I believe that a PLC5/25 is a "classical".
That means it does not have a serial port.
It does have a DH+ port.
You can connect to it via a KF2 box, but such a thing is expensive and wont bring the full throughput of DH+.
Alternatives can be:
A PKTX card in a PC for direct access over DH+ (cheaper and faster than KF2).
Exchange the PLC5/25 for an "enhanced" CPU with a serial port.
By the way. I am writing this in an airport lounge on the way to Minsk.
Funny isn't it ?