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.

GTYI

Device number 70 (octal). CONI 10 means data ready. CONO low three bits is the PI channel. DATAI gets a an ASCII character in the right half, and console number in the left half.

GTYO

Device number 750. CONI 100 means 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.

The output is some kind of communication protocol to the controller. The bulk of the work here will be to reverse engineer the protocol.

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.

The text consists of display 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