FDB Specification Rev B Proposal

""""'' wr·,.,,, ,• 062-0267 Specltlcation, Front Desk Bus Revision B Proposal M.R. Ctark UNRELEASED I CONFIDENTIAL INTRODUCTION The Front Desk Bus is a method and protocol for interconnecting computers with human input and other devices. This specification covers the l'hyslca1,Datallnk, and Network 1ayers of the Front Desk Bus. In this specification the computer is referred to as the host. Peripherals connected to the bus are referred to as devices. The host is the undisputed bus master. It controls the flow of data by issuing Commands and it is the only device permitted to issue them. Talk is the command used for a data transaction from a device to the host. Listen is the command used for a data Lransa<.;Lluu frow lhc hu~l lu ts elev i\;~. PHYSICAL LAYER Interconnection: All devices will communicate with the host via a 3.5 mm mini phone jack, as specified in Apple Specification,T.B.D., with the following connector assignments; Tip-Power, Riu~·Data, SlceV'c-Powcr Return They will be interconnected with three conductor cables terminated with 3.5 mm mini phono plugs, as specified in Apple Specification, T.B.D. Signal Levels: Input Signals: Data: The data line will be pulled up by the host with a 10 K Ohm resistor to power. A "High" is 2.4 V minimum . A "Zero" is 0.8 V maximum . Power: The Host will supply 5.0 Vdc + 10% to the devices. The power line will be current limited by the host to prevent systems damage in the event of a Power to Power Return short. Output Signals: A "High" is the voltage on the Tip connection. Each device in the inactive or "High" state must source less than SO uA. A "Zero" is 0.4 V maximum at 1.6 mA minimum. Devices will provide current limiting on the data line to prevent damage to the device in the event of a Power to Data line short. Page 1 of 12 © Apple Computer Inc., 1985 June 13,1985 062·0267 Specification, Front Desk Bus Revision B Proposal M.R. Ctark UNRELEASED I CONFIDENTIAL Modulation: There are three f onns of modulation on the bus, Normal modulation which transmits commands and data, Hi&h Speed modulation which transmits data, and Signals which broadcast global messages such as s~rvk~ Rcquc~l auu Rt:set. Normal Modulation: An RZ code for modulation has been adopted for the Front Desk Bus. Each bit ce 11 boundary is signified by a falling edge on the bus. The period of each bit cell is the time between two falling edges on the bus. The time for a normal modulation bit cell, Teye' is 100 usec + 30%. All devices must support nonnal modulation for data transactions. The data is encoded as the ratio of low to high time of each bit cell. Thus a "O" is ~ncuc.le<l ct~ a hit cell in which thP. lnw timP. i~ 2reatcr thPtn th~ hieh t1me.. C.onver.~ely. a "l" is encoded as a bit cell in which the low time is kss than the high time. A Start is defined as a" l ".A Stop is similar to a "O", in that it has a low time of TO , but it does not have another negative edge to define the bit cell time. It is used to s…