A PLC communicates with a wide range of external electrical and electronic signals. These signals might be alternating current (AC) or direct current (DC) currents or voltages. They typically vary from 4 to 20 milliamperes (mA), or 0 to 120 volts alternating current (VAC) and 0 to 48 volts direct current (VDC). I/O (input/output) points are the names given to these signals. There can be from 1 to 2 dozen I/O points on a single PLC module.
The number of I/O required for a specific application is determined by how the signal is interpreted. For example, if it's an ON/OFF switch then obviously you need a binary system, so each bit requires its own I/O. But if the signal is an intensity level between no light and full light then it could be one voltage source feeding into an amplifier which outputs a different voltage at each end of its operating range. In this case, only two levels are needed but there are still two I/O per module because both ends of the line are open when either one is not powered.
There are various ways to represent I/O on a diagram. The most common way is to show each point as a small circle with a line coming out of it. Each time that signal passes through a logical "1" it comes out of one of these circles and into your circuit. Logical "0"s have no such effect on their circuits. Sometimes other symbols are used instead.
PLCs are often referred to as high-level microcontrollers. They consist primarily of a processing module, a power supply, and I/O modules. The central processing unit (CPU) and memory make up the processor module. This module communicates with other internal and external components using the I/O modules. Examples of common PLC components include: transmitters, receivers, encoders, motors, sensors, and actuators.
The PLC is responsible for controlling machinery, equipment, or devices such as valves, solenoids, and lights. It receives instructions through its input buttons or switches. Based on these instructions, it can open or close circuits within itself or between itself and other connected devices. For example, if it senses that there is no water flow at a certain point, it will turn off a valve so that water cannot escape from that area. Through this simple action, the PLC can protect people from being hurt by falling objects or trapped air, or avoid financial loss due to abandoned equipment.
PLCs can also control machinery or devices that are not part of a networked system. For example, they can open or close circuits within themselves to perform tasks such as testing functions, storing data, or providing outputs such as signals or lights. These self-contained capabilities provide PLCs with an advantage over conventional computers which require connections to other computers in order to function properly.
Input modules or points, a central processing unit (CPU), output modules or points, and a programming device are the essential components of a PLC. As it runs a stored program, the CPU examines the state of inputs, outputs, and other variables. The CPU then sends signals to the outputs to update their state. A restart condition can also trigger a PLC to run its programs again.
A basic PLC has input modules that detect changes in conditions such as temperature, pressure, and humidity and send a signal to the CPU when they sense something is wrong. The CPU interprets these signals and takes appropriate action, such as turning on or off devices like valves and motors. Output modules transmit data back to the programming device indicating what action was taken by the PLC. The programming device reprograms the PLC if needed and starts it running again.
More advanced PLCs have additional functionalities such as memory storage and analysis of sensor data. These PLCs are used in control systems such as factory automation and water treatment plants.
The term PLC is often used interchangeably with PC-based control system, but they are not the same thing. A control system consists of the PLC along with other hardware devices such as sensors, actuators, and interfaces for connecting them together. A control system can be designed and built completely in house or acquired as a package from a single vendor.
Logic Controllers with Programmable I/O Addresses The PLC must be able to recognize each individual input and output. It accomplishes this by assigning addresses to each input and output. With a tiny PLC, this is most usually merely a number preceded by a letter indicating whether it is an input or an output. For example, the first input on device #1 would be labeled "IN1". The second input on device #1 would be labeled "IN2", and so forth.
For large systems, however, these address labels are insufficient. Instead, the PLC assigns addresses in accordance with a protocol that ensures that inputs and outputs can be identified regardless of their location within the system. The two main protocols are called "Sequential Input Memory" (SIM) and "Function Select Memory" (FSM). SIM is used where there is a need to identify inputs in a specific order; for example, a panel operator might only be given access to certain parts of the machine when he or she enters a code into the controller. FSM is used where several sets of inputs may need to be accessed simultaneously; for example, a panel operator could open all the feeders at once by entering the correct code into the controller. Either method can be used with either small serial ports or large parallel ports.
The PLC determines which protocol should be used by looking at the type of port on board. If it is a small serial port, then SIM will be used.
What exactly is a PLC? PLC is an abbreviation for "Programmable Logic Controller." A programmable logic controller (PLC) is a computer that is specifically built to work consistently in tough industrial settings such as high temperatures, wet, dry, and/or dusty circumstances. These controllers use hardware and software components designed to provide optimal performance under harsh conditions.
Controllers are used in industry in many applications including but not limited to: manufacturing; automation; robotics; power generation; water treatment; food processing; mining; construction; etc. The word "controller" comes from the ability of these devices to control processes or machines without continuous human intervention. They do this by monitoring variables within the system and taking appropriate action if necessary. For example, a PLC can detect when a machine tool fails to stop moving when it reaches its end point, at which time it will activate alarms and prevent other parts of the machine from being damaged.
The first PLCs were large, expensive units capable of controlling multiple systems. Modern PLCs are much smaller and less costly, usually measuring about 1" × 1" × 1". Some small enough to be mounted on their own 2" diameter circuit board. They typically cost up to $15000.
These days, most industrial controllers are microprocessors based with memory and input/output (I/O) circuits integrated onto one chip.