The present invention relates to a small computer system interface (hereinafter referred to as SCSI), and more particularly to a selection response circuit according to SCSI which realizes a selection response function by a digital hardware.
Generally, SCSI is the regulations affecting the interface for connecting a small computer, i.e., a personal computer, and external peripheral units. Currently, the regulations are standardized with respect to a magnetic disc unit such as a floppy disc driver and a hard disc driver, and an optical disc unit such as a compact disc ROM, and a printer.
FIGS. 1A to 1C show systems connected according to SCSI, where one computer 1A can be coupled to one controller 2A by a mutual SCSI bus (as shown in FIG. 1A), or one computer 1B can be coupled to two controllers 2B and 2C by an SCSI bus (as shown in FIG. 1B). Also, two or more computers 1C to 1E can be coupled to a plurality of controllers 2D to 2H by an SCSI bus. That is, one or a plurality of computers 1A to 1E control one or a plurality of controllers 2A to 2H as a target.
FIG. 2 is a functional diagram with respect to phase conversion in SCSI. A bus free phase P1 used in FIG. 2 is the phase in which all SCSI units do not use a bus. An arbitration phase P2 is for an SCSI unit to obtain the use of a bus, and a selection phase P3 or a reselection phase P4 is for an initiator (computer) to select an SCSI unit operated as a target and winning in arbitration. An information transmission phase P5 (message out, comment, data in, data out, status, message in) is to transmit/receive actually needed information between an initiator and a target.
Meanwhile, FIG. 3 shows an SCSI bus intermediating data transmission between a computer 1F and a controller 2I, in which the SCSI bus has data bits, a parity bit, and control signal bits.
The selection phase among the respective bus phases is for an initiator (computer) to select an SCSI unit operated as a target, and winning in arbitration. The response sequence in the selection phase of SCSI is as follows.
That is, when a bus is not used by all SCSI units and data on the bus is equal to identification (hereinafter referred to as ID) of the SCSI unit itself, and IDs of three or more SCSI units are not active over a predetermined time, it is automatically indicated that the target is selected with respect to the initiator.
Conventionally, such a selection response function of SCSI units is performed using the software of a computer. That is, after it is identified that there is a selection phase, it is identified that the ID of the SCSI unit itself is active by reading data on a data bus and the active bit is one bit (i.e., ID of the itself) or 2 bits (i.e., IDs of SCSI unit itself and its host computer), using a switch or a switching device, and then the response with respect to this is performed.
Also, there is a system using an SCSI controller IC to obtain a selection response, but the internal constitution of such an SCSI controller IC and the selection response type have not been known.