Wrap Plugs for Diagnostics

Ever tried to test a serial or parallel port and wondered what this "Wrap Plug" is they mention in some publications or within the diagnostics?
Ever wondered where to get such a thing or how to build an own?

Here are some diagrams that might help to build own wrap plugs and some hints how to use them.


"What's a Wrap Plug anyway?"

In brief it is a plug with some connections, where data lines of serial/parallel port are shorted to create loopback connection.

"What's the use of a Wrap Plug?"

It is used to test if the external communication from a port works as well, i.e. that port drivers and the connector itself is in working order.

"Can you give an example?"

I sure can. Here's the most common:

Assumed you have a PS/2 and have some trouble with i.e. the serial port. You start your machine with the reference disk or from the reference partition. At the reference' Main Menu you press [CTRL]+[A] to start the advanced diagnostics and select "Run system checkout" from the follow-up menu.
You get a device list to confirm and can then select "Run the tests one time" or "Run the tests continously", "Log or display the errors" and "Display the device list".

Use "Run the tests one time". Then select the (first) serial port, sometimes named as "Async 1".

After some time a window similar to this below appears:

|               Question      Page   1 of 1 |
|                                           |
| Do you want to use a wrap plug to test    |
| system-board async port 1?                |
|                                           |
|                                           |
|                                           |
|                                           |
|                                           |
| Press Y or N                              |

If you answer with "N" only the internal part of the serial port is tested (the UART and the Interrupt requests) but not the external transceivers. If they are damaged -by what reason- the diagnostic will end with no errors, but the port may however be defective and unusable.

If you answer with "Y" you get the next window:

|               Question      Page   1 of 1 |
|                                           |
| Is an IBM communications cable attached   |
| to system-board async port 1?             |
|                                           |
|                                           |
|                                           |
|                                           |
|                                           |
| Press Y or N                              |

If there is none attached (in most cases) answer with "N".

Now the next window shows up:

|             Instructions    Page   1 of 1 |
|                                           |
| Connect the wrap plug to                  |
| system-board async port 1.                |
|                                           |
|                                           |
|                                           |
|                                           |
|                                           |
| Enter                                     |

Assumed you don't have a wrap plug attached: the system will send data to the serial port and wait for the incoming data ... but there is none coming in, because the wrap plug is missing.

A red error message in a window similar to that below shows up:

|               Error         Page   1 of 1 |
|                                           |
| 10:35:00    Slot = 0                      |
|    System Board Async Port 1              |
|   Error code = 00111200                   |
| A system-board async-port 1 error         |
| occured. Replace the system board         |
| (type #E1FE)                              |
|                                           |
| F3=Exit                                   |

"Okay - I can see that. Where can I get a Wrap Plug ?"

You can buy one from IBM. There is a 3-way wrap plug available, which has one 25-pin wrap plug for the parallel port, one 25-pin and one 9-pin for the serial port. It is IBM P/N 72X8546 and looks like that:

           ++                      ++
25-pin     ||                      ||      25-pin
Serial --- ||       72X8546        || --- Parallel
Plug       ||                      ||      Plug
           ++                      ++
            +-----------+     +----+
                        |     |
 9-pin                  ++---++
Serial Plug -----------  +---+

It is a nice little thing - with the possible disadvantage that it won't fit on every machine and sometimes i.e. video-, network- or keyboard-cables come in the way.

"Are there alternatives to this IBM part ?"

There are surely alternatives. I just wanted to be fair and mention that IBM part, because it is referenced in the handbooks very often.
If you are familiar with a soldering iron and can solder some wires to a plug without hurting and tattooing yourself ... consider to build your own set of wrap plugs.

The next section gives the details.

Wrap Plug Schematics

9-pin serial plug

You need one Sub-D female 9-pin plug and some short wires (minimum of 30AWG).

DCD 1 -----+-- 7 RTS
           +-- 8 CTS

RXD 2 -------- 3 TXD

DTR 4 -----+-- 9 RI
           +---6 DSR

    1 o o o o o 5

    6  o o o o  9

(View onto rear pins)

25-pin serial plug

You need one Sub-D female 25-pin plug, one 3.9 Kilo-Ohms resistor and some short wires (minimum of 30AWG).
This plug is also intended to test the IBM SDLC- and BSC-Adapters, which are not that common for the average PC-user but if you stumble over one you have at least a plug to test it too.

FGND 1 ----------------- 7  GND

TXD  2 ----------------- 3  RXD

RTS  4 --------------+-- 5  CTS
                     +-- 8  DCD

DSR  6 ---+------------- 20 DTR
          +--[R3K9]--+--*11 RES
                     +-- 22 RI

TCK 15*--------------+--*17 RCK
                     +--*23 CHCI

RES 18*-----------------*25 BUSY

*) used on synchronous ports
        (SDLC / BSC)

 1 o o o o o o o o o o o o o 13

 14 o o o o o o o o o o o o  25

     (View onto rear pins)

25-pin parallel plug

You need one Sub-D male 25-pin plug and some short wires (minimum of 30AWG).

STRB  1 ----------------- 13 SLCT

D0    2 ----------------- 15  ERR

ACK  10 ----------------- 16 INIT

BUSY 11 ----------------- 17 SLCT

PE   12 ----------------- 14 FDXT

 13 o o o o o o o o o o o o o  1

 25  o o o o o o o o o o o o  14

      (View onto rear pins)

That's all. For easier handling of the plugs you can mount the appropriate plastic housings. No need to fumble around with them and ruin the fingernails on the attempt to unplug them after testing.

These 3 plugs are enough to test either serial or parallel ports from IBM planars or IBM Adapter-cards with the IBM Advanced Diagnostics.

"What if I want to test i.e. Non-IBM serial cards ?"

Many card manufacturers have own testing software available on their option disks to test at least the internal card functions.
But that's not enough as we have learned.
The IBM Advanced Diags tests only IBM components which have appropriate Diags-files.

One simple way to test the external communication on a Non-IBM serial card is to plug the wrap plug to the card and run a good terminal program like Telix
(not Hyperterminal - I said "A good one" !).
This program must be configured to the corresponding port and be able to start a "Chat"-mode, with a split window. The one half shows the outgoing characters, the second half shows the incoming character.
If all character you type are echoed back (at least after pressing [Enter]) your card works fine.

"Can I use these wrap plugs with other testing software ?"

The serial plugs might work with other testing software as well - the parallel wrap plug most likely not.

The manufacturers of the software have own preferences which output-signals they use to test the input-lines. And these are in most cases others than IBM used for the Advanced Diagnostic. Consult your testing software documentation, READ.ME-files or manuals if there is a pin-out for the required wrap plugs.

Content created and/or collected by:
Louis F. Ohland, Peter H. Wendt, David L. Beem, William R. Walsh, Tatsuo Sunagawa, Tomáš Slavotínek, Jim Shorney, Tim N. Clarke, Kevin Bowling, and many others.

Ardent Tool of Capitalism is maintained by Tomáš Slavotínek.
Last update: 08 May 2024 - Changelog | About | Legal & Contact