I got my hands on an IBM PS/1 computer and it provides a 25-pin serial port connector. Whith the help of an adapter I can get a 9-pin serial port on COM1 and transfer data that way. I would like to know wether the PS/1 implements a secondary COM port on the 25-pin connector or if it is left unconnected.

  • 2
    The RS232 connector standard allows both DB9 and DB25 connectors. I would suspect you just have a single port.
    – Chenmunka
    Commented Jun 28, 2016 at 8:09
  • Could you tell as the exact model? Serial ports seem to differ from PS/1 model to PS/1 model, there are even PS/1 models with no serial ports (if the information I googled is correct). That said, I've never seen any computer actually using the secondary COM port on an 25-pin connector, so it's very unlikely.
    – dirkt
    Commented Jun 28, 2016 at 9:08

3 Answers 3


According to the IBM PS/1 Technical Reference Manual, the pin assignment of the serial port is:

Technical Reference for PS1 Computer PN 57F1970, p. 9-22

So to answer your question: no, there is neither a second port nor a secondary channel for the first port. All the secondary lines are not connected.


DB25 in its very early stage provided for a secondary port on the same connector (using pins 12,13,14,16,19).

Since then, those secondary and other additional signals became obsolete and even DB25 ports only use up to 9 signals - same as on in DB9 connector.

It is possible that your motherboard provides another port and you would need to provide a cable for it, or you can install a PCI or ISA card with additional ports.

The easiest way to verify would be to check if a system sees any port other than COM0 (or ttyS0), if it does - short pins 14 with 16 and see if you get echo of signals sent to that port.

OR - check in the manual for your exact motherboard - it will give you definite answer.

Original RS232 pinout

  • The pins that are labeled "secondary" in your image can be used for a second COM port. The question was if these pins are actually used on a PS/1. And while I, too, have never seen this done, I really doubt the statement "all DB25 serial ports only provide a single port" is true, because somebody must have used the assignments for the secondary COM port (or they wouldn't be there).
    – dirkt
    Commented Jun 28, 2016 at 11:50
  • @dirkt I don't think the term "secondary" here means another port, it might simply mean - secondary pin for the same signal. Rationale - only 5 signals (TX,RX,RTS,CTS,DCD) have their corresponding secondary pins. If thore were actually meant for another port then the second port would be somewhat impaired with less signals available (but still enough to function as a simple RS232). Commented Jun 28, 2016 at 12:00
  • 7
    Nope, it really means secondary channel, not "another pin for the same signal".
    – dirkt
    Commented Jun 28, 2016 at 12:03
  • You'd need a way to both identify and access that secondary port in software, and then (as it suggests here) see if you can get an echo from it. That is, the box needs to have these wires actually hooked up such that the OS is able to identify and provide access to some abstract notion of a "comms" port that simple serial software could send and receive data on.
    – user12
    Commented Jun 28, 2016 at 16:35
  • I don't think IBM ever used a DB25 for a double serial port, but there were as sufficient variety of I/O cards produced that I wouldn't be surprised if, in the days before the standardized DB9 serial-port pinout, someone made a dual serial card which used a DB25 that could communicate with one device using a commonplace DB25 cable, or with two devices using a "hydra-headed" cable.
    – supercat
    Commented Jul 1, 2016 at 22:43

Two-in-one serial ports on DB25 have been implemented in some unix workstations, but the use of a DB25 connector on 1980s PC style computers for single serial ports was perfectly common and is not an indicator for such a port.

As the above post of the full pinouts shows, the original RS232 standard allows for a lot of optional signals that were not implemented in PC hardware, so the 9-pin connector slowly became the preferred implementation. Serial modems often had the 25 pin connector and came with a 9-to-25 pin cable.

Finding an undocumented (but present from a software/OS view) secondary serial port on some connector on any computer system would be most easily done by sending a constant stream of data out that port (serial ports have no idea whether anything is connected to them!) and checking pins with an oscilloscope. Once the output signal is found, the input pin could be found by connecting that output signal to any pin that is NOT ground, NOT a part of another port, and NOT showing a voltage reading - and checking whether an echo can be established. Using a safety resistor of a few 100 ohms could be prudent here.

Also, the number of implemented serial ports could be estimated by looking for the common line driver/receiver ICs (75189, 75188, 75232, MC1488, MC1489 etc.) and calculating how many signal lines these make possible.

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