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Parallel SCSI connectors

Physical connectors

Connector SCSI standards Notes

DB25 - external
Apple narrow SCSI Legacy SCSI interface designed by those lunatics in California. Doesn't adhere to any sane standard, doesn't allow the use of twisted pairs for the data conductors and hence the only other people to use it are Iomega and the occasional scanner manufacturer :-) Avoid wherever possible!

Centronics 50 - external
SCSI 1 external Proper chunky connector resembling a parallel printer connector with a few more pins. Flat out, this monster can shift a whopping 5MB every second! WOW!

DB50-HD - external
SCSI 2 external This connector allows for 10MB/sec SCSI-2 and is generally found on things like Iomega Jaz drives, external DDS streamers etc.

HE50 - internal
SCSI 1/2 internal Insulation displacement connector for 50-way internal ribbon connectors. Fairly robust, but an absolute bastard to remove without damaging the ribbon when they've been seated for a while...

DB68-HD - internal / external
(Ultra / Ultra160 / Ultra 320)Wide SCSI 68-pin connector that finds itself on the rear face of plenty of drive arrays. This is a current connector and handles all the way up to the latest Ultra-320 spec.

SCA-80 - internal
(Ultra / Ultra160 / Ultra 320)Wide SCSI Midi-centronics connector that carries both SCSI data and power to an individual drive. Used to allow hot-swapping of SCSI drives in systems that support it. The drives are usually semi-enclosed by a 'sled' that guides the connector accurately into a matching SCA backplane. Note that, unless you really know what you're doing, 'hotplugging drives' is usually a synonym for 'breaking stuff in a horribly expensive manner'.

Bus speeds

Speed Frequency Narrow (8-bit) speed Wide (16-bit) speed
SCSI 5MHz 1.5 to 5 MB/s [no wide device]
Fast SCSI 10MHz 10MB/s 20MB/s
Ultra SCSI 20MHz 20MB/s 40MB/s
Ultra2 SCSI 40MHz 40MB/s 80MB/s
Ultra 160 80MHz [no narrow device] 160MB/s
Ultra 320 80MHz with clever sideband addressing [no narrow device] 320MB/s

SCSI connector pinouts

DB25 (Apple) Connector

As mentioned above, Apple decided to use a DB25 connector for narrow SCSI on the earlier Macs, but this doesn't allow twisted data pairs in the cable (bad idea) and as such is not complient to SCSI standards. Daisychaining DB25 devices is not the best idea in the world and a cable longer than 75cm seems to introduce so many errors it's simply not worth bothering with. This interconnection format should have been put to bed a long time ago!

Description Name Pin   Pin Name Description
GroundGND720DPData parity bit
Data bit 0D0821D1Data bit 1
GroundGND922D2Data bit 2
Data bit 3D31023D4Data bit 4
Data bit 5D51124GNDGround
Data bit 6D61225TermPwrTermination power
Data bit 7D713 

Narrow external (LD50 & HD50)

LD50 and HD50 use the same pinout, so this table apply to both of them.

LD50 is SCSI-1 standard and is still used for many narrow (formerly called 8bit SCSI) devices. Consider using the HD50 for new designs.
HD50 is SCSI-2 standard, fully compatible with LD50, support higher frequency such as UltraSCSI and is smaller.
DescriptionNamePin PinNameDescription
Twisted pair GroundGND1 26D0Data bit 0
Twisted pair GroundGND2 27D1Data bit 1
Twisted pair GroundGND3 28D2Data bit 2
Twisted pair GroundGND4 29D3Data bit 3
Twisted pair GroundGND5 30D4Data bit 4
Twisted pair GroundGND6 31D5Data bit 5
Twisted pair GroundGND7 32D6Data bit 6
Twisted pair GroundGND8 33D7Data bit 7
Twisted pair GroundGND9 34DPData Parity bit
GroundGND10 35GNDGround
GroundGND11 36GNDGround
Reserved 12 37 Reserved
13 38TermPwrTermination Power (+5V)
Reserved 14 39 Reserved
GroundGND15 40GNDGround
Twisted pair GroundGND16 41ATNAttention
GroundGND17 42GNDGround
Twisted pair GroundGND18 43BSYBusy
Twisted pair GroundGND19 44ACKAcknowledge
Twisted pair GroundGND20 45RSTReset
Twisted pair GroundGND21 46MSGMessage
Twisted pair GroundGND22 47SELSelection
Twisted pair GroundGND23 48C/DCommand/-Data
Twisted pair GroundGND24 49REQRequest
Twisted pair GroundGND25 50I/OInput/Output
Note: signals positions are the same as on HE50 internal narrow connector, only pin numbers are different because of the connectors differences.

