SCSI Bus A Connector Pin-Out

Differential Data

The "A" cable is used with SCSI-1, SCSI-2, or SCSI-3 to provide "FAST" SCSI with an 8 bit [Parallel] data bus.
With SCSI-2 the "A" may be used alone or with the "B" cable to provide "WIDE" SCSI allowing a 16 or 32 bit data bus. The pin-out for the 'A' Single-ended data cable is listed below. The Single-ended pinout for the A Cable is shown on the Single-ended A Cable page. The A cable has 50 conductors and provides an 8 bit data bus. The interface is Differential (Twisted-pair) with a cable length at 25 meters maximum. A 1 foot minimum length cable shall be used between devices. SCSI-I used a Centronics 50 pin connector, 0.10 inch spacing, with a 0.05 inch ribbon cable. While SCSI-II provided 8 bit data over a Micro D 50 pin, high density (50 mil). The 'A' cable impedance is between 90 and 140 ohms. The minimum conductor size is 28 AWG solid or stranded wire. For Connector and Cable Assembly manufacturers refer to the main SCSI page. Or refer to the main Connector manufacturers page, Cable manufacturers page, and Cable Assembly manufacturers

SCSI A Cable PinOut
Pin # Pin Function Pin # Pin Function
1 Ground 26 Ground
2 D0+ 27 D0-
3 D1+ 28 D1-
4 D2+ 29 D2-
5 D3+ 30 D3-
6 D4+ 31 D4-
7 D5+ 32 D5-
8 D6+ 33 D6-
9 D7+ 34 D7-
11 Ground 36 Ground
12 Reserved 37 Reserved
13 Open 38 TRMPWR
14 Reserved 39 Reserved
15 ATN+ 40 ATN-
16 Ground 41 Ground
17 BSY+ 42 BSY-
18 ACK+ 43 ACK-
19 RST+ 44 RST-
20 MSG+ 45 MSG-
21 SEL+ 46 SEL-
22 C/D+ 47 C/D-
23 REG+ 48 REG-
24 I/O+ 49 I/0-
25 Ground 50 Ground

On SCSI-3, the "A" cable may be used for an 8 bit data bus. However the new "P" cable is used to provide a 16 bit data path (wide SCSI). To allow 32 bits of data the "P" cable is combined with another new cable called "Q" (wide SCSI). The "Q" cable pin-out is listed on the SCSI Bus 'Q' Single-Ended connector or SCSI Bus 'Q' Differential connector page. The pin-out for the 'P' cable is listed on the SCSI Bus 'P' connector page.
The "B" Single-Ended cable pinout is listed on the SCSI Bus 'B' connector page.

Back to the main SCSI Interface Bus page, which along with the table of signal assignments is out-dated and obsolete.
The SCSI [parallel] bus width is either 8 bits or 16 bits [Wide bus]. Also the bus may be either Single ended or Differential; however the two are mutually exclusive. SCSI is a chained parallel bus, cables start at the Host and run from device to device in a chain. SCSI may be used for asynchronous and synchronous transfers; Asynchronous transfers using Start and Stop bits and synchronous transfers using system timing (Hand-Shaking). The data bus also carries one parity bit.

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Modified 2/5/12
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