May 1, 2018

How to Choose your PLC or PLC Choosing Criteria

Choosing a PLC or Controller for your Process is very important as it helps in cost optimization. If you choose a wrong PLC it may happen that you are out of I/O's while programming or maybe you have the excess of I/O's left which are of no use. So it becomes very important that you choose a right PLC for your Automation Process.There are several criteria that help in choosing a PLC. Before knowing the Criteria of choosing a PLC you should know basics of PLC. Subscribe to Automation-Talk by Email.

Below are some points which will help in choosing a PLC but keep in mind that you should know exactly the need of the Customers and should know how many Input and output he wants for present and for future.

PLC are chosen generally on the basis of some requirement of the company or individual person. We should keep in mind the following below point while choosing a PLC.

How to Choose your PLC or PLC Choosing Criteria

Some of the PLC choosing criteria are

1. Number of I/O's:

It is very important that you know the exact number of input and output that are going to be used in the process for best and economical use.

2. Type of I/O's:

The Automation Engineer should also know the type of I/O's whether he needs a digital input like Sensor, Push Buttons etc. or an analog input like RTD, thermocouple etc. The requirement for Output should be also known as what type of Output is required whether you required a digital Output like Relay, Contactor, Lamp etc. or you need an Analog output like Drive and Control Valves.

3. Memory size of PLC:

Choosing PLC memory size is also important as you may be out of space in PLC while writing a PLC program if you choose less memory size PLC.

4. Compact or Modular PLC:

Compact Type PLC are those which has Power supply and controller in the same housing while Modular type PLC has power supply and controller in a different housing. So it is good to choose compact or Modular type PLC according to your requirement.

5. AC or DC PLC:

AC PLC's are those which are operated on 220 V i.e. they require 220 V for running. Whereas DC PLC requires only 24V for Running. So you should know which type of PLC will be good for you according to the Power Supply available in your Control Panel.

6. Sinking and Sourcing PLC:

Making a PLC source at Input or making a PLC sink at input depends upon the requirement of the user so PLC should be chosen in that way. Some PLC may support both Sink and Source at Input and Output Both.

Recommended Article: Understanding Difference Between Source & Sink in PLC's

7. PLC Scan Time:

PLC scan time is a time needed for PLC to completely Scan one cycle of PLC. It may consist of reading PLC input status, clearing PLC memory, Executing the PLC program and updating the Output. The PLC with less Scan time are found to be more Costly.

8. Number and type of Communication ports in PLC:

The number of communication Port or type of communication port is also very important in choosing a PLC. Suppose you want to communicate your PLC in MODBUS but you don't have a MODBUS port so your PLC is of no use. Also, take an example where you want to communicate an HMI with RS232 port but you don't have one. So communication Port should also be in mind while choosing a PLC.

So these were some of the Criteria for Choosing a PLC which should be kept in mind while you buy a PLC. PLC choosing criteria of Allen Bradley, ABB, Siemens, Omron, Mitsubishi, GE Fanuc, Delta, Fuji, GE or any other PLC are same in all cases. Subscribe to Automation-Talk by Email for more Tips and Tutorials.

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  1. Is #3 MEMORY SIZE actually an issue anymore? even entery-level PLCs like the AB Micrologix, OMRON CP1, and CLICK PLCs all start at 2,000 steps and go to 10,000 programming-steps of logic. These boxes only have 20-60 physical points of I/O, the only way you could possibly use up 2,000 program steps for 20 points of I/O is you code entirely in contacts and coils, Instruction-sets exist too.
    #6 Syncing/Sourcing PLC. Does it give anyone new information to say "___ is dependent on user requirement"? Pretty much everything from AB, OMRON, Seimens, etc is Sync or Source depending if connect your 0VDC to COM IN & +24VDC to COM OUT, or vice-versa. Sync of Source comes into place for transistor INPUTS and OUTPUTS, not RELAY OUTPUTS.
    You also may need AC OUTPUTS & ISOLATED OUTPUT COMMONS if you're outputting to a 120 VAC contactor-coil for example.

  2. #5 AC or DC PLC: AC has a built-in DC-Power supply, while DC doesn't so you need to provide it. A seperate DC Power supply for INPUTS & OUTPUTS is useful to isolate the CPU from going down with a shorted output


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