Tymshare Base-Host Protocol

Humorous visualization of Tymnet protocol

This documents the protocol, as used around 1975-1982, between a PDP-10 host and Varian 620 Tymbase. At a low level, the interface consists of shared memory; one page (512 words) at address 2000 (octal). The memory has some metadata and ring buffers for input and output. The host and the Tymbase 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 200 (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 ouptut 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	Requst 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.
200+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.