Tymshare Base-Host Protocol

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Humorous visualization of Tymnet protocol

This documents the protocol, as used around 1975-1982, between a PDP-10 host and Varian 620 TymBAS. At a low level, the interface consists of shared memory; one page (512 words) at address 02000 (octal). The memory has some meta-data and ring buffers for input and output. The host and the TymBAS regularly poll this memory for updates.

Messages are transmitted from the PDP-10 as 36-bit words with four octets in bits 0-31. Bits 32-35 are unused.

Message types A, B, C, and D are just 4 octets.

Message number Port Subtype or data Data

Type F messages total 8 octets:

Message number Port Data Data
Data Data Unused Unused

Type G messages have a variable number of octets. The message number is 0200 (octal) plus the number of data octets following the 16-bit header. The last word is padded out to a 36-bit boundary.

200+n Port Data Data
Data as per n
... Padding

Type H messages are for sending or receiving a block of words.

Message number Port Word count, high 8 bits Word count, low 8 bits
Address, high 8 bits Address, middle 8 bits Address, low 8 bits Unused

Messages. Any number not in this list is illegal. Numbers 1-41 were present in 1975. Numbers 42-61 were added around 1982.

Number Type Name Comment
1 A ANS System is answering.
2 A SHT System is up but shut.
3 A CRS Sender is crashed.
4 A DIE Recipient should crash.
5 A NSP Base taken over by new supervisor.
6 B LOG Login, next 4 data characters are the info about terminal type, and port or origin, then name, etc.
7 B AUX Supervisor response to establishing auxiliary circuit.
10 B NOP Back pressure on.
11 B OUP Back pressure off.
12 B GOB Character gobbler.
13 B ZAP Circuit zapper.
14 B EDC Enter deferred echo mode.
15 B LDC Leave deferred echo mode.
16 B GRN Green ball.
17 B RED Red ball.
20 B YEL Yellow ball.
21 B ORG Orange ball.
22 B HNG Hang character - not used.
23 B ETM Enter 2741 transparent mode.
24 B LTM Leave 2741 transparent mode.
25 C LOS Lost ball, data has been lost from buffers. The data filed may tell how many were lost.
26 C SUP Supervisor request (aux circuits).
27 C SUR Supervisor response (aux circuits).
30 C AXC Supervisor string character.
31 F TSP Test pattern probe.
32 F TSR Test pattern response.
33 F SAD Host sad.
34 B ECN Echo on.
35 B ECF Echo off.
36 D TCS Term characteristics, first data byte indicates which characteristics second data byte indicates value to set to.
37 C TCP Term characteristics probe, data byte indicates which terminal characteristic were requested.
40 D TCR Term characteristics response, data is just like TCS, comes in response to a probe; also is reflected by remote when terminal characteristics are sent.
41 C HSI Host up and answering with # of ports in port byte, and host # in data byte.
42 A CLP Super clock info request.
43 F CLR Super clock info.
44 H BKO Start block output.
45 B BOC Block output complete.
46 H BIN Start block input.
47 D INB Block input done, out of buffer.
50 D INE Block input done, completed.
51 D INT Block input done, timeout.
52 B IRQ Request block input termination.
53 D IHR Block input done, host request.
54 B ORQ Request block output termination.
55 B OHR Block output done, host request.
56 C NEG Block IO port negotiation.
57 B BRK Break character received.
60 C PSR Port status.
61 B EAO Enter alt output device mode.
0200+n G Data.

This information comes from the SUMEX-AIM monitor file TYMSRV.MAC, written by Michael A. Heathman in 1975. Additional updates from the SRI-NIC version from 1982.

External links