Allscott linked to the "classic" Modbus module that Prosoft makes. They work and work well.
I'd recommend using the newer (well, 5 years old) platform, the "Multi-Vendor Interface" from Prosoft.
The
MVI71-MCM is Prosoft's current technology module to allow a PLC-5 controller to communicate over serial Modbus.
I think the MVI-series are also a little less expensive than the older 3100 series modules.
I like the MVI series because the same internal x386 system-on-chip platform and firmware are used in several form factors (SLC, PLC, FLEX, Compact, ControlLogix). That makes it easier for me to program and troubleshoot, and easier for Prosoft to build and support.
If you have a network connection (ControlNet or EtherNet/IP or DH+) there are more options from Prosoft, HMS Fieldbus, Real-Time Automation and Datalink. But the in-rack Prosoft card is the most straightforward way to add Modbus communications to a PLC-5 controller.