3 PLCs and Processing I/O
Introduction
After an introduction of PLCs in chapter one, various characteristics of the PLC need to be discussed. The purpose now is to discuss what parts constitute a modern PLC and how these parts interface. The topology of a PLC system is also discussed as to how PLCs are distributed in a manufacturing environment to best control a process efficiently.
Since each is electronic and operates using one or more microprocessors, a 5 volt power supply and CPU (central processing unit) are the core of the PLC. Included in the CPU is a computer with memory and communications hardware to communicate to a programming panel, the I/O, and to a network which is either peer-to-peer or a multimode network.
Many PLC vendors divide the work of the PLC between multiple microprocessors with coordination handled by a master microprocessor. One processor may be assigned to handle the I/O. Another may handle the networking and communication to the programming panel. A supervisory microprocessor handles the logic, scan, arithmetic, and other instructions solution of the program.
Inputs and outputs complete the PLC with inputs reporting the status of the system and outputs controlling the sequencing of the process. Inputs and outputs are of many types and forms. A simple switch can be an input. Also, a high-speed pulse input can be an input providing speed information from a motor. Inputs and outputs alike can be simple or complex in nature. Both the simple I/O as well as the more complex will be discussed in the chapter and through the rest of the book.
Overview of the PLC
Inputs form the portion of the PLC connecting switches, sensors, transducers and other devices to the processor. Typically, the input is tied to a screw terminal. The PLC program reads the status of the inputs and solves logic based on this status.
The CPU stores the program and controls communication with all peripherals including programming devices as well as the I/O. The CPU executes the programs in an orderly manner and guarantee that I/O responds per the program. The guarantee is not trivial if one is experienced with most computer operating systems.
Outputs are connected to devices that control the process. Relays, motors, solenoids and other outputs are some examples. A pulse wave PWM is one that controls stepper motors and positional movement. The PLC program can control the status of this output and thus control the motor speed and movement.
In the figures below, the simplified PLC is shown first followed by an expanded view of the PLC. Each view shows the importance of the CPU (Central Processing Unit) as well as the interconnection of the CPU to I/O (Inputs and Outputs). Other devices interact in such a way that the program executes and solves logic in a timely manner.
The second view gives a greater detail of data flow into and out of the PLC. As the devices and programs become more involved, the flow of data must both increase while being as secure as with