PLC are available in different price and performance. In each industry experts want to select the best kind of PLC and control system for best efficiency and price.For criteria that are important about PLC selecting can mention as following parameters:
Reliability :
The most important parameter is reliability. If PLC uses in industries such as oil&gas power or special industry that it is critical it is better to choose PLC with better reliability and PLC which has prove efficiency. in this case sometime PLC with redundancy use for better reliability . some kind of PLC can configure as redundancy .
Speed :
different industries need different speeds. This parameter depend on the speed of control loop and desire control loop speed.
I/O:
Number of I/O is important to choose . when engineer design plc and number of I/O it is clear that The plc must be selected that support this number of I/O.with desire speed In scan time .
Price: PLC are available in different price
Accessories: when engineer design a PLC and SCADA system the number of network and especial accessories design for network . There are different kind of network in PLC such as Etehrnet , Profibus , Modbus and… The PLC that has been selected must support this kind of network for designed SCADA system.Different network module for PLC are available , The p;c must be selected that support this network and module with desirable network speed.