Hello Porky.
Like Mickey asked for in post #3, is the PLC5 a "classic" PLC5/10, PLC5/12, PLC5/15 or PLC5/25 ?
If this is the case, then there is no serial port to connect to. What looks like a serial 9-pin SUB_D port is actually a DH+ port.
If the PLC5 is an "enhanced" PLC5/11, PLC5/20 ... then there is a 25-pin SUB-D port to connect to.
You need to check that the dip-switches on the CPU are set for RS232 and not for RS422 or RS485.
The serial online cable needs the following pins connected:
9-pin PC --- 25-pin PLC5
RXD 2 --- 2 TXD
TXD 3 --- 3 RXD
GND 5 --- 7 GND
And then in RSLinx you have to carefully set the same serial parameters as set in the PLC5 program for CH0.
With a USB/serial adapter you cannot count on that the autoconfigure function works. Sometimes it does, sometimes not.
I think that you can actually select just any USB/serial adapter as the DF1 protocol is quite forgiving. I have a cheap model that works well.
Try to search for "USB serial" as there was a quite comprehensive thread some time ago.