PLC, Modbus, engines and sensors...

As Collin has said, use the serial port to communicate with the MODBUS device. You'll probably need an adapter, though, as the usual implementation is RS-485, not the RS-232 port provided on a PC. As you'll discover when you Google around, MODBUS uses some weird data types, especially bit data, which are mapped to specific memory addresses in the RTU. The RTU/PLC processor reads and writes to these memory addresses to obtain configuration settings, receive commands, and record data. Your application should be able to write to these memory bits and to select specified slices of data from the RTU memory.

Depending on the device, it might be able to generate alarms, in which case you'll want to monitor the port closely. I'd stick that function in a separate thread that can interrupt the rest of the application when an alarm comes in. Otherwise, in the RTU slave mode, the device will simply execute its PLC program until you poll it.

This is something I spent a lot of time looking into a few months back, as my company has 57 sewer lift stations which are controlled by Allen Bradley PLCs using a MODBUS protocol for control. My goal is to one day link them via radio, and write a SCADA system to monitor and control them from the office. I'll look forward to the article you'll surely be writing for us when it's done, and plan to learn from it how this is done properly.:-)