PDP-11 Controller needed to use a couple of RX33 drive

tiggerlasv at aim.com tiggerlasv at aim.com
Sun Jun 14 23:23:48 CDT 2009

A while back, I posted instructions for making a cable
to go directly from an RQDX controller to RX33's an RX50's.

While I didn't test it with more than 2 drives,
I don't see why it wouldn't support 4 drives.

For some reason (possibly my mail editor),
the archive copy had ?'s in place of _spaces_
so I am reposting it below.


If anyone is interested, here are the pinouts, and instructions
for constructing such a cable.  The instructions were created
with the lay-person in mind.  Copying this text into Windows Notepad
should restore proper formatting.

This cable was constructed using a BA23 distribution panel as a guide,
and has been tested with both RX50's and RX33's, attached to an RQDX3.

Cable diagram:

Floppy  RQDX    Description
1       21      Gnd
2       22      Density Select
3       NC      (Cut)
4       NC      (Cut)
5       NC      (Cut)
6       24      Drive Select 4
7       25      Gnd
8       26      Index
9       NC
10      28      Drive Select 1
11      NC      (Cut)
12      29      Drive Select 2
13      NC      (Cut)
14      30      Drive Select 3
15      NC
16      32      Floppy Motor
17      33      Gnd
18      34      Direction
19      35      Gnd
20      36      Step
21      37      Gnd
22      38      Floppy Write Data
23      39      Gnd
24      40      Write Gate
25      41      Gnd
26      42      Track 0
27      NC      (Cut)
28      8       Write Fault
29      45      Gnd
30      46      Floppy Read Data
31      47      Gnd
32      48      Head Select 0
33      49      Gnd
34      50      Ready

Stuff you'll need:

1.)     A suitable length of 34-conductor ribbon cable.
        Make sure you have enough wire to reach the controller,
        and enough slack on the other end to allow the drives
        to slide out of the bay.  Add 6" onto this measurement.

2.)     A 50-pin IDC-style "Berg" connector for the RQDXn.

3.)     A 34-pin IDC-style "Berg" connector for an RX50.

4.)     One or two 34-pin IDC-style card-edge connectors for RX33's.

What to do:

1.)     Split one end of the ribbon cable as follows.
        DON'T separate ALL of the wires individually; keep them paired!
        Peel each grouping of conductors back about 4 or 5 inches.

        6 through 10
        14 through 26
        29 through 34

2.)     Cut conductors 3, 4, 5, 11, 13, and 27.
        They won't be used, and there won't be a convenient place
        to terminate them on the connector.

        These wires will need to be insulated at some point.
        If you have wide plastic sleeving, then put a section of it
        onto the cable.  Heat shrink will NOT work well.
        A simple loop of electrical tape should suffice.

3.)     Conductor 28 needs to be fairly long, since it
        lands on the opposite side of the 50-pin connector.

        Trim about 3 inches off ALL of the OTHER wires.

4.)     Using a pair of needle-nose pliers,
        REMOVE the following contacts from the 50-pin IDC connector:

        7, 9, 19, 20, 23, 27, 31, 43, and 44.

        This will prevent certain RQDXn signal lines from
        being inadvertently grounded, and will help properly
        identify your landing positions for some of the connections.

5.)     Line up the ribbon cable with the connector,
        and check to make sure that conductor 28 is long enough
        to reach across the cable, without placing it under any strain.

6.)     Press the ribbon cable onto the 50-pin IDC connector.
        Refer to the chart at the top of this text; You'll see
        that they are grouped exactly as they should sit on
        the actual connector, with the exception of conductor 28.

        Note that SOME groupings start on ODD connector numbers,
        and some start on EVEN numbers.

7.)     If you haven't done so already, refer back to step 2,
        and insulate the wires that were cut, to prevent them
        from shorting-out against the chassis.

8.)     Determine where you need your floppy connectors,
        and press the connectors onto the 34-pin cable.

        Make sure that you observe the position of Pin 1.

        I made a "Universal cable" for my BA123.
        It has a 34-pin berg, and TWO card-edge connectors
        near the end (spaced about 4" apart), and then a THIRD
        card-edge connector about 10" down.

        This will allow double-stacked RX33's in a single bay,
        individual RX33's in separate bays, or a single RX50.

9.)     Test, and enjoy !

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