Serial communication between a PLC and a microcontroller

I'm performing a project in which I'm using a PLC from "altus" the brazilian Co., an Exter T100 touch panel from "Beijer" and an Atmel microcontroller.

As I could not send the instructions directly from the touch panel to the microcontroller I'm using the PLC as an intermediatte stage between them. Then I'm going to send the data and the instructions sent from the touch panel to the PLC to the microcontroller via serial port.

The problem is that these data is concerned with different process parts. How could I make the microcontroller know that the received data is for a specific part of the process?

Typically, the HMI would have memory locations mapped to specific registers or memory locations in the PLC. Since you don't have that structure, you would need to create it. I am not familiar with the capabilities of that PLC, but I would assume that you have some ascii in or out capabilities. You will then have to create your own protocol. When data changes in the PLC, the PLC will have to send a series of words, containing the data location and then the data. It is going to be complicated and slow.


The ideal situation is to write a driver for the serial protocol supported by the HMI in your micro-controller. I am assuming that the HMI already supports your PLC protocol. PLC manufacturers usually publish this. If you are lucky, the HMI may support modbus and you may find examples in "C" for this on the Net. You will find this method much easier and the response quicker, because the HMI will only communicate when data changes.