PLC Hints and Tips

The digital I/O in Siemens PLCs is arranged into groups of 8 bits, calleda byte (see Appendix). A signal is identified by its bit number (0–7) andits byte number (0–127). Inputs are denoted I<byte>. <bit> and outputsby Q<byte>. <bit>. I9.4 is thus an input with bit address 4 inbyte 9, and Q63.6 is an output […]

The Allen Bradley PLC-5 can have up to eight racks in its 5/25 version.The rack containing the processor is automatically defined as rack 0,but the designer can allocate addresses of the other racks (in the range1–7) by set-up switches. The racks other than rack 0 connect to theprocessor via a remote I/O serial communications cable.There […]

The PLC program is concerned with connections to the outside plant,and the input and output devices need to be identified inside the program.Before we can examine how the program is written we will first discusshow various manufacturers treat the I/O.
A medium-sized PLC system consists of severalracks, each containing cards, with each card interfacing generally to […]

A PLC program can be considered to behave as a permanent runningloop similar to that in Figure (a). The user’s instructions are obeyedsequentially, and when the last instruction has been obeyed the operationstarts again at the first instruction. A PLC does not, therefore, communicatecontinuously with the outside world, but acts, rather, by taking ‘snapshots’.
The action […]

Introduction
 In addition to the standard logical operations that a PLC can perform, seasoned PLC programmers are aware that, by taking advantages of some of the unique features and characteristics of a PLC, some very powerful operations can be performed. Some of these are operations that would be very difficult to realize in hardwired relay logic, […]