FBM204 FOXBORO
It should be noted that the power supply of the CPU does not supply power to on-site devices such as sensors and coils.This power connection must be established separately with the card.
Central Processing Unit(CPU)
CPU is the actual”brain”of PLC,making it a computer.Even small non modular PLCs include a CPU.The input signal comes from the I/O card,and the logic program makes decisions based on the signal.If necessary,the CPU then commands the output to turn on and off as the signal and conditions change
The program may include advanced functions such as mathematical operations,timing,counting,and sharing information through modern network protocols.For older relay systems,many of these operations are extremely difficult,if not impossible.
Input/output card
Sensors,buttons,switches,relays,solenoids,and even network devices share information with the I/O signals connected to the screw terminals on the I/O card.These cards can be selected according to the needs of each machine and facility.In some small PLCs,I/O terminals are not modular cards,but are conveniently installed on the side of the PLC.
Due to the wide variety of I/O device structures and discrete and analog modules,the terms for analog models include”source”and”sink”,”voltage”and”current”.
Programming equipment and human-machine interface
There are two very important components outside of the PLC itself:programming devices and human-machine interfaces(HMI).Programming devices can be desktop computers,laptops,or handheld instruments from the same manufacturer.Some small PLCs even have buttons on the front that can program very basic logical operations without the need for a computer.
Although programming devices allow users to view and modify the code running on the PLC,HMI provides a way to display information and obtain input,modeling the control system as a whole.HMI usually does not provide any methods for modifying logical programs.
Figure 3 shows an HMI touch screen that can be used in the control room or in a”field”closer to the process.These types of interactive displays are very common and are usually installed directly on or near the PLC casing for operator use.
Programmable logic controllers or PLCs are ubiquitous in various processes and manufacturing industries today.PLC was originally designed to replace electromechanical relay systems in order to provide simpler solutions for modifying the operation of control systems.No need to reconnect a large number of relays,just quickly download from a PC or programming device and change the control logic in a few seconds.
PLC is an industrial grade digital computer designed to perform control functions,particularly suitable for industrial applications.
