Daisy-chaining a Pair of Disk ][ Drives

from The Apple Blossom V3N3, July 1997

Special to The Apple Blossom by Vincent Quinn


This article owes a great deal to the information contained in David Empson's (dempson@actrix.gen.nz) piece on attaching a 19-pin 5.25" drive to a 20-pin Disk ][ controller card ("Making a Disk ][ adapter cable" ), which can be consulted on-line at


Thank you David. I'm happy to acknowledge this information debt!

Description and disclaimer:

This article describes a procedure I have used to modify three Disk ][ drives. Previously, all three were standard Disk ][ drives, attaching to a Disk ][ controller card in slot 6 of an Apple IIGS, via a 20-strand flat ribbon cable with IDC connector s at each end. Now they have 19-pin plugs and pass-through sockets, allowing any two of them to be daisy-chained to each other and to one or more Apple 3.5-inch drives on the IIGS smartport, where they are recognised as Slot 6 drive 1 and Slot 6 drive 2. Any one of the drives can also be plugged into the external disk port of a //c (the //c will accept only one external 5.25-inch drive) where it is recognised as Slot 6 drive 2.

Although the procedure is straightforward once you have the cable pinouts (provided that you can correctly identify the pin numbers and wires of all the connectors), it is very easy to get the wiring wrong. If you cross-wire any of the pins, you risk damaging your drive, your computer or doing other serious damage. So although I have got it right each time, I cannot guarantee that you will--and therefore I will not be responsible for any such damage. If you follow this procedure, you do so entirely a t your own risk.

What you will need: the tools involved are:

  • Philips head and flat blade screwdrivers
  • hand or power drill and drill bits
  • baby hacksaw or 'abrafile'
  • fine file
  • fine-nosed electrician's pliers
  • soldering iron
  • continuity tester

For each drive you intend to modify, you will need:

  • one 19-pin male D plug
  • one 19-pin female socket
  • one 20-pin IDC connector
  • about six inches of 20-strand flat ribbon cable, with strand 1 marked (this is usually in red)
  • about 15 inches of round 20-core-and-shield cable.

The pinouts:

The ones that concern us are the drive control signals that are common to the IIGS smartport, the //c external disk port and the Disk ][ pins. They are as follows (again, thanks to David Empson):
Signal           IIGS           //c             Disk ][

ground           1,2,3          1,2,3,4         1,3,5,7

-12V             5              5               9

+5V              6              6               11,12

+12V             7,8            7,8             13,15,17,19

write protect    10             10              20

stepper motor    11,12,13,14    11,12,13,14     2,4,6,8

write request    15             15              10

drive 1 enable   17             17              14

drive 2 enable   9              -               14

read data        18             18              16

write data       19             19              18

To complete the picture, there are some other signals which, in the case of 5.25" drives, we can safely ignore. They are:

Signal                IIGS       //c      Disk ][

external interrupt     -          9        -

3.5 drive enable       4          -        -

3.5 drive select      16          -        -

no connection          -         16        -

From the above, we can see straightaway that the significant signals at the IIGS smartport and the //c external disk port are practically the same—the major difference being the lack of a drive 2 enable signal on the //c. Note also that there is only one enable pin--pin 14--on a Disk ][. We are going to use this to our advantage.

A note before we begin: the most difficult thing I found about this conversion was finding 19-pin D plugs and sockets--I couldn't! What I had to do was to get 23-pin ones and cut them down with a hacksaw (at least, I cut the plugs down--where the sock ets were concerned, I simply blocked up the last hole at the left and right ends of each row with superglue). They work fine...

Phase one--the physical work

Obviously, you need to carry out all work with everything switched off and with your Disk ][ drive(s) disconnected from the controller card.

First you need to cut a mounting hole in the Disk ][ casing to take the pass-through socket. Turn the drive upside down and remove the four black philips-head bolts. Then slide the main part of the drive forward out of the casing.

Carefully unplug the 20-pin IDC connector that attaches the ribbon cable to the drive's logic board (important--note carefully how this plug and cable are attached: the ribbon cable comes out of the IDC plug on its upper surface). Also note tha t the pin numbers of the connector are marked on the drive's logic board--this is again very important. Detach the ribbon cable from the cable restraint on the back plate of the drive casing and put it aside--you should keep it in case you ever need to u se your drive with a Disk ][ controller card again.

Now turn the drive over again and remove carefully the four countersunk screws that hold the mechanism itself to the bottom part of the case. Lift the mechanism gently away. If you wish, you can give it a clean and recalibrate its speed while you have the drive apart (instructions on adjusting speed and drive alignment can be found on-line--check the Apple II FAQ).

Decide where on the back of the casing you want to locate the pass-through port. I've found that the best place for cutting the hole is (if you look at the casing from the rear, holding it the right way up) around 15 mm down and 15 mm in from the top left corner (peel off the red sticker that says ‘Warning--connect this cable only to the Disk ][ controller card': I mean, you ain't gonna need that anymore!). If you put the socket there, there is little danger of fouling other connections inside the dr ive when it is reassembled.

Use paper and pencil to draw a cutting template around your 19-pin socket. In my case, to make sure that any rough edges would be concealed (and to make the job a little easier!) I mounted the flange of the socket on the outside, not the inside, of th e drive casing--bear this in mind when I talk about feeding the cables through the mounting hole from inside to outside below: you don't need to do this if you are mounting the flange of the socket inside the casing. Then scribe the hole you have marked onto the back of the drive casing. Use the drill to make a series of small holes, joined together so that you can get the hacksaw or abrafile through them to begin cutting. Carefully cut around the template hole (the back casing of all three of my drives was aluminium, so this wasn't too difficult) and remove the waste; smooth any rough edges with the file.

