9,4 Other Protocols

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I/O			\
REQ	\	/		\	/
							/
ACK	\	/			\
Data	< from Target v		From	/ ' r-	i -i- i	\	indicator

Time ^

Figure 9.15. SCSI Timing

The IEEE-488 bus is well suited to remote control of instrumentation and is becoming available on many of the instruments being designed at this time. You will probably see a lot of the IEEE-488 bus in your design experiences.

9.4.4 The Small Computer System Interface (SCSI)

The microcomputer has made the intelligent I/O device economical. In a lot of systems today, a personal computer communicates with a printer that has a microcomputer to control it, or a disk that has its own microcomputer. Communications between a personal computer and the intelligent I/O device can be improved with an interface protocol specially designed for this application. The small computer system interface (SCSI) is designed for communications between personal computers and intelligent I/O devices. Many systems that you build may fit this specification and thus may use an SCSI interface.

The asynchronous protocol is quite similar to the IEEE-488 bus, having a 9-bit (8 data plus parity) parallel bus, a handshake protocol involving a direction signal (I/O), a request (REQ), and an acknowledge (ACK). (See Figure 9.13.) (There are also six other control signals, and the interface uses a 50-pin connector.) Up to eight bus controllers can be on an SCSI bus, and they may be initiators (e.g., microcontrollers) or targets (e.g., disk drives). A priority circuit assures that two initiators will not use the SCSI bus at the same time. After an initiator acquires the bus, a command stage is entered. The310-bytecommand packet selects a target controller and is capable of specifying the reading or writing of up to 256 bytes of data on up to a 1024-GB disk. After the command packet is sent, data are transferred between the initiator and target.

Each command or data byte is transferred using the IO, REQ, and ACK signals. IO is low when the initiator wishes to write a command or data into the target, and high when it wants to read data from the target. If the initiator is sending data or acommand, it begins a transfer by putting the eight-bit data or command and its parity bit on the