SCSI I Bus Detailed Description and 'A' Connector Pin Out

SCSI General Description

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SCSI-I originated in 1986 as X3.131-1986, and was superseded by SCSI-II in 1990 [SCSI II approved in 1994].
The SCSI-1 term stands for Small Computer Systems Interface [-Revision 1] and is obsolete, SCSI-1 should not be used for new designs.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However even the current version is obsolete, as parallel SCSI is no longer used.

Data Transmission Description

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].

SCSI Bus Active / Passive Termination methods
SCSI-I Passive Termination Methods

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).

SCSI Cable Description

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

SCSI Connector Description

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.

50 pin scsi centronics connector, panel mount
SCSI-I Champ Connector

SCSI Connector Pinout

SCSI A Cable PinOut
Single-Ended Data Pinout
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.

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Modified 3/05/12
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