PLC families

Although there are several families of Modicon controllers, we can segregate them into two groups.

The first group of controllers is the one managed with Concept IDE. Those include Momentum family (ex. 171 CCC 960 30), Quantum family (ex. 140 CPU 534 14), Compact and Atrium families. Those are old and mainly obsolete.

The second groups of controllers is the one managed with Unity Pro IDE or, at the time of writing this article, Control Expert. Those include Momentum family (ex. 171 CBU 98090), Quantum family (ex. 140 CPU 652 60), M340 family (ex. BMX P34 2020), MC80 and Premium families and the most recent M580 family (ex. BME P58 5040).

Located memory details

Located memory in Modicon PLCs is basically the memory type that can be access with Modbus protocol.

The memory is not share between different function codes which means reference to ex. 3088 Modbus address with Function Code 3 and Function Code 4 will result in getting two different values.

Here, I would like to mention that there are devices with shared memory, for example, SEPAM Protection Relay. In SEPAM, for some memory parts, it is possible to read specific register (lets stick to 3088) with both Function Code 3 and 4 and You are going the get the same value. What is even more interesting, You can refer to the bit of the register with Function Code 1 and 2 by indicating the address multiplied by 16 plus bit offset depending which bit to read. In our example, referring to 49408 would be register 3088 bit 0. Please check the documentation of the device You are planning to use for details.