Several basic factors need to be considered:
Necessary input/output capabilities
Required I/O type
Required memory size
The speed and power required for CPU and instruction set
Manufacturer’s support and backup
- Quantity of inputs/outputs:
It is very important that you know the exact number of inputs and outputs to be used in the process in order to achieve optimal and economical utilization. The Programmable Logic Controller 59 system must have sufficient terminal points to connect all signals and control lines of the process. - Input/output type:
When choosing a PLC, we must consider what type of I/O is needed, such as digital inputs like sensors and buttons, or analog inputs like pressure and temperature. Just as we consider outputs, digital outputs include relays, contactors, lights, and so on. Or you may need an analog output, such as a drive and control valve. - Memory and programming requirements:
The memory size is usually related to the number of I/O points required by the system. Another factor that affects the required amount of memory is the control program to be installed.
The size of the program is also related to the number of I/O points, as it must include instructions read from or written to each point.
- Compact or modular PLC:
PLCs can be chosen between modular and compact models. Modular PLCs allow us to design PLCs for our purposes and have power and controllers in different enclosures, while compact PLCs are those that have power and controllers in the same enclosure. - Instruction set/CPU:
All PLCs handle logic control, sorting, etc., but differ in data processing, special functions, and communication. In these fields, larger programmable controllers often have more powerful instructions than smaller controllers. - PLC scanning time:
The PLC scanning time is the time required for a complete PLC cycle to be scanned by the PLC. It may include reading PLC input status, clearing PLC memory, executing PLC programs, and updating outputs. Sometimes it is necessary to scan PLCs with shorter scanning times, but the cost is higher.
