Reverse Engineering / Unknown RS-232 pinouts

I have a device with an RS-232 interface. The socket on the device itself is a PS/2 style one with the only connections being to pins 1, 2, 3 and 4. Pins 5 and 6 are not connected.

It is not a PS/2 device even though the socket is a PS/2 type.

I need to work out how to hook this up to a normal PC serial port (DB9 connection). Trouble is I do not know the pinouts.

I've measured the voltage between various pins to see if that would point me in the correct direction. The only test gear I have at hand is a digital multimeter. I do not know if it is enough to determine what is what but the results are:

Pin 2 is at about -5V in respect to pin 3.
Pin 4 is at about -5V in respect to pin 3.
Pin 1 is at about -8V in respect to pin 3.
There is 0V between pins 2 and 4.
Pin 1 is at about 1.4V in respect to pin 2.
Pin 1 is ar about 1.4V in respect to pin 1.

I imagine that there will need to be Tx, Rx and signal ground. Hell, I don't even know if this device uses an Rx as it is possible that it just sits there spewing out data and doesn't have the need to take commands from the PC.

So, I guess all we can say for sure is that it will have a Tx pin and a signal ground. What the other two connections are used for I could not say.

But I'm guessing something like Rx and CTS or RTS.

My question is this.

Can you help me with step by step instructions on how I would go about working out the pinouts given the above information and / or other measurements that you specify? Ultimately I need to solder up a cable with a PS/2 style plug on one and and a DB9 serial connection on the other.

I do not know the baud rate, number of bits, parity etc. Hints in working this out (other than just try cominations randomly) will be appreaciated as well

Solution: Reverse Engineering / Unknown RS-232 pinouts

Just guessing at this mind you but I've seen a similar serial port on an old 8-bit computer.

Pin 1 = CD (carrier detect)
Pin 2 = RD (receive data)
Pin 3 = Gnd
Pin 4 = TD (transmit data)

I picked pin 1 as CD because its the only one with that voltage.
I picked pin 2 as RD and pin 4 as TD because they are the same voltages. Also I picked their properties based on the original serial port I referenced earlier - they may be reversed.

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