Tymshare Base-Host 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 total number of octets in the message, including header.
| 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.
