Front Desk Bus Rev 2.1

To: Bob Bailey Date: Sept. 26, 198-4 Gary Butts - APG Dave Christensen - APG Mike Clark - APG Burrell Smith From: Peter Ashkin Subj: Front Desk Bus - Rev 2.1 To make the "Front Desk Bus" a more flexible and powerful interface, I believe that it should have the follo"'1ing properties: 1. restrictive .J The bus shall be bidirectional. [An input only bus is too 2. Each device on the bus has a unique address. For practical purposed the address range should be o - 14. Some or these addresses may be reserved for broadcasting universal messages. [This seems like a sane number or devices, particularly since there exists today only three devices; keyboard, keypad and mouse .J 3. All command transactions shall be eight bits long. All data transactions shall be 16 bits long. [This facilitates the decoding of commands by devices. of limited iiltelligence.J 4. The host shall be the undisputed bus master. [This removes any question of who's controlling the bus.J 5. There shall be a limited number of commands. Commands should be broken int.o two groups, basic commands (TALK and LI STIR) Which all devices on the bus shall understand; and advanced commands vvhich only intelligent devices (as appropriate) should understand. [This makes the command interpreter, be it hardv.1are or soft"'1'8.re, Simple. It also allo'NS more complex devices to used some of tl:te "fancier" features of the bus.] There shall be only one active talker on the bus at any time, this may be the host or an addressed device. [A device addressed to TALI ¥\Tith data to send "untalks" itself after it sends its 16 bits of data or if it has no data to send "untalks" itself immediately and allo'\AIS the bus to time-out.] 6. Front Desk Bus September 26, 1984 7. The bus prot:ocol must accept devices that talk at different speeds. The host, at a minimum, must be able to listen at various speeds. (This implies that the data on the bus must be "self-clocked". By not rigidly fixing the speed of transmission. the bus does not need to be crystal (etc.) controlled.] There shall be only one active listener on the bus at any time, this may be tlle host or an addressed device. [A device addressed to LI STIR "unlistens" itself-after it receives 16 bits of data or if it receives a new command before receiving 16 bits of data.] 8. 9. An interrupt mechanism must be available which circumvents the needs to poll devices that need service. [Since the bus is relatively slow. tlle interrupt latency time in a polled environment is long. The ability to interrupt the master for service is important.] 10. There shall exist a mechanism that sends a unique signal that puts all devices on the bus into the command (reset) mode. [This is important if for some reason the bus gets "hung".) 11. There should be a minimum number of "time-outs" needed on the bus. The only needed time out should be to time out a non-responsive talker. [Timers are ugly, but "'1aiting for a dead device is uglier. The length of this time-…