Introduction

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This is the first episode in an 8 part series.

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This series is on programmable logic controllers, or PLCs as they are commonly known.

What is a PLC you ask?

In short, it is a general purpose programmable industrial control device.

They are used all through industry in factories, utilities, and anywhere industrial machines need to be controlled.

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They are a fully integrated system of hardware, software, and development environment tailored specifically for automatic industrial equipment.

It is probably easiest to describe them by first giving a bit of historical background as to what preceded them.

04 The Early Days of Automation

You have probably heard of the Jacquard loom of the early 19th century and its use of punch cards with respect to it being one of the technologies which eventually lead to computing.

However, for our purposes here, its significance is that it was an early form of industrial automation control system, since it was after all controlling a machine in a factory.

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For other types of machines, a common purely mechanical means of having the various parts of a complex machine move in a coordinated manner was through the use of shafts and cams.

A series of cams located on a shaft or set of shafts connected by gears, could move parts of a machine, turn valves on or off, and generally coordinate the parts of a machine.

With the spread of the use of electricity in factories in the early 20th century, it was now possible to use the new electrical technology to perform control and automation functions.

06 The Background of Relay Logic

I will need to explain some electromechanical terminology here as it is necessary to understand these terms and the concepts behind them in order to understand PLCs, as the latter is an evolution of its predecessors and uses the same terms.

What preceded PLCs was what was commonly known as "relay logic".

07 What is a Relay

A relay is an electromechanical device that responds to and controls the flow of electric current.

A relay has a "coil" which as the name implies is a coil of conductive wire.

When you energize the coil, that is you apply electric current to it, it forms an electromagnet.

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This electromagnet draws in an armature.

The armature is a moving piece of metal which is attracted to the coil when the latter is energized.

The relay being activated is referred to as closing.

Deactivating is referred to as opening.

Alternative names for these states are "pulls in" and "drops out" respectively.

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The armature in turn is attached to one or more contacts.

You can think of a contact as being like a switch.

When the switch is turned one way, the electrical path is closed and current can flow.

When it is turned the other way, the electrical path is broken and current flow is interrupted.

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Contacts can be normally open, in which case the path is closed and current can flow when the relay turns on.

Alternatively, they can be normally closed, in which case the path is closed and current can flow when the relay turns off.

A relay can, and typically does, have multiple contacts, including both normally open and normally closed.

Remember these terms, coil, contact, normally open, and normally closed.

11 What is a Latching Relay

There is a special type of relay known as a latching relay.

This was only occasionally used in relay control circuits, but the concepts behind it will become more important when we talk about actual PLCs.

A latching relay has two coils.

One coil is used to turn on or "set" the relay.

The other coil is used to turn off or "reset" the relay.

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The relay has either a mechanical latching mechanism or a magnet which is used to hold the relay in the set (or in other words, closed) position even if the set coil is de-energized.

You must energize the reset coil in order to reset or "open" the relay.

A latching relay provides the equivalent of one "bit" of memory which retains its last state even if the power to the machine is turned off.

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Latching relays were used in applications where it was important that the relay logic circuit remember its last state.

While not frequently used in relay control circuits due to their greater cost and complexity, the concept was to be more extensively used in PLC programs when set and reset instructions were provided which performed the equivalent function in software where there were no similar cost considerations to worry about.

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Thus set and reset were to be much more frequently used in PLC programs than they were when they were actual relays.

Add these terms to your list of things to remember - latch, unlatch, set and reset.

15 What is a Contactor

I will mention another term now in case it happens to come up later.

This is "contactor".

A contactor is basically just a large relay.

It is typically used to control large electrical loads such as motors and heaters.

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Smaller relays are typically just called "relays", or sometimes "control relays".

These be used to create either logic circuits or control smaller electrical loads such as lights or pneumatic or hydraulic valves.

This may seem like a lot of jargon, but bear with me, I will make analogies to computers when appropriate.

17 Input and Output Devices

If you want to do some useful work, you will need some I/O.

Typical input devices include the following.

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Push buttons.

These are buttons which are pressed by the operator to command the machine to do something.

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Selector switches.

These are switches which are typically rotated to turn on or off and maintain their position.

These can have multiple positions, each of which can activate a separate input.

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Pilot lights. These are lights which are used to provide feedback to the operator.

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Limit switches.

These are mechanical switches which parts of the machine activate, rather than the operator activating them.

This can be used to determine what position the various parts of the machine are in at any given time.

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Proximity sensors.

These are essentially solid state limit switches which are more reliable than mechanical switches as they are less subject to wear and tear.

These are often colloquially abbreviated as "proxies".

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Solenoid valves.

These are like relays in that an electromagnetic coil is activated.

However, instead of activating another electrical contact, it activates a pneumatic or hydraulic valve.

