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 with bit address
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 are three different ways in which an Allen Bradley rack can beconfigured, but we shall discuss the simplest (and possibly the most logical)method.
Each rack contains 16 card positions which are grouped in pairscalled a ‘slot’. A rack thus contains eight slots, numbered 0–7. A slot cancontain one 16-way input card and one 16-way output card or two eightwaycards usually (but not necessarily) of the same type. For example, slot 1 contains a 16-way input card and 16-way outputcard, and slot 2 contains two eight-way output cards. Read the rest of this entry »
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 8, 16or 32 devices. I/O addressing is usually based on this rack/card/bit idea.
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 of Figure (a) is called a program scan, and the period ofthe loop is called the program scan time. This depends on the size of thePLC program and the speed of the processor, but is typically 2–5ms perK of program. Average scan times are usually around 10–50 ms.Figure (a) can be expanded to Figure (b). The PLC does notread inputs as needed (as implied by Figure (a)) as this would bewasteful of time. At the start of the scan it reads the state of all the connectedinputs and stores their state in the PLC memory. When the PLCprogram accesses an input, it reads the input state as it was at the start ofthe current program scan.As the PLC program is obeyed through the scan, it again does notchange outputs instantly. An area of the PLCs memory correspondingto the outputs is changed by the program, then all the outputs areupdated simultaneously at the end of the scan. The action is thus: readinputs, scan program, update outputs.The PLC memory can be considered to consist of four areas asshown in Figure (c).
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 of the size of the machine and the I/O requirements(how many inputs and outputs). Read the rest of this entry »
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 also make commissioning and faultfinding rather difficult, as signals can only be monitored effectively ata point possibly some distance from the device being tested.In all bar the smallest and cheapest systems, PLC manufacturerstherefore provide the ability to mount I/O racks remote from theprocessor, and link these racks with simple (and cheap) screened singlepair or fibre optic cable. Racks can then be mounted up to severalkilometres away from the processor.
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 loads themselves are often inductive (e.g. solenoid and relay coils). There are two basic types of output card. Eight outputs are fed from a common supply, which originates local to the PLC cubicle (but separate from the supply to the PLC itself). Read the rest of this entry »
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 from voltage spikes on signal lines, or from load currents flowing in AC neutral or DC return lines. Differences in earth potential between the
PLC cubicle and outside plant can also cause problems.
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 command, (2) the object of the command and (3) a terminator (the ENTER key). In the case of the first line, LD is the command, IN1 is the object of the command and the ENTER key is the terminator. Read the rest of this entry »
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 enter the ladder diagram as drawn. The software then takes care of Read the rest of this entry »
