Product type
A programmable logic controller is the device that runs a machine or a process: it reads its inputs, executes a program and drives its outputs, repeatedly and in a fixed cycle. Where a single-loop controller handles one measurement and one action, a PLC handles the whole sequence, the interlocks, and the coordination between them.
A programmable logic controller is the device that runs a machine or a process: it reads its inputs, executes a program and drives its outputs, repeatedly and in a fixed cycle. Where a single-loop controller handles one measurement and one action, a PLC handles the whole sequence, the interlocks, and the coordination between them.
PLC systems are specified in a different way from instruments. The instruments elsewhere on this site are chosen by what they measure; a PLC is chosen by how many signals it has to handle and of what kind, how quickly it has to respond, and how it has to communicate. The programming environment and who will maintain the program matter as much as the hardware, and they are frequently the part left until last.
The range runs from a compact unit with a fixed set of inputs and outputs, through modular systems expanded by adding cards, to controller platforms that handle large plants and redundancy. This page covers how the point count, the signal types, the communication and the support requirements translate into a specification, and the boundary against the neighbouring control devices.
The controller runs a cycle: it reads every input into memory, executes the program, writes every output, and starts again. The time that cycle takes is the scan time, and it sets the fastest response the system can give to any single event. A short scan time matters where a signal has to be caught and acted on within a few milliseconds - a registration mark on a fast line, or an interlock - and it matters much less on a slow process such as a tank level.
Inputs and outputs are counted and typed separately, because an analogue channel and a digital one cost differently and consume different card types. Digital points are usually counted with an allowance for spare capacity, since a plant gains signals over its life. Analogue channels bring their own decisions about resolution, isolation, and whether the card accepts a standard process signal directly.
Communication is what connects the controller to everything else: the operator interface, the drives, the instruments and the plant network. The protocol, the number of ports and the network topology are all part of the specification, and they are usually fixed by what already exists on site. A controller that cannot speak to the installed systems is a controller that needs an additional gateway, which is both a cost and a point of failure.
Most of the specification is fixed by these decisions rather than by the accuracy class. Each one constrains the next, which is why they are settled in order.
| Configuration | Options to fix | What it actually decides |
|---|---|---|
| Form factor | A compact unit with fixed I/O, a modular system expanded by cards, or a larger controller platform | How the system grows, and whether the point count is expected to change over the life of the machine. |
| Digital input and output count | The number of each, with spare capacity allowed for | Whether the installed signals fit with room to add, and whether spare channels are wired or only reserved in the program. |
| Output type | Relay, transistor or triac outputs | What the output can drive. A relay suits a mixed load and is slower; a transistor switches quickly and suits pulse work. The choice follows the load rather than a preference. |
| Analogue channels | The number, and whether they accept standard process signals directly | Whether the instruments connect without signal conditioners, and how much resolution the process needs. |
| Communication ports and protocols | Ethernet, serial buses and fieldbus options, and how many of each | What the controller can talk to without a gateway, which is normally decided by what is already installed on site. |
| Processor performance and memory | The instruction speed, and the program and data memory | The shortest scan time achievable at the final program size, and how much expansion the memory allows for. |
| Programming environment and licensing | The software used, and how the licence is held | Who can maintain the program after handover, which is a support question rather than a technical one. |
| Environmental and mounting | Enclosure rating, ambient temperature and panel space | Whether the unit can be mounted where the machine is, or needs a conditioned enclosure. |
| Factor | Why it decides the answer | How to settle it before enquiring |
|---|---|---|
| Number and type of signals | The count drives the size of the system, and the types drive the cards | Provide an input and output list, with digital and analogue counted separately. |
| Spare capacity wanted | Plants gain signals; a system sized exactly to today’s list has no room to grow | State the spare proportion required, and whether spare channels are to be wired. |
| Required response time | It sets a ceiling on the scan time, and therefore on the processor | Give the fastest event the system has to catch and act on. |
| What it must communicate with | Existing drives, instruments and networks decide the protocols | List the devices and the protocols already in use on site. |
| Who will maintain the program | The programming environment and the licence determine whether the customer’s own staff can support it | State who will own and maintain the software after handover. |
| Power and environment | Supply available, panel space, ambient temperature and any classification | Describe the panel and the location, not the plant in general. |
| Growth and future phases | A system specified for one phase may have to accept a second | State what is planned, so that memory, I/O and network are sized for the route rather than for the first step. |
If any of these are still open, send what is known. A quotation can be issued against a duty description and refined once the gaps are filled.
