PLC Selection Criteria

PLC Selection Criteria

PLC selection criteria consists of:

  • System (task) requirements.
  • Application requirements.
  • What input/output capacity is required?
  • What type of inputs/outputs are required?
  • What size of memory is required?
  • What speed is required of the CPU?
  • Electrical requirements.
  • Speed of operation.
  • Communication requirements.
  • Software.
  • Operator interface.
  • Physical environments.

System requirements

  • The starting point in determining any solution must be to understand what is to be achieved.
  • The program design starts with breaking down the task into a number of simple understandable elements, each of which can be easily

Application requirements

  • Input and output device requirements. After determining the operation of the system, the next step is to determine what input and
    output devices the system requires.
  • List the function required and identify a specific type of device.
  • The need for special operations in addition to discrete (On/Off) logic.
  • List the advanced functions required beside simple discrete logic.

Electrical Requirements

The electrical requirements for inputs, outputs, and system power; When determining the electrical requirements of a system, consider three items:

– Incoming power (power for the control system);

– Input device voltage; and

– Output voltage and current.

Speed of Operation

How fast the control system must operate (speed of operation).

When determining speed of operation, consider these points:

– How fast does the process occur or machine operate?
– Are there “time critical” operations or events that must be detected?
– In what time frame must the fastest action occur (input device detection to output device activation)?
– Does the control system need to count pulses from an encoder or flow-meter and respond quickly?


If the application requires sharing data outside the process, i.e. communication. Communication involves sharing application data or status with another electronic device, such as a computer or a monitor in an operator’s station.

Communication can take place locally through a twisted-pair wire, or remotely via telephone or radio modem.

Operator Interface

If the system needs operator control or interaction. In order to convey information about machine or process status, or to allow an operator to input data, many applications require operator interfaces.

Traditional operator interfaces include pushbuttons, pilot lights and LED numeric display. Electronic operator interface devices display messages about machine status in descriptive text, display part count and track alarms. Also, they can be used for data input.

Physical Environment

The physical environment in which the control system will be located. Consider the environment where the control system will be located. In harsh environments, house the control system in an appropriate IP-rated enclosure.

Remember to consider accessibility for maintenance, troubleshooting or reprogramming.

Vendor selection

The range of PLC suppliers is vast and many offer a number of alternative product ranges with any number of modules, boasting special features.

Our choice must meet the application requirements, provide extra capacity for future development and provide a cost effective solution.

Price is the most commonly stated reason for making a choice, but the true price of a PLC to meet the requirements of a particular application is often much the same over a wide range of supplier equipment.

The final choice of supplier for our PLC will depend upon functionality, support available, customer preferences, user knowledge and price.

These are the issues that must be addressed:


We have to match the application requirements with the features of each of the contending suppliers’ equipment to identify the one that best meets our requirements.


Before any purchase is made the following points should be confirmed with any manufacturer:


  • Technical support (on site and over the phone);
  • Application support to configure and design a system;
  • Rapid exchange/repair of failed equipment;
  • Guaranteed support for any products for at least 10 years from purchase.

Allen Bradley PLC Vs Siemens PLC – Which is better for industrial automation?

Right from the time when Dick Morley invented the first Programmable Logic Controller (PLC) in 1969 to till date, PLCs have been of great help to the industrial segment. A PLC which is also known as the industrial computer control system greatly enhances an industrial process while allowing you to easily replicate or change the process. Secondly, it allows you to mix input and output systems for a modular environment.

Rockwell and Siemens are two of the biggest companies that are dominating the PLC market. Here is a comparison of these PLC heavyweights.

Siemens PLC Vs Allen Bradley PLC


When it comes to hardware, both these automation tools work the same way and are reliable. They only differ in fewer areas. For instance, when you install the Allen Bradley PLC, you need to connect the Allen Bradley power supply and the Allen Bradley rack as well. Moreover, you need to install additional cards for safety communication ports.

Siemens works with any 24V DC power supply. They come with built-in safety communication ports. AB uses American native protocols such as EthernetIP, DeviceNet and ControlNet. Siemens uses European protocols such as ASI and Profibus.

Automation procedure management

Rockwell PLC is easy to configure and manage. The intuitive interface allows users to easily monitor and manage the system. Features such as communication with 3rd party hardware, mass production of code and export/import tags from Excel to scada databases make it popular among endusers.

On the other hand, Siemens seems to be complex for regular maintenance staff and require more computer programming background. However, it offers more options to program and customize the process.

Siemens PLC offers two safety runtime groups. It means you can divide the infrastructure into two groups and set controller parameters with different priorities and time cycles. This ability allows you to program blocks and download them to the PLC without stopping any process.

With Rockwell PLC, this is not possible as it supports only one safety group. Siemens also offers more programming techniques such as FBD, SCL, STL, LAD etc. However, high-end AB systems support function blocks as well as multiple programming techniques.

Support & Pricing

While pricing is not the first aspect to compare, it has significant important as well. The AB Rockwell automation system is expensive when compared with that of Siemens PLC solutions For instance, the Compact GuardLogix which is the least expensive product from AB is still priced at a higher level.

Siemens offers standard technical support for free. When it comes to Rockwell, technical support charges are based on the amount of hardware installed.


While both these automation systems are similar in most areas, the difference lies in the software segment. AB is easy to use and manage but offers limited customization options.

Siemens is comparatively inexpensive, allows you to program the software to customize different program blocks but requires more computer programming knowledge. Rockwell is more popular in the US while Siemens enjoys an international market.

Choosing the correct processor

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.


After the planning phase of the design, the equipment can be ordered. This decision
is usually based upon the required inputs, outputs and functions of the controller. The
first decision is the type of controller; rack, mini, micro, or software based.

This decision will depend upon the basic criteria listed below.

• Number of logical inputs and outputs.

• Memory - Often 1K and up. Need is dictated by size of ladder logic program. A ladder element will take only a few bytes, and will be specified in manufacturers documentation.

• Number of special I/O modules - When doing some exotic applications, a large number of special add-on cards may be required.

• Scan Time - Big programs or faster processes will require shorter scan times. And, the shorter the scan time, the higher the cost. Typical values for this are 1 microsecond per simple ladder instruction

• Communications - Serial and networked connections allow the PLC to be programmed and talk to other PLCs. The needs are determined by the application.

• Software - Availability of programming software and other tools determines the programming and debugging ease.

The process of selecting a PLC can be broken into the steps listed below.

  1. Understand the process to be controlled

• List the number and types of inputs and outputs.
• Determine how the process is to be controlled.
• Determine special needs such as distance between parts of the process.

  1. If not already specified, a single vendor should be selected. Factors that might be considered are, (Note: Vendor research may be needed here.)

• Manuals and documentation
• Support while developing programs
• The range of products available
• Support while troubleshooting
• Shipping times for emergency replacements
• Training
• The track record for the company
• Business practices (billing, upgrades/obsolete products, etc.)

  1. Plan the ladder logic for the controls. (Note: Use the standard design sheets.)

Free PLC Courses