PLC programming, tutorial, training, hints and tips.

As a programming example, consider the simple AND circuit shown in Figure 1 consisting of two momentary pushbuttons in series operating a lamp. Although it would be very uneconomical to implement a circuit this simple using a PLC, for this example we will do so.

When we convert a circuit to run on a PLC, we first remove the components from the original circuit and wire them to the PLC as shown in Figure 2. One major difference in this circuit is that the two switches are no longer wired in series. Instead, each one is wired to a separate input on the PLC. As we will see later, the two switches will be connected in series in the PLC program. By providing each switch with a separate input to the PLC, we gain the maximum amount of flexibility. In other words, by connecting them to the PLC in this fashion, we can “wire” them in software any way we wish.
The two 120V control voltage sources are actually the same source (i.e., the control transformer secondary voltage). They are shown separately in this figure to make it easier to see how the inputs and output are connected to the PLC, and how each is powered.

Once we know how the external components are wired to the PLC, we can then write our program. In this case we need to connect the two switches in series. However, once the signals are inside the PLC, they are assigned new reference designators which are determined by the respective terminal on the PLC. Since SWITCH1 is connected to IN1, it will be called IN1 in our program. Likewise, SWITCH2 will become IN2 in our program. Also, since LAMP1 is connected to OUT1 on the PLC, it will be called relay OUT1 in our program. Our program to control LAMP1 is shown in Figure 3.

The appearance of the PLC program may look a bit unusual. This is because this ladder rung was drawn by a computer using ASCII characters instead of graphic characters. Notice that the rails are drawn with vertical line characters, the conductors are hyphens, and the coil of OUT1 is made of two parentheses. Also, notice that the right rail is all but missing. Many programs used to write and edit PLC ladder programs leave out the rails. This particular program (TRiLOGI by TRi International Pte. Ltd.) Leaves out the right rail, but puts in the left one with a rung number next to each rung.
When the program shown in Figure 3 is run, the PLC first updates the input image register by storing the values of the inputs on terminals IN1 and IN2 (it stores a one if an input is on, and a zero if it is off). Then it solves the ladder diagram according to the way it is drawn and based on the contents of the input image register. For our program, if both IN1 and IN2 are on, it turns on OUT1 in the output image register (careful, it does NOT turn on the output terminal yet!). Then, when it is completed solving the entire program, it performs another update. This update transfers the contents of the output image register (the most recent results of solving the ladder program) to the output terminals. This turns on terminal OUT1 which turns on the lamp LAMP1. At the same time that it transfers the contents of the output image register to the output terminals, it also transfers the logical values on the input terminals to the input image register. Now it is ready to solve the ladder again.
For an operation this simple, this is a lot of trouble and expense. However, as we add to our program, we will begin to see how a PLC can economize not only on wiring, but on the complexity (and cost) of external components.

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When learning PLC programming, one of the most difficult concepts to grasp is the difference between physical components and program components. We will be connecting physical components (switches, lights, relays, etc.) to the external terminals on a PLC. Then when we program the PLC, any physical components connected to the PLC will be represented in the program as program components. A programming component will not have the same reference designator as the physical component, but can have the same name. As an example, consider a N/O pushbutton switch S1 named START. If we connect this to input 001 of a PLC, then when we program the PLC, the START switch will become a N/O relay contact with reference designator IN001 and the name START. As another example, of we connect a RUN lamp L1 to output 003 on the PLC, then in the program, the lamp will be represented by a relay coil with reference designator OUT003 and name RUN (or, if desired, “RUN LAMP”).

1. Processor. The processor (sometimes call a CPU), as in the self contained units, is generally specified according to memory required for the program to be implemented. In the modularized versions, capability can also be a factor. This includes features such as higher math functions, PID control loops and optional programming commands. The processor consists of the microprocessor, system memory, serial communication ports for printer, PLC LAN link and external programming device and, in some cases, the system power supply to power the processor and I/O modules.

2. Mounting rack. This is usually a metal framework with a printed circuit board backplane which provides means for mounting the PLC input/output (I/O) modules and processor. Mounting racks are specified according to the number of modules required to implement the system. The mounting rack provides data and power connections to the processor and modules via the backplane. For CPUs that do not contain a power supply, the rack also holds the modular power supply. Read the rest of this entry »

Programmable controllers (the shortened name used for programmable logic controllers) are much like personal computers in that the user can be overwhelmed by the vast array of options and configurations available. Also, like personal computers, the best teacher of which one to select is experience. As one gains experience with the various options and configurations available, it becomes less confusing to be able to select the unit that will best perform in a particular application.

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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 6 in byte 63.Like Allen Bradley, Siemens use card slots in one or more racks. Thecards are available in 16-bit (2-byte) or 32-bit (4-byte) form. Read the rest of this entry »

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).

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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.

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