For Selecting Modular Processors the following Criteria examined include:
I/O points (local I/O points and expandable points).
Each PLC processor will only be capable of working with a limited number of each type of I/O modules.
Memory size (for data storage or program storage) and Performance (scan time depends on the processor).
The size of program is dependent upon the complexity of the control problem and the skill and style of the programmer.
The required operating speed for all the I/O must be determined, with a PLC selected to match. This requires the estimation of the program size and the proportion of slow instructions. The scan speed is normally expressed in terms of
ms/K for a stated mix of simple and complex instructions. A PLC with an appropriate memory capacity and speed can be selected.
For any particular application it is essential to ensure that the
PLC selected can handle the required operations.
When a communications facility is required we need to determine whether the built-in port is adequate for the application, or whether a separate module will be required.