In order to avoid reflections or cross-talking between the signal lines, there are some requirements which must be satisfied by SCSI cabels. For instance, the characteristic impedance should be between 90 and 140 ohms. Therefore, cables made for serial or parallel ports must not be used for connecting devices by 25-pin SCSI connectors, even if the connectors are the same and all 25 wires are connected. Nevertheless, it may be possible to use such a simple cable practically under the following circumstances:
A SCSI cable consists of a 50-conductor flat cable, with nearly every second conductor connected to ground, or a 25-signal twisted-pair cable, each signal conductor twisted with a ground conductor. On 25-pin connections, all ground lines are connected together at the connector.
Even when using "good" cables, the over-all length of the SCSI bus (i.e. summed length of all cabels and devices) is restricted to 6 meters.
There are different type of 50-pin connectors used for internal connections within devices and for external connections. Although the mechanical order of the signals is the same for internal and external connectors, the pin numbers are different because different numbering systems are used.
| Signal | external connector |
internal connector or cabel conductor number |
external connector |
Sub-D 25 | Signal | ||
|---|---|---|---|---|---|---|---|
| GROUND | 1 | 1 | | | 2 | 26 | 8 | /DB0 |
| GROUND | 2 | 3 | | | 4 | 27 | 21 | /DB1 |
| GROUND | 3 | 5 | | | 6 | 28 | 22 | /DB2 |
| GROUND | 4 | 7 | | | 8 | 29 | 10 | /DB3 |
| GROUND | 5 | 9 | | | 10 | 30 | 23 | /DB4 |
| GROUND | 6 | 11 | | | 12 | 31 | 11 | /DB5 |
| GROUND | 7 | 13 | | | 14 | 32 | 12 | /DB6 |
| GROUND | 8 | 15 | | | 16 | 33 | 13 | /DB7 |
| GROUND | 9 | 17 | | | 18 | 34 | 20 | /DBP |
| GROUND | 10 | 19 | | | 20 | 35 | 7,9,18 | GROUND |
| GROUND | 11 | 21 | | | 22 | 36 | 14,16,24 | GROUND |
| RESERVED | 12 | 23 | | | 24 | 37 | RESERVED | |
| N.C. | 13 | 25 | | | 26 | 38 | 25* | TERMPWR |
| RESERVED | 14 | 27 | | | 28 | 39 | RESERVED | |
| GROUND | 15 | 29 | | | 30 | 40 | GROUND | |
| GROUND | 16 | 31 | | | 32 | 41 | 17 | /ATN |
| GROUND | 17 | 33 | | | 34 | 42 | GROUND | |
| GROUND | 18 | 35 | | | 36 | 43 | 6 | /BSY |
| GROUND | 19 | 37 | | | 38 | 44 | 5 | /ACK |
| GROUND | 20 | 39 | | | 40 | 45 | 4 | /RST |
| GROUND | 21 | 41 | | | 42 | 46 | 2 | /MSG |
| GROUND | 22 | 43 | | | 44 | 47 | 19 | /SEL |
| GROUND | 23 | 45 | | | 46 | 48 | 15 | /CD |
| GROUND | 24 | 47 | | | 48 | 49 | 1 | /REQ |
| GROUND | 25 | 49 | | | 50 | 50 | 3 | /IO |
*Note: TERMPWR (Terminator Power, Pin 25) is not connected on SCSI connectors of Multiplayers.