MIT AI lab IO bus interface for GE Datanet 760
The MIT AI lab PDP-6 had a GE Datanet 760 controller with four consoles. The controller was attached through two IO bus devices. This article documents the IO bus interface as used by ITS 138.
Contents
GTYI
Device number 70 (octal). CONI 10 means "done", data ready to receive. CONO low three bits is the PI channel. DATAI gets an ASCII character in the right half, and console number in the left half.
ITS 138 code is consistent with these theories:
- DATAI clears "done".
- Interrupts are raised or cleared according to "done".
GTYO
Device number 750. CONI 100 means "done", ready to accept data. CONO 200 is some kind of reset or enable, and the low three bits is the PI channel. DATAO sends an 7-bit character.
ITS 138 code is consistent with these theories:
- DATAO and CONO clears "done".
- Interrupts are raised or cleared according to "done".
- CONO 200 sets "done" and thus raises an interrupt if PIA is not 0.
Communications protocol to Datanet 760
Data is sent to the 760 as framed messages. A message starts with an SOM/SOH (octal 001). Next comes a console selection code which is 140, 150, 160, or 170, and then a status code which ITS sets to 0. Next comes an STX (002), text data, and ETX (003). The message ends with a "longitudinal parity" character, which is an XOR checksum computed over the selection code through the ETX.
For transmission, all characters have their seven bits inverted, and rotated one bit to the right.
Text and control codes
The text portion of a message consists of ASCII characters in the range 040 to 137, and may embed control characters.
| Code | Name | Description |
|---|---|---|
| 010 | BS | Backspace marker one character. Stops at left margin. |
| 012 | LF | Line feed. Moves marker down one line. Wraps around to top line. |
| 014 | FF | Form feed. Erase display and move marker to top left. |
| 015 | CR | Carriage return. Move marker to leftmost position. |
| 021 | RLF | Reverse line feed. Moves marker up one line. Stops at top line. |
| 022 | FS | Forward space marker one character. Does a CR at right margin. |
| 024 | PR | Page return. Goes to top left without erasing the display. |
External links
- US patent 3,500,338 Party Line Use and Superwisory Control of Multiple Terminal Data Editing Display Systems
- Honeywell Series 6000 - GECOS Time-Sharing System General Information Manual, page 51.
- Auerbach Data Communications Reports, page 6321:00.
Further reading
This documentation is part of MIT's Tapes of Tech Square (ToTS) collection, but not yet available to the public. A copy of the latter is in Computer History Museum's catalog.
- The General Electric DATANET-760 Keyboard/Display, sales brochure.
- DATANET-760 Keyboard/Display Subsystem Manual, technical documentation.
