Okay, so you had a 1336 Plus drive with Remote I/O (not DH+, even though the wiring is identical). That makes sense.
RIO was the principal remote I/O protocol for the PLC-5, though you can use ControlNet on the models equipped with that, or DeviceNet using a scanner card.
PLC-5 controllers of course don't run I/O over Ethernet or over their serial port.
You *could* send MSG instructions repeatedly to the drive over the PLC-5's Channel 0 serial port using DF1 messaging, and that would work with the 20-COMM-S.
But that's slow, and difficult, and not nearly as reliable as a true I/O network.
What sort of physical connection do you have set up, and what kind of logic and protocol settings have you tried ?