Narrow internal (HE50)

Internal Narrow SCSI cable, similar to external LD50 and HD50.

DescriptionNamePin PinNameDescription
Twisted pair GroundGND1 2D0Data bit 0
Twisted pair GroundGND3 4D1Data bit 1
Twisted pair GroundGND5 6D2Data bit 2
Twisted pair GroundGND7 8D3Data bit 3
Twisted pair GroundGND9 10D4Data bit 4
Twisted pair GroundGND11 12D5Data bit 5
Twisted pair GroundGND13 14D6Data bit 6
Twisted pair GroundGND15 16D7Data bit 7
Twisted pair GroundGND17 18DPData Parity bit
GroundGND19 20GNDGround
GroundGND21 22GNDGround
Reserved 23 24 Reserved
25 26TermPwrTermination Power (+5V)
Reserved 27 28 Reserved
GroundGND29 30GNDGround
Twisted pair GroundGND31 32ATNAttention
GroundGND33 34GNDGround
Twisted pair GroundGND35 36BSYBusy
Twisted pair GroundGND37 38ACKAcknowledge
Twisted pair GroundGND39 40RSTReset
Twisted pair GroundGND41 42MSGMessage
Twisted pair GroundGND43 44SELSelection
Twisted pair GroundGND45 46C/DCommand/-Data
Twisted pair GroundGND47 48REQRequest
Twisted pair GroundGND49 50I/OInput/Output
Note: signals positions are the same as on HD50 and LD50 connectors, only pin numbers are different because of the connectors differences.


Wide external (HD68)

This connector is used for external Wide devices.
For 8bit devices, the HD50 should be used instead.

DescriptionNamePin PinNameDescription
Twisted pair GroundGND1 35D12Data bit 12
Twisted pair GroundGND2 36D13Data bit 13
Twisted pair GroundGND3 37D14Data bit 14
Twisted pair GroundGND4 38D15Data bit 15
Twisted pair GroundGND5 39DP1Data Parity bit 1
Twisted pair GroundGND6 40D0Data bit 0
Twisted pair GroundGND7 41D1Data bit 1
Twisted pair GroundGND8 42D2Data bit 2
Twisted pair GroundGND9 43D3Data bit 3
Twisted pair GroundGND10 44D4Data bit 4
Twisted pair GroundGND11 45D5Data bit 5
Twisted pair GroundGND12 46D6Data bit 6
Twisted pair GroundGND13 47D7Data bit 7
Twisted pair GroundGND14 48DP0Data Parity bit 0
GroundGND15 49GNDGround
GroundGND16 50GNDGround
Termination Power (+5V)TermPwr17 51TermPwrTermination Power (+5V)
Termination Power (+5V)TermPwr18 52TermPwrTermination Power (+5V)
Reserved 19 53 Reserved
GroundGND20 54GNDGround
Twisted pair GroundGND21 55ATNAttention
GroundGND22 56GNDGround
Twisted pair GroundGND23 57BSYBusy
Twisted pair GroundGND24 58ACKAcknowledge
Twisted pair GroundGND25 59RSTReset
Twisted pair GroundGND26 60MSGMessage
Twisted pair GroundGND27 61SELSelection
Twisted pair GroundGND28 62C/DCommand/-Data
Twisted pair GroundGND29 63REQRequest
Twisted pair GroundGND30 64I/OInput/Output
Twisted pair GroundGND31 65D8Data bit 8
Twisted pair GroundGND32 66D9Data bit 9
Twisted pair GroundGND33 67D10Data bit 10
Twisted pair GroundGND34 68D11Data bit 11

Wide internal (HD68)

This connector is used for internal Wide devices, for 8bit devices, the HE50 should be used instead.

Documentation not available yet.

This page was last updated: 30th September 2004 at 2:06pm BST
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