The SCSI-1 interface defined the electrical, physical and protocol layers
for a parallel bus which operates using either 8 bits or 16 bits.
SCSI-I originated in 1986 as X3.131-1986, and was superseded by SCSI-II in 1990
[SCSI II approved in 1994].
The SCSI-1 term stands for Small Computer
Systems Interface [-Revision 1] and is obsolete, SCSI-1 should not be
used for new designs.
The description of the current SCSI version, and a
comparison of all the SCSI revisions is located on the main SCSI Bus page.
However even the current version is obsolete, as parallel SCSI is no longer used.
The SCSI-1 Hard Drive Interface is a Single ended data bus transmitting
data referenced to ground [unbalanced]. SCSI-1 may be call narrow SCSI
[or just SCSI] when the 8-bit data path is implemented, or Wide SCSI
when operating with a 16-bit data bus. SCSI is an 8-bit / 16-bit
[Parallel] data bus, with all data bits sent at the same time over 8 or
16 data lines.
The SCSI bus is a chained Interface, cables start at the Host
and run from device to device in a chain. A total of 7 devices may be
connected on the bus.
The two devices at each end of the chain require
terminations, either added external to the devices or provided internal
to the devices.
SCSI I devices only used passive terminations [see below].
Passive Resistor Terminations reside on both
sides of the bus, and define the ends of the bus. Some SCSI devices have
internal terminators. The voltage is normally provided by the bus line:
'TERMPWR', and also requires a Schottky diode to handle reverse currents.
Decoupling capacitors (ranging between 2.2uF
and 10uF) should reside on the 'TERMPWR' line at each termination point.
Passive Termination provided reliable operation in SCSI-1 systems, how
ever for systems using SCSI-2 and above require active termination
schemes. The primary problem is double clocking on the Strobe lines,
which may occur because of a signal reflections. Of course
the passive approach also has a constant resistive path from TERMPWR to
ground, and is not regulated so varies with TERMPWR. SCSI I devices only
used passive terminations.
SCSI I transactions may use either asynchronous and synchronous data
transfers across the SCSI interface. SCSI-1 asynchronous transfers would
operate up to 1.5 MB/s. SCSI-1 synchronous transfers would operate up to
5 MB/s. SCSI commands are sent at the asynchronous rate. SCSI data is
transferred either at the asynchronous rate (worst case) or a negotiated
synchronous rate (with 5 MB/sec being the best case). SCSI-1 asynchronous
transfers use Start and Stop bits and synchronous transfers using
system timing (Hand-Shaking).
The SCSI-1 bus used the "A" cable to interconnect between devices. The 'A' cable interface is Single-ended (unbalanced) and has a maximum cable length at 6 meters. The 'A' cable may also used with the SCSI-2, or SCSI-3 buses to provide "FAST" SCSI. The 'A' cable impedance is between 90 and 140 ohms. The minimum conductor size is 28 AWG solid or stranded. The pin out for the 'A' Single-ended data cable is listed below. The differential pinout for the A Cable is shown on the Differential A Cable page. For SCSI Cable or Cable Assembly manufacturers refer to the main SCSI bus page. Or refer to the main Cable manufacturers page, and Cable Assembly manufacturers
The A cable has 50 conductors and provides an 8 bit data bus. SCSI-I used a Centronics 50 pin connector, having a 0.10 inch pin spacing, with 0.05 inch ribbon cable. Connectors used Internal to a SCSI device used 50-pin IDC headers. Refer here for a definition of an IDC Connector, or here for a definition of Header Connector. For SCSI Connector and Cable Assembly manufacturers refer to the main SCSI bus page, or refer to the main Connector manufacturers page. | SCSI-I Champ Connector |
Pin # | Pin Function | Pin # | Pin Function |
1 | Ground | 26 | D0- |
2 | 27 | D1- | |
3 | 28 | D2- | |
4 | 29 | D3- | |
5 | 30 | D4- | |
6 | 31 | D5- | |
7 | 32 | D6- | |
8 | 33 | D7- | |
9 | 34 | DPARTY- | |
10 | 35 | Ground | |
11 | 36 | Ground | |
12 | Reserved | 37 | Reserved |
13 | Open | 38 | TRMPWR |
14 | Reserved | 39 | Reserved |
15 | Ground | 40 | Ground |
16 | 41 | ATN- | |
17 | 42 | Ground | |
18 | 43 | BSY- | |
19 | 44 | ACK- | |
20 | 45 | RST- | |
21 | 46 | MSG- | |
22 | 47 | SEL- | |
23 | 48 | C/D- | |
24 | 49 | REG- | |
25 | 50 | I/O- |
This is a sub topic relating to the SCSI standard.
Refer to the main SCSI Interface Bus page for a description of the computer bus.
The "B" Single-Ended cable pinout is listed on the SCSI Bus 'B' connector page.
There are other cable types and connector types defined in the new revisions of SCSI.
Navigation: Engineering Home > Interface Buses > Cabled Interface Standards > SCSI Bus Description > SCSI I
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