PLC Selection Guidelines

First, click here to download and print the worksheet. PDF Document

This worksheet is intended to function as a checklist of things to consider when researching a PLC. It is broken down into the most important areas to consider when choosing a PLC. Following are guidelines for completing the checklist:

1) Determine whether your system is new or existing: Will your system be installed from scratch or are there existing products already installed that the rest of your system will need to be compatible with?

Why this is important: Certain PLC products will not be compatible with others.. Making sure your existing products are compatible with any PLC products you are researching will save you time and money.

2) Define any environmental issues that will effect your application: Are there specific environmental issues that will effect your application (temperature, dust, vibration, codes specific to your facility, etc.)?

Why this is important: Certain environments may effect the operation of a PLC. For example, typical PLCs have an operating temperature of 0-60 degrees Celsius. If your application will include any extreme environmental conditions, or you have specific codes at your facility that must be met, you will need to research products that meet those specifications, or design the installation to meet requirements.

3) Determine how many discrete and analog devices your system will have: How many discrete and analog devices will you have? Which types (AC, DC, etc.) are needed?

Why this is important: The number and type of devices your system will include is directly linked to the amount of I/O that will be necessary for your system. You will need to choose a PLC model that supports your I/O count requirements and has modules that support your signal types.

4) Determine whether your system will require any specialty features: Will your application require high-speed counting or positioning? What about a real-time clock or other specialty feature?

Why this is important: Specialty functions are not necessarily available using standard I/O modules. Planning ahead to determine whether or not your application will require any features such as these will help you determine whether or not you will need to purchase additional specialty modules for your system.

5) Determine the type of CPU you will need: How much memory will your system require? How many devices will your system have (determines data memory)? How large is your program, and what types of instructions will your program include (determines program memory)?

Why this is important: Data memory refers to the amount of memory needed for dynamic data manipulation and storage in the system. For example, counter and timer instructions typically use data memory to store setpoints, current values, and other internal flags. If the application requires historical data retention, such as measured device values over a long period of time, the size of the data tables required may determine the CPU model you choose. Program memory is the amount of memory needed to store the sequence of PLC program instructions that have been programmed to perform the application. Each type of instruction requires a specific amount of program memory, typically defined in a PLC's programming manual. Applications that are basically sequential in nature can rely on the I/O device rule of thumb to estimate program memory; complex applications will be more difficult to judge, but memory has become relatively inexpensive and fairly easy to upgrade if necessary.

6) Determine where your I/O will be located: Will your system require only local I/O, or both local and remote I/O locations?

Why this is important: If subsystems will be needed at long distances from the CPU, you will need a PLC model that supports remote I/O. You will also have to determine if the remote distances and speeds supported by the PLC will be adequate for your application.

7) Determine your communication requirements: Will your system be communicating to other networks or systems?

Why this is important: Communication ports are not necessarily included with a PLC. Knowing ahead of time whether or not your system will be communicating with other systems will help you choose a CPU that supports your communication requirements, or additional communication modules if necessary.

8) Determine your programming requirements: Does your application require only traditional programming instructions, or are special instructions necessary?

Why this is important: Certain PLCs may not support every type of instruction. You will need to choose a PLC that supports all instructions that you may need for a specific application. For example, built-in PID functions are much easier to use than writing your own code to perform closed-loop process control.

What now?

Once you have recorded the information on the worksheet and determined your requirements, use this sheet to find a PLC that meets your requirements. With your requirements outlined, it will be much simpler to find a product with the necessary number of I/O points, features, memory, etc. that your application requires.

For more information on PLCs in general on selecting an AutomationDirect PLC, see our PLC Selection Guide pdf (PDF document).