Technical articles on Electronics and Electrical Engineering topics focused on Basics of EE, Electronic components, power systems and equipment used for generation, transmission, operation, and controls of electricity.
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 time (ET) registers, both of which are typically integer number values. Many other PLC instructions are designed to receive and manipulate non-Boolean values such … Read more
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 “off-delay” timer instructions have single inputs triggering the timed function. An “on-delay” timer activates an output only when the input has been active for a … Read more
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 1 (“false” to “true”). Counter instructions come in three basic types: up counters, down counters, and up/down counters. Both “up” and “down” counter instructions have … Read more
The most elementary objects in Ladder Diagram programming are contacts and coils, intended to mimic the contacts and coils of electromechanical relays. Contacts and coils are discrete programming elements, dealing with Boolean (1 and 0; on and off; true and false) variable states. Each contact in a Ladder Diagram PLC program represents the reading of … Read more
In the United States, 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 logical relationships between inputs and outputs as though they were contacts and coils in a hard-wired electromechanical relay circuit. This language was invented for the … Read more
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 PLC is organized. This is sage advice for any programmer, especially on systems where memory is limited, and/or where I/O has a fixed association with … Read more
Perhaps the most important yet elusive concept to grasp when learning to program PLCs is the relationship between the electrical status of the PLC’s I/O points and the status of variables and other “elements” in its programming. This is especially true for Ladder Diagram (LD) programming, where the program itself resembles an electrical diagram. Making … Read more
Every programmable logic controller must have some means of receiving and interpreting signals from real-world sensors such as switches, and encoders, and also be able to effect control over real-world control elements such as solenoids, valves, and motors. This is generally known as input/output, or I/O, capability. Monolithic (“brick”) PLCs have a fixed amount of … Read more
Many different digital network standards exist for PLCs to communicate with, from PLC to PLC and between PLCs and field devices. One of the earliest digital protocols developed for PLC communication was Modbus, originally for the Modicon brand of PLC. Modbus was adopted by other PLC and industrial device manufacturers as a de facto standard5 , … Read more
Input/output capability for programmable logic controllers comes in three basic varieties: discrete, analog, and network. In the early days of programmable logic controllers, processor speed and memory were too limited to support anything but discrete (on/off ) control functions. Consequently, the only I/O capability found on early PLCs were discrete in nature. Modern PLC technology, … Read more