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Understanding Software PLC Systems

by Matt Edwards, Director of Professional Services

The software PLC (soft PLC) was introduced with the aim of enhancing interoperability and compatibility among various industrial systems and products. It is a digital computer that facilitates electromechanical process automation, and a control mechanism that is used to enhance production lines and machine functions and processes. The soft PLC monitors the state of input devices continuously and makes appropriate decisions based on a custom program that controls the state of output devices. PLCs are required to operate flawlessly, especially in industries that are hazardous to the electric components used to make modern PLCs.

Since PLCs are hard real-time systems, output ought to be produced as a response to input commands in the least time possible. Otherwise, undesired operations may result and cause damage.

Let’s review the basics of soft PLC systems.


The PLC was invented as a response to the American automotive manufacturing industry requirements. Before its invention, the safety interlock, control and sequencing logic among manufacturing lines depended on hundreds or thousands of relays, drum sequencers, cam timers and dedicated closed-loop controllers. The relay systems were unreliable because they often failed and caused delays. In such scenarios, engineers were forced to troubleshoot the entire relay’s wall in a bid to find and fix problems. Additionally, system updates were tedious because technicians had to separate and rewire each relay manually. The PLC was intended to replace electromechanical parts and substitute them with digital computers containing a saved program that imitates the relays’ interconnections to perform different logical tasks.


A basic PLC must be flexible and configurable to meet different application needs. All soft PLCs have similar components that work together to process information into the PLC from the source plant as instructed and send the output back to the source plant. These components are input and output modules, co-processor modules, peripheral devices, power supply, and the central processing unit.

Scan Cycle

A PLC executes an initialization step when in run mode. If no problems are detected, the PLC executes the scan cycle sequence repeatedly. The scan cycle takes a few milliseconds and consists of four steps:

1. Input scan: the status of every input module is copied to a memory area known as the input image table. This action avoids scenarios where input changes from the beginning of a program to the end.
2. Program scan: any data in the input image memory section is applied within the user program. The user program is then performed and the output image memory section is updated.
3. Output scan: data from the output image section is transferred to all system output modules.
4. Housekeeping: this includes system checks, timer values, and current internal counter updates.

These fundamentals hopefully provide a better understanding of soft PLC systems.

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