What is the difference between a PLC and an RTU?" PLC means Programmable Logic Controller. RTU can represent Remote Telemetry Unit or Remote Terminal Unit (like Tetragenics offers). These two kinds of RTUs are compared against PLCs across the industry.

But, knowing what PLC and RTU stand for is not enough. You need to know what each provides. In this article I discuss the differences between a PLC and the two types of RTUs, starting with the least sophisticated and working towards the most sophisticated device.

Remote Telemetry Units

Remote Telemetry Units are usually nothing more than a multiplexed addressable I/O device with communications. They have input and output points, and they are connected to a more intelligent controller. The controller is responsible for the control algorithm. This kind of RTU has very little computing power and is specified for use in installations like water and wastewater automation.

The Remote Telemetry Unit is strictly a slave device. It is not programmable and cannot be used as a stand-alone controller, but it is usually addressable. You can use it to relay status and values both from the remote site to a controller and from the controller down. But it cannot communicate with other devices at or below its own level.

Programmable Logic Controller (PLC)

PLCs use what the industry calls Ladder Logic. [Ladder Logic is a representation of relay logic and consists of two vertical lines with contact symbols along the rungs in between (hence, the ladder look).]

The first PLCs were just a software representation of relay logic. In other words, they were designed to duplicate the functionality of a rack of interconnected relays. In the last few years higher end models have been supplemented with analog inputs and outputs. A wide range of pricing is available from low end PLCs at $150, to high end models that sell for around $100,000. The low end PLCs are not even addressable (they cannot be used as a slave to another device or as a component in a control system).

PLCs scan their I/O by electrically reading each I/O point. This is done quickly, but in a system with lots of I/O points it can take some time to completely scan all the points.

PLCs can be used as stand-alone devices but they are difficult to configure. You must use ladder logic to program them. Normally, they are not usable as the master controller in a control system. If your system contains lots of I/O that must be monitored or controlled, PLCs are usually not the best choice. They are not appropriate for use as protocol converters or for controlling other intelligent electronics devices (IEDs).

Intro2U - Programmable logic control or PLC is the most commonly used industrial automation technique in the world. It is universally applied for factory automation, process control and manufacturing systems. Programmable logic control originated from the creation of computerized versions of relay control systems used to control manufacturing and chemical process systems. The programming is done using a special technique called ladder logic, which allows sequences of logical actions to be set up, inter-linked and timed. A standard task in logic control is batch control and sequencing in a process system.
PLC Introduction

A PLC or Programmable Logic Controller is a user friendly, microprocessor specialized computer that carries out control functions of many types and levels of complexity. Its purpose is to monitor crucial process parameters and adjust process operations accordingly. It can be programmed, controlled and operated by a person unskilled in operating computers. Essentially, a PLC's operator draws the lines and devices of ladder diagrams with a keyboard onto a display screen. The resulting drawing is converted into computer machine language and run as a user program.

PLC will operate any system that has output devices that go on and off (Discrete, or Digital, outputs). It can also operate any system with variable (analog) outputs. The Programmable Logic Control can be operated on the input side by ON/OFF devices or by variable (analog) input devices.

Control engineering has evolved over time. In the past humans was the main method for controlling a system. More recently electricity has been used for control and early electrical control was based on relays. These relays allow power to be switched on and off without a mechanical switch. It is common to use relays to make simple logical control decisions. The development of low cost computer has brought the most recent revolution, the Programmable Logic Controller (PLC). The advent of the PLC began in the 1970s, and has become the most common choice for manufacturing controls.

Programmable Logic Controllers have been gaining popularity on the factory floor and will probably remain predominant for some time to come. Most of this is because of the advantages they offer.

Remote Terminal Units

The most sophisticated of the three devices is the second kind of RTU the Remote Terminal Unit. This is the kind of RTU that Tetragenics helped pioneer.

These RTUs actually have the intelligence needed to control a process (or multiple processes) without intervention from a more intelligent controller or master.

Tetragenics RTUs offer SCADA capabilities and features that lesser RTUs cannot begin to offer, such as interrupt driven digital inputs, time stamped sequence of events, data logging, intelligent communications, multitasking sequential control, PID control, alarm logging, modular construction, easy programming; well, the list just goes on and on.

The Remote Terminal Unit is a fast and flexible solution to most control needs. It serves both as the master controller or a slave controller. In fact, it can be used as both a slave and master simultaneously in a vertically deployed control system. Also, it is easy to use as a protocol converter or for controlling IEDs. And it can be expanded as the control system grows.

Face Off

The face off between PLCs and RTUs really depends on what you need and can afford now and what you need in the future. Sometimes a quick look at where you want your system to be 2, 5, or even 10 years from now can help you decide what you want to buy today.

If you have a small system and need only a slave device with little computing power, a Remote Telemetry Unit will probably work. But if you need to interface with other devices, you will have to look to PLCs or Remote Terminal Units.

If you need a stand-alone device that has power, a PLC might work for you. But be prepared for some programming training, PLCs can be difficult to configure. And because of the wide price range, you get the functions you pay for.

If you need an intelligent unit to control multiple processes without intervention from a controller or master, you need a Tetragenics Remote Terminal Unit. These intelligent units also provide advanced control functions and are suited for expansion.

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