Making a diskette drive adaptor

Some earlier PS/2s (including some or all Model 50s, 60s, 70s and 80s) used diskette drives with 40 way card edge connectors. These fitted into a frame which contained the female connector. Later machines used a 34 way 2 row header instead.

There is an adaptor cable that connects the 40 pin card edge connector to the newer 34 pin header connector. It is used (for example) when installing the Internal Tape Backup Unit (ITBU) or the External Diskette Drive adaptor (the dummy drive cage) to machines that do not use the edge connector interface. It might also be useful in extremis if it were necessary to fit an older drive to one of the newer Model 80s (the actual drive retention cage is the same).

The IBM version of this cable uses about 3cm of 34 way ribbon cable, with the (female) edge connector on one end and a (male) header connector on the other. However, wiring is not 'straight through'; the power connections differ between the two interfaces.

For completeness, here are all of the connections required:

Edge connector pin Header connector pin Signal
1 1 Ground
2 2 -High Density Select
3 No connection Reserved
4 4 Drive Type ID 1
5 5 Ground
6 No connection Reserved
7 7 Ground
8 8 -Index
9 9 Ground
10 10 Reserved
11 11 Ground
12 12 -Drive Select
13 13 Ground
14 14 Reserved
15 15 Ground
16 16 -Motor Enable
17 17 Ground
18 18 -Direction In
19 19 Ground
20 20 -Step
21 21 Ground
22 22 -Write Data
23 23 Ground
24 24 -Write Enable
25 25 Ground
26 26 -Track 0
27 27 Ground
28 28 -Write Protect
29 29 Ground
30 30 -Read Data
31 31 Ground
32 32 -Head 1 Select
33 33 Ground
34 34 -Diskette Change
35 No connection Ground
36 No connection Ground
37 No connection Ground
38 3 +5V DC
39 No connection Ground
40 6 +12V DC

In practice, manufacture of this cable is fairly straightforward. Cut a short piece of 34 way ribbon cable (about 6cm). Fit the 34 way header connector in the normal way, remembering that the edge with the red stripe goes to pin 1. Separate out wires 3 and 6 for most of the length of the cable, leaving about 1cm intact nearest the header connector. Cut the three main parts of the cable (wires 1-2, 4-5, and 7-34) back to about 3cm in length. Trail wires 3 and 6 across the surface of the rest, and hold temporarily in place with a small piece of sticky tape. Fit the edge connector loosely, making sure that the wires all go in the right connectors (a gap of one wire between each of the first three sections, counting from pin 1, a 3 wire gap to the single wire from pin 3 on the header, and a 1 wire gap between that and the final wire (from pin 6 on the header). When all is in place, crimp it all home.

It may be worth winding some wide tape round the cable, transversely, to avoid the two single wires snagging on something when the cable is fitted.

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Last Updated: 14th April 1999
© 1999 by Bob Eager, Tavi Systems