Simple PC - PLC Communications Part I

A simple PC application structure that communicates to PLC

In most cases PLC is enough to solve automation tasks with its programming interface and special designed architecture.

However, it is not always possible to solve all problems such as database support. Some PLC brands have command sets supporting table search but it is not intended for complex operations.

In this post, we will focus on a PC application that supports PLC by exchanging data.

Most common PC – PLC communication methods are RS 232 and Ethernet, they do not differ in
basic software structure, so you can apply this structure for both RS232 and Ethernet communications. Also it is possible to use parallel hardware I/O cards on PC for communication, the same structure also fits for I/O cards.

The most convenient way for communicating is continuously exchanging snapshots of memory areas between PC and PLC.

Actually, all PLC systems do the same thing at the background:

Reading inputs, evaluating them to calculate necessary outputs and writing outputs continuously in a loop.

If we adopt this to a PC application, same algorithm could be as follows:

This structure simplifies PC evaluation algorithm.

Next post is going to be a simple case study, we will design a station that gives feedback to PC for
recording a products information to maintain traceability.