Now bring up the socket to the hole and check it fits. Mark the positions of the two mounting bolt/rivet holes, take the socket away again and drill the two holes out. Now wash any metal swarf off the casing. The physical part is done!

Phase two--the electronics

First strip the outer insulation from both ends of your 20-core-and-shield cable. We are going to use only 19 of the cores, so select one core and cut it back to the outer insulation at each end. Tin (coat with solder) the other 19 cores at both ends, and tin the solder cups of both the 19-pin plug and socket.

Now solder the 19 cores to the 19-pin male D plug--it does not matter which colour wire goes to which pin, but all 19 pins must have a wire going to them.

Pass the free end of the cable through the hole that you have cut in the drive casing, passing it from inside to outside of the casing. Now here comes one of the critical parts. Using your continuity tester, check which core you have soldered to which pin of the 19-pin plug. I'd recommend checking three times and writing the wire colours down carefully so that there is no mistake. Now fold the wires that run from plug pin 9 and plug pin 17 back out of the way, and solder the remaining 17 cores to the female 19-pin socket like to like: i.e., the wire running from male plug pin 1 goes to female socket pin 1, 2 goes to 2, etc.

Now you have 19 cores attached to the plug and 17 to the socket: at the socket end, there are no wires running to pin 9 or pin 17 and you have two cores unattached. Here is another critical bit. Take the wire that runs from male plug pin 9 and solder it to female socket pin 17. The wire that runs from male plug pin 17 remains unattached for the moment.

Identify the pins on the 20-pin IDC connector. The easiest way of doing this is as follows: hold the IDC connector horizontal, with the pin holes down and the notch that indicates the side of the connector that the cable is to come out of facing you. Pin 1 is at the left front (i.e., the one nearest to you). Using a vise and two bits of wood, clamp the 20-pin IDC connector to the six inch flat ribbon cable, taking care that the red stripe that indicates strand 1 of the cable actually runs to pin 1 of the connector. If you do this right, the strands of the cable will run to the correct pin numbers: ie, the red stripe will run to pin 1, the strand on its right will run to pin 2, the strand on ITS right to pin 3, etc. Using a piece of wire pushed in to the holes of the IDC connector, use your continuity tester to make sure that everything is OK and also that there are no shorts between adjacent pins caused by faulty clamping. Again, this is one of the critical points: you must know what wire runs to w hat pin.

At the other end of the ribbon cable, bare and tin the ends of the wires. Now carefully split the insulation so that you can peel wires 14, 17 and 19 back and away from the rest. Cut off 17 and 19--they are not needed. Remember the wire coming from th e male plug pin 17 that we left loose? Solder it to wire 14 coming from the IDC connector, cover the joint with tape or heatshrink and gently move it aside. Feed the remaining 17 wires that come from the IDC connector through the socket mounting hole, fr om the inside to the outside, and solder them to the pins exactly according to the following plan:

Signal        IDC 20 wire       female D socket pin

ground            1                   1

stepper motor     2                  11

ground            3                   2

stepper motor     4                  12

ground            5                   3

stepper motor     6                  13

ground*           7                   4

stepper motor     8                  14

-12V              9                   5

write request    10                  15

+5V              11                   6

+5V**            12                  16

+12V             13                   7

+12V             15                   8

read data        16                  18

write data       18                  19

write protect    20                  10
*3.5-inch drive enable on the IIGS;
** 3.5 head select on IIGS; not connected on //c.
There should be nothing connected to pin 9 of the female D socket.

That's the electronics finished!
Put it back together: first bring the 19 pin female socket up to the mounting hole and fix it in place with two small bolts or rivets. Now attach the 20-pin IDC connector to the logic board of the drive, taking care that the red strand goes to pin 1 as marked on the drive's board (ie, if you have done everything correctly, the ribbon cable should come out of the IDC connector on top). Secure the mechanism to the base of the casing with the four countersunk screws. Clamp the round 20-core cable into the cable restraint on the back of the drive. then slide the mechanism backwards into the casing, feeding the cable out as you go. Secure the casing with the four black philips-head bolts. Finally, put the cable restraint and the shells on to the male 19 -pin D plug.

Now repeat this procedure exactly with a second (and a third, and a fourth...) drive, so that you have identical devices. Now they can be daisy-chained on a IIGS smartport: as they are identical, it does not matter which one is first or second: whiche ver it is, it will be identified as 'Slot 6 drive 1' or 'Slot 6 drive 2', as appropriate.

How it works

It is very simple really. As you can see from the pinouts above, the wiring means that the signals from the IIGS or the //c ports are passed straight to the correct pins at the IDC connector on the drive's logic board. The clever bit is where the two drive enable signals end up.

The Disk ][ requires its drive enable signal on pin 14. The IIGS (and the //c) puts out a drive 1 enable signal on pin 17. As is clear from the above, the signal on pin 17 at the computer is transmitted directly to pin 14 on the Disk ][, because we co nnected the wire from pin 17 of the 19-pin male plug directly to pin 14 of the IDC connector.

The IIGS puts out its drive 2 enable signal on pin 9. We connected pin 9 of the 19-pin male plug to pin 17 of the female pass-through socket. So when you daisy-chain two of these devices together, the second one sees the drive 2 enable signal at pin 1 7 of the first drive's female pass-through socket and passes it to pin 14 at the IDC connector on the logic board. Daisy-chainable Disk ][s. Simple, isn't it?

By the way, if you are lucky enough to come across 19-pin plugs and sockets, would'ya mail me some??

Vincent Quinn :-)

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