Upgrading and Repairing PCs

SCSI-1

SCSI-1 was the first implementation of SCSI and was officially known as ANSI X3.131-1986. The major features of SCSI-1 were as follows:

• 8-bit parallel bus

• 5MHz asynchronous or synchronous operation

• 4MBps (asynchronous) or 5MBps (synchronous) throughput

• 50-pin cables with low-density pin-header internal and Centronics-style external connectors

• Single-ended (SE) unbalanced transmission

• Passive termination

• Optional bus parity

SCSI-1 is now considered obsolete; in fact, the standard has been withdrawn by ANSI and replaced by SCSI-2.

Table 8.2. SCSI Types, Data-Transfer Rates, and Cables

SCSI Standard	SCSI Technology	Marketing Term	Clock Speed (MHz)	Transfer Width	Transfer Speed (MB/s)	Max. No. of Devices[1]	Cable Type	Max. Length (SE)	Max. Length (HVD)	Max. Length (LVD)
SCSI-1	Async	Asynchronous	5	8-bit	4	7	A (50-pin)	6M	25M	-
SCSI-1	Fast-5	Synchronous	5	8-bit	5	7	A (50-pin)	6M	25M	-
SCSI-2	Fast-5/Wide	Wide	5	16-bit	10	15	P (68-pin)	6M	25M	-
SCSI-2	Fast-10	Fast	10	8-bit	10	7	A (50-pin)	3M	25M	-
SCSI-2	Fast-10/Wide	Fast/Wide	10	16-bit	20	15	P (68-pin)	3M	25M	-
SCSI-3/SPI	Fast-20	Ultra	20	8-bit	20	7	A (50-pin)	3/1.5M[2]	25M	-
SCSI-3/SPI	Fast-20/Wide	Ultra/Wide	20	16-bit	40	7	P (68-pin)	3/1.5M[2]	25M	-
SCSI-3/SPI-2	Fast-40	Ultra2	40	8-bit	40	7	A (50-pin)	-	-	12M[3]
SCSI-3/SPI-2	Fast-40/Wide	Ultra2/Wide	40	16-bit	80	15	P (68-pin)	-	-	12M[3]
SCSI-3/SPI-3	Fast-80DT	Ultra3 (Ultra160)	40[4]	16-bit	160	15	P (68-pin)	-	-	12M[3]
SCSI-3/SPI-4	Fast-160DT	Ultra4 (Ultra320)	80[4]	16-bit	320	15	P (68-pin)	-	-	12M[3]
SCSI-3/SPI-5	Fast-320DT	Ultra5 (Ultra640)	160[4]	16-bit	640	15	P (68-pin)	-	-	12M[3]
Cable Lengths are in meters: 25M = 80ft., 12M = 40ft., 6M = 20ft., 3M = 10ft., 1.5M = 5ft. SE = Single-ended signaling. HVD = High Voltage Differential signaling; obsolete. LVD = Low Voltage Differential signaling. SPI = SCSI Parallel Interface; part of SCSI-3. DT = Double transition, or two transfers per clock cycle; 16-bit only.

^[1] = Not including the host adapter.

^[2] = Ultra SCSI cable total length is restricted to 1.5M if more than three devices exist on the bus (not including the host adapter). A maximum of seven devices is allowed.

^[3] = A 25M cable can be used if only one device exists (point-to-point interconnect).

^[4] = Ultra3 (Ultra160), Ultra4 (Ultra320), and Ultra5 (Ultra640) SCSI transfer twice per clock cycle and are 16-bit only.