Front Desk Bus Rev 3.1

•• , I To: Bob Belleville Date: October 29, 1984 Bt11 Bu11 -c;ary· Butts Mike Clark Jerome Coonen Dan Hil1man Larry Kenyon Burre11 Smith ?et, From: Peter Ashkln Subj: Front Desk Bus - rev 3.1 Enclosed is the latest version of the Front Desk Bus (rev 3.1) specification. It is divided into three sections: Preface - which contains the 12 fundamental properties of the bus; Main - which contains the commands which devices on the bus must execute and deta11s on the timing and the moduJat ion of the bus; and the Appendix - which describes the interface between the FOB "modem· and the Macintosh digital subsystem. There are some things this specification does not contain, there is no description of the connectors nor is there any mention of how the bus should be "used". Please read this over and feel free to make any changes or improvements. I'm interested in a robust (and useful) spe~ification. 1911 contact each of you the first week in_·November to discuss your comments. Thanksm •• rtrrae - rroat Int .. I•• To mate the "Front Delk Bua" 1 more ne1ible and powerful interface, it should have the f ollowina properties: l. The bus shall be bidirectional. (An input only bus is too restrictive.) 2. Bach device on the bus has a unique address. For practical purposed the address range should be 0 - 1.f. Some of these addresses may be reserved for broadcasting universal messages. (This seems lite a sane number of devices, particularly since there eiists today only three devices; keyboard, keypad and mouse.) 3. All command transactions shall be eight bits Iona. All data transactions shall be 16 bits long. (This facilitates the decoding of commands by devices of limited intelligence.) The host shall be the undisputed bus master. (This removes any question of who's controlling the bus.) 4. 5. There shall be a limited number of commands. Commands should be broken into two groups, basic commands (TALI: and LISTBN) which all devices on the bus shall understand; and advanced commands which only intelligent devices (as appropriate) should understand. (This makes the command interpreter, be it hardware or software, simple. It also allows more comple1 devices to used some of the fancier" features of the bus.) · 0 6. 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: with data to send untalts itself after it sends its 16 bits of data or if it has no data to send "untalts" itself immediately and allows the bus to time-out.I 0 0 The bus protocol 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 filing the speed of transmission, the bus does not need to be crystal (etc.) controlled.) 7. 0 There shall be only one active listener on the bus at any time, this may be the host or an addressed device. (A device addressed to LISTBN…