SCSI Bus A Connector Pin Out

Single-Ended Data Pinout

The "A" cable is used with SCSI-1, SCSI-2, or SCSI-3 to provide "FAST" SCSI with an 8 bit [Parallel] data bus. Refer here for the main SCSI Bus page.
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 differential pinout for the A Cable is shown on the Differential A Cable page.
The pin out for the 'A' Single-ended data cable is listed below, which is of course still valid.
However keep in mind that these iterations of the SCSI interface are obsolete, but listed for reference.

The A cable has 50 conductors and provides an 8 bit data bus. The interface is Single-ended (unbalanced) with a cable length at 6 meters. SCSI-I used a Centronics 50 pin connector, 0.10 inch spacing, with 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. For Connector and Cable Assembly manufacturers refer to the main SCSI page. With the advent of SCSI-4 in 2002, and then SCSI-5 in 2003, the 'A' cable is obsolete and should not be used for new designs.
Also refer to the main Connector manufacturers page, Cable manufacturers page, and Cable Assembly manufacturers [Cables with the connectors installed].
Providing generic cable and connector styles from various companies.

SCSI A Cable 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-

SCSI MD-50 Male Connector

The "B" Single-Ended cable pinout is listed on the SCSI Bus 'B' connector page.
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.

Back to the main SCSI Interface Bus page
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.

Keep in mind that the SCSI interface discussed here is obsolete.
The signal assignments would still be valid as found on some legacy computer gear, but the interface is dated.
However the protocols may be found on other devices, but the physical and electrical layers that SCSI defined are out-dated.

SCSI Electrical Interface, Single Ended SCSI [SE].

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Modified 3/05/12
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