These are the types an enquiry for a PLC system usually turns out to be, and the point at which the choice is genuinely decided.
| Nearest alternative | What separates it from a PLC system |
|---|---|
| PID controllers | Control one loop each, taking a measurement and driving an output to hold it at a setpoint. They are self-contained, need no program, and are the right answer where a single variable has to be held. A PLC takes over where several loops interact, or where the sequence matters as much as the value. |
| Remote I/O | Extends the inputs and outputs of a controller into the field over a network, without providing a processor of its own. It reduces wiring and lets the cabinet sit away from the machine, but it does nothing alone - there has to be a controller at the other end. |
| Control modules | Add a specific function to a system, such as a dedicated motion or weighing capability, rather than general logic. Where an application has a demanding specialised requirement, a module built for it may serve better than writing the same function into the main program. |
| Industrial communication devices | Connect systems that would otherwise not speak to each other, converting one protocol to another. They are the answer where the controller cannot be changed but the networks must be joined, and they add a device between the two ends. |
| Signal isolators | Separate circuits electrically so that a fault or a ground loop on one side cannot affect the other. They sit between the field and the controller, and they solve an electrical problem rather than a control one. |
| Modbus devices | Speak a specific and widely used protocol, and may be instruments or gateways. They matter to the specification because the controller has to support that protocol natively or through a gateway. |
The decision most often deferred, and most often regretted, is who maintains the program. A system that the customer’s own engineers cannot support ties them to whoever wrote it, whatever the hardware specification says.
The first reply can be a quotation rather than a set of questions if the enquiry carries these. Where a figure is not known, send the duty description and we will confirm what it implies.
Available products
AMX-HM · PLC Systems
Programmable logic controllers with an integrated touch screen, in 4.3, 7 and 10 inch formats, combining discrete and analogue I/O, PT100 inputs, high-speed counting and relay outputs in one panel unit.
Also in Industrial Automation & Control
Every type we quote for under Industrial Automation & Control. A type with no link yet is still quoted for - it is described on the Industrial Automation & Control overview, and gets a page of its own as soon as a model is published against it.
Frequently asked
It depends on whether the loops are independent. Where several variables each have to be held and they do not interact, separate loop controllers are simple, self-contained and need no program. Where the loops interact, where a sequence has to run, or where interlocks span several signals, a PLC is the practical answer - the coordination is the reason it exists.
Enough that the next change does not require a new rack or a new card. Plants gain signals over their life: an interlock is added, an instrument is fitted, a signal is brought back for monitoring. The proportion is a judgement about the plant rather than a fixed rule, and it is worth stating explicitly, because a system sized exactly to today’s list has no room for tomorrow’s.
That depends on what is being bought, and it should be settled in writing. Hardware supply, program development and commissioning are separate activities. Where the program is developed by a third party, the environment and the licence still have to allow the customer’s own staff to maintain it afterwards, which is a commercial term as much as a technical one.
Fast enough for the fastest event the system has to catch, and no faster. Scan time is set by the processor and by the final size of the program, and a shorter one costs money that buys nothing on a slow process. The requirement is best stated as the event to be detected and the time available to act on it, rather than as a scan time figure on its own.
Send the specification and we will confirm the matching model, availability and lead time. Minimum order quantity is 1 pc.
Request for quotation
Send the technical detail and we will confirm availability, lead time and terms. In-stock items ship within 48 hours; customized specifications typically take 2-10 days. Minimum order quantity is 1 pc.