![]() ![]() This seems intuitive to me sinking in, sourcing out. In the U.S., most PLC/PAC system are designed with sinking input modules and sourcing output modules. The sinking device is the device that’s connected to the DC- and the sourcing device is the device that’s connected to the DC+ side of the circuit. You have to have a complete circuit for current to flow, right? Sinking and sourcing just has to with which side of the circuit you are completing to detect your input or activate your output. Basic electrical theory says that DC current must flow from DC+ through a load to DC. Unfortunately this topic causes a lot of confusion. NOTE: Many people have questions about sinking versus sourcing discrete I/O. Other ways you can describe a discrete signal are to say it is either true or false, 1 or 0, open or closed. Because of this, discrete signals are simple to process for a computer or PLC. Just think of discrete I/O as always either on or off.There’s no in between. Some specific applications of discrete outputs would be closing or opening circuit breakers, starting or stopping generators, opening or closing water valves, or turning on and off alarm lights.Īgain, this concept is really simple so I won’t belabor it. Some specific real-world examples of discrete inputs to a PLC would be open or closed circuit breakers, running or stopped generators, a conveyor belt position sensor, or a water tank level sensor. Some of the discrete outputs would be the water fill valve, the water drain valve and the heating element. From our dishwasher example in Part 1, some of the discrete inputs would the start button, the door switch and the water level switch. Some examples of discrete input devices would be things like light switches, push-buttons and proximity switches.Įxamples of discrete output devices are lights, relays and motor starters. The concept is simple, discrete I/O are signals that are either on or off. Sometimes discrete I/O is referred to as digital I/O. ![]() The most common type of PLC I/O is discrete I/O. An output module controls devices such as relays, motor starters, lights, etc. An input module detects the status of input signals such as push-buttons, switches, temperature sensors, etc. In a PLC system there will usually be dedicated modules for inputs and dedicated modules for outputs. PLC I/O is the part of the PLC that connects the brain of the PLC, the CPU, to the outside world, the machines. Now it’s time to get a little more specific with what the PLC CPU controls: PLC I/O! In this post we will discuss discrete I/O, analog I/O and briefly introduce industrial communications protocols. We talked about what PLCs are, their history, and how they work compared to a dishwasher. In part 2 we looked deeper into what the PLC processor does, the relationship between the PLC processor and the human brain, and we looked at a few of Allen-Bradley’s most popular PLC/PAC processors. In part 1 I gave an introduction to PLCs. But before we do that, let’s do a little review. Ok, moving on, in this post we are jumping into PLC inputs and outputs, often referred to simply as I/O. Welcome to part 3 of the “Beginner’s PLC Overview” series! Before we get started, if you have not read Part 1 and Part 2, I recommend that you do that now before continuing this post. It's time to get a little more specific with what the PLC CPU controls: PLC I/O! This article will discuss discrete I/O, analog I/O and briefly introduce industrial communications protocols.Ī Beginner's PLC Overview, Part 3 of 4: PLC Inputs and Outputs (I/O) ![]()
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