I think the best question for automation engineer is how choose the best module & PLC & Interments for any system ?

well answers i will add you further

PLC scan time is more important as selecting according to Operation of Process.

Additional Spare I/Os availability in PLC when  choosing for  future use if Company want to extend the process.

User friendly Control panel with HMI for easy understanding everyone in the field.

Communication Module for easy compatible with other control system already use in Plant.

PLC With PC,WIFI and HMI Support is most and best for Automation systems..................

processing speed , no of inputs  & outputs , way of communication , software used , upgrading and updatin availability , cables used in communication , price

the criteria for choosing a PLC can be based on certain key points such as  the number of i/o , processor speed and availability of modules to add.

the choice of the PLCmust be well studied especially the question of the compatibility of other systems to be installed as the control panel or control systems.

Best means ,which fits perfectly on your need of control task .

and there are some basic things to find a competable plc for your system .

1-you should know the exect number of your I/O s

2-the type of sensors ,you are goig use (and also you should know that your plc librrary have the    

   blocks to accumodate that sensors or input devices and check that ,can your plc accomudate that

    kind of interfacing modules)

3-the comunnication needs ( scada ,dcs or stand alone system OR it will be master or slave of any 

   other system )

4-competability with the3rd party components (if using)

5-A matching power supply (there was no need to add this(it should be understood and common) but once i got faild in interview because i never mantioned that .

Thank for your answer , also some parameter must take i n consideration in this field :1- I f the system required to connect for SCADA system the PLC must have some specification as communication support master / slave , modbus.2- If operator need to change parameter or make any process must have HMI screen on the panel connected to PLC .

Some parameters are taken into account:

1) the choosen component must support all the functions to be executed.

2) reliability

3) upgradable features

4) efficiency required

5) cost and budget

6) availability and maintenance figures