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 [...]
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 [...]
Any control system goes through four stages from conception toa working plant. A PLC system brings advantages at each stage.The first stage is design; the required plant is studied and the controlstrategies decided. With conventional systems design must be completebefore construction can start. With a PLC system all that is needed isa possibly vague idea [...]
So far we have assumed that a PLC consists of a processor unit anda collection of I/O cards mounted in local racks. Early PLCs did tend tobe arranged like this, but in a large and scattered plant with this arrangement,all signals have to be brought back to some central point inexpensive multicore cables. It will [...]
Output cards again require some form of isolation barrier to limit damage from the inevitable plant faults and also to stop electrical ‘noise’ corrupting the processor’s operations. Interference can be more of a problem on outputs because higher currents are being controlled by Computers and industrial Computers and industrial control 25 the cards and the [...]
Internally a computer usually operates at 5 V DC. The external devices (solenoids, motor starters, limit switches, etc.) operate at voltages up to 110 V AC. The mixing of these two voltages will cause severe and possibly irreparable damage to the PLC electronics. Less obvious problems can occur from electrical ‘noise’ introduced into the PLC [...]
The complete command listing for this ladder rung including termination commands is: LD IN1 ENTER AND IN2 ENTER STO OUT1 ENTER The commands may be entered using a hand-held programmer, dedicated desktop programmer or a computer containing software that will allow it to operate as a programming device. Each controller command line contains (1) a [...]
Introduction All discussions in previous sections have considered only the ladder diagram in all program example development. The next thing to be considered is how to get the ladder diagram into the programmable controller. In higher order controllers, this can be accomplished through the use of dedicated personal computer software that allows the programmer to [...]