8AA | PDP8A 16K MOS Memory (hex wide) |
| M8417 | MSC8AB | PDP8A 32K MOS Memory (hex wide) |
| M8417 | MSC8DJ | PDP8A 128K MOS Memory (hex wide) |
| M842 | XY8E | X/Y Plotter Control |
| M843 | CR8E | Card reader control |
| M8430 | KW8EW | Alarm timer |
| M8433 | RL8A | Disk controller for RL01/02 (hex wide) |
| M847 | MI8E | Hardware Bootstrap Loader |
| M847 | MI8EN | Hardware Bootstrap Loader for RK8E |
| M847 | MI8EP | Hardware Bootstrap Loader for RX01/RX02 |
| M848 | KP8E | Power fail and auto-restart |
| M849 | KK8E | RFI shield |
| M860 | DK8ED | Programmable Real Time Clock |
| M860 | DK8EF | Programmable Real Time Clock |
| M860 | DK8EP | Programmable Real Time Clock |
| M863 | DR8E | 12 Channel Buffered Digital I/O |
| . | DR8EA | 12 Channel Buffered Digital I/O |
| M866 | DP8EA | Synchronous Modem (Bell series 200 modem) |
| . | DP8EB | Synchronous Modem (Bell series 300 modem) |
| M865 | . | Console Teletype Control (current loop only) |
| M8650 | KL8E | Asynchronous Data Control (current loop or RS232) |
| M8653 | KL8M | Modem Control, used with KL8* to provide data ready stuff |
| M8655 | KL8JA | Terminal Control (UART based substitute for M8650) |
| M868 | TD8E | DECtape Control |
| M869 | VC8E | (requires M885) |
| M882 | DK8EA | Real Time Clock (Line Frequency) |
| M8830 | DK8EC | Real Time Clock (Crystal, 100Hz) |
| M884 | KG8E | Parity Generator Detector for DP8E |
| M885 | VC8E | (requires M869) |
| M935 | OMNIBUS bridge (joins two adjacent backplane segments) |
Credit: Doug Jones [original document]