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 02000 (octal). The memory has some meta-data 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 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 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.