This page breifly describes an adapter to connect a
machine that has a MODU connector to an RJ-45 plug.

The information presented here is not guaranteed in any way.
Corrections are welcomed.

What is a MODU connector?

A MODU connector is a 10-pin RS-232 connector with the individual pins arranged in a 2x5 configuration. The pins are spaced exactly like that in a common IDE harddrive, in a 40-pin IDC connector, 2.54mm or 0.10" apart.

This connector is used on various IBM machines including 6150, 320H, and according to one source (David Luner), the PC Jr. Having been given a 320H is what possed me to make this write-up.

Information for pinouts can be had in a couple of different places. One is a FAQ that includes the key words "IBM RT PC hardware FAQ 6150. I can seem to find the needed information on only 2 places on the Internet:

I quote a section from H.10  What is the pinout of the 6150 serial ports?
Looking INTO the connector pins in the back of the RT tower,
    here is the pin-out assignments and functions.  The connector on the
    back of the RT tower model is a male MODU connector.  This pinout
    is the same for the IBM 4-port card, and the fan out box of the IBM
    8-port card.

                               @   pin  abb   function
    +-----------------+        T    1   TX  = Transmited data from RT
    |                 |        T    2   DTR = Data Terminal Ready   
    |  1  2  3  4  5  |        T    3   RTS = Ready to send
     |               |         R    4   RI  = Ring Indicate
     | 6  7  8  9 10 |         -    5   -     No connection
     |               |         R    6   RX  = Received data to RT
     +---------------+         R    7   DSR = Data Set Ready
                               R    8   CTS = Clear to send
                               R    9   CD  = Carrier Detect
                               -   10   GND = Signal Ground

       @ means   T = RT transmits  R = RT receives (listens)


Another source is Usenet by searching with By using keywords like "RS/6000 320H Serial ports modu", Google will show a couple of posts from comp.unix.aix from people sharing information on this connector.

Yet another source, cited by the above sources, is the adapter cable itself made by IBM. The adapter is a 14 inch cable with a female MODU connector on one end and a male DB-25 connector on the other. The cable has an IBM part number (or an FRU) of either 00G0943 or 59F3740. To quote again from the IBM RT FAQ, question H.10:
NOTE: Metering the IBM RS6000 stubby cable P/N 00G0943 matches this pinout.

     Pin#   1   2  3   4  5  6  7  8  9  10      10-PIN MODU  female
            |   |  |   |  |  |  |  |  |  |
            |   |  |   |     |  |  |  |  +--+
            |   |  |   |     |  |  |  |  |  |
            2  20  4  22     3  6  5  8  7  1    DB25 male connector

       NOTE: pin 5 of the MODU connector is unused.

[picture of FRU 00G0943]
[picture of FRU 00G0943 connectors]

Why an RJ-45 plug?

I came across an article written in MAKE Magazine published by O'Reilly. In short, if you administer or troubleshoot serial devices, you can cut many headaches and hassles using this method. See the 5-in-1 network admin's cable.

My adapter is a little different in 2 respects. 1) The modular connectors featured in Michael Ossmann's site have RJ-45 /sockets/. My adapter uses an RJ-45 plug. And 2) perhaps the more controversial, my adapter INCLUDES a crossover. My reasoning for doing this is because I have intensions of using my IBM 320H as a headless server. No keyboard, no monitor,
just acceess to a serial line when the network is not up.

I realize having the builtin crossover may present some disadvantages. The loopback plug may no longer work, and having to connect a real serial device will require an extra coupler and crossover cable.

The Adapter

Now for the meat of this article, the pinout of the adapter:

    RJ-45         MODU
    1 (wht/org) --  6
    2 (org) ------ 10
    3 (wht/grn) --  1
    4 (blu) ------  8
    5 (wht/blu) -- 10
    6 (grn) ------ 7,9
    7 (wht/bro) --  3
    8 (bro) ------  2

Notice that this adapter has a couple of double-connections. MODU pin 10 connects to both RJ-45 pins 2 and 5. And RJ-45 pin 6 connects to both MODU pins 7 and 9.

To help make this adapter, I ripped off a connector from a IDE harddrive ribbon cable. I used my trusty pocket knife to carve out a piece 5 pins long. I cut off an end from an ethernet cable and proceeded to solder the wires on the my newly created MODU plug. After all of the soldering was finished, I got creative with some epoxy resin to protect the solder joints.

[picture of rj45-to-modu adapter]

All picture are my own. Various pieces of information are copyrighted by their owners. This web page copyrighted (c) 2006 by Curtis Brown <> .