PLC Pump Permissive Interlocks
The following PLC program was written to control the operation of a large electric motor-driven pump. A variety of “permissive” inputs protect the pump from damage under abnormal conditions, and…
What is Ladder Diagram Programming ?
Ladder Diagram (LD) Programming The most common language used to program PLCs is Ladder Diagram (LD), also known as Relay Ladder Logic (RLL). This is a graphical language showing the…
Basics of Fail-safe Circuits
Logic circuits, whether comprised of electromechanical relays or solid-state gates, can be built in many different ways to perform the same functions. There is usually no one "correct" way to…
History of Programmable Logic Controllers (PLC)
Every control system may be divided into three general sections: input devices (sensors), controllers, and output devices (actuators). The input devices sense what is happening in the process, the controller…
PLC Electric Motor Interlocks
Programmable Logic Controller Questions and Answers The following PLC program was written to control the operation of a large electric motor-driven pump. A variety of “permissive” inputs protect the pump…
PLC Data Comparison Instructions
As we have seen with counter and timers, some PLC instructions generate digital values other than simple Boolean (on/off) signals. Counters have current value (CV) registers and timers have elapsed…
PLC Math instructions
The IEC 61131-3 standard specifies several dedicated ladder instructions for performing arithmetic calculations. Some of them are shown here: PLC Math instructions As with the data comparison instructions, each of…
PLC Timer Instructions
A timer is a PLC instruction measuring the amount of time elapsed following an event. Timer instructions come in two basic types: on-delay timers and off-delay timers. Both “on-delay” and…
PLC Counter Instructions
A counter is a PLC instruction that either increments (counts up) or decrements (counts down) an integer number value when prompted by the transition of a bit from 0 to…
PLC Memory Mapping and I/O addressing
A wise PLC programmer once told me that the first thing any aspiring programmer should learn about the PLC they intend to program is how the digital memory of that…