This valve in turn typically allows air or hydraulic fluid to move a piston within a cylinder, which then moves some mechanical part of the machine.

A typical machine will have lots and lots of proximity sensors and solenoid valves.

24 Relay Logic

All of these inputs and outputs require some sort of logic to coordinate them.

This is where control relays came in.

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If having the output of one electrical device being able to control another electrical device sounds a bit like a transistor, then yes relays are analogous to electronic transistors.

However, while transistors are a mid 20th century invention, relays date from the mid 19th century.

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Like with transistors, it is possible to encode logic into a network of wires connecting relays together, along with inputs from switches and other input devices.

By wiring sensors and relays together in the right order, it is possible to create reasonably complex sequences of operation to control a machine in an automatic manner.

27 Relay Boolean Logic

Wiring relay contacts in series creates "and" conditions.

Wiring them in parallel creates "or" conditions.

Using normally closed relay contacts creates "not" conditions.

By feeding a relay's own contacts back into the circuit leading up to its coil, it is possible to have a relay remember its own state.

Each relay therefore could be thought of as one bit of logic in a boolean logic circuit.

28 Timers and Counters

Special timing relays could be used to create a time delay between the relay energizing or de-energizing, and the contacts closing or opening.

A timing relay which imposed a delay after being energized is an "on delay timer".

A timing relay which imposed a delay after being de-energized is an "off delay timer".

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Timing relays were often pneumatic.

They had a small rubber bellows that leaked air slowly through a adjustable orifice.

The bellows would prevent the relay from opening or closing, depending on the type, until enough air had leaked out for it to close or open.

For more accuracy or for longer time delays, motor driven clock timers could be used.

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Special counter relays could be used to require an input to turn on or off a specified number of times before activating the relay outputs.

Counters typically used a ratchet mechanism to count up to a preset before activating.

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Later timing and counting relays used electronic timers and counters, but these arrived relatively late at a time when relay logic was on its way out.

32 Electrical Panels

Large numbers of relays would be mounted on panels in large electrical enclosures, with the wires running between them, and also out to the limit switches, valves, and other devices mounted on the machine.

33 Designing the Logic Circuit

The logic or program would be encoded in the selection of the devices and in the wires running between them.

To design and document this logic, the designer would create electrical drawings.

These drawings would follow one of two different styles.

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In one style, the two power wires, either hot and neutral in the case of AC power, or positive and negative in the case of DC power, were drawn as vertical lines down each side of the page of the drawing.

These vertical lines are known as the "rails".

Wires would then be drawn horizontally across the page from left to right showing the connections between push buttons, switches, and relay contacts across to the relay coils or pilot lights on the right.

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The physical connections between relay coils and relay contacts were not normally shown on the drawings, although they may in the case of push buttons or selector switches with multiple contacts.

Instead you would rely on the labels or names to see which contacts were associated with which relay coils, limits switches, or push buttons.

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There are standard naming conventions for devices which I won't go into here.

However, this means that the relay coils could be on one page of the drawing set, and the contacts could be anywhere else in the drawing set.

It was convention to write a cross reference beside the relay coil to list where its contacts are used in the drawing.

37 Ladder Rungs

In practical terms, what this meant is that the logic drawing tended to take the form of many independent horizontal sets of wiring and contacts looking rather like the rungs of a ladder.

Thus these drawings came to be known as "ladder diagrams" or "ladder drawings".

The horizontal elements were known as "rungs".

38 Standard Symbols

Each type of device had a standard symbol so you could see at a glance what it was.

For example a relay coil was a circle.

A normally open relay contact was two short vertical lines separating a break in the wire circuit.

A normally closed relay contact was just a normally open contact with a diagonal line drawn across it.

39 Logic Flow

By convention, drawings were read from top to bottom, left to right.

In actual practice operations could happen in parallel and you had to be careful to avoid introducing what was called a "relay race" where the outcome of an operation depended upon which of two parallel operations completed first.

A relay race could produce unpredictable results depending upon which of the operations completed first and therefore must be avoided.

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Engineers, technicians, and electricians were expected to become proficient in reading and understanding these ladder diagrams in order to design and troubleshoot equipment.

To someone who was experienced in the field, reading and understanding these diagrams became second nature.

41 DIN Drawings

I mentioned there were two styles of drawings.

The other style is known as DIN, which stands for Deutsches Institut fur Normung, or German Standards Institute in English.

This is basically just a ladder diagram turned on its side, with the rails running horizontally from left to right, and the rungs running vertically from top to bottom.

I have mentioned DIN drawings here for completeness, but even those countries which still use DIN style drawings for documenting wiring use vertical ladders when ladder logic went electronic.

42 Comparison to Electronic Logic Gates

If you are familiar with electronic and, or, nand, and nor gates some of the above concepts should familiar to you.

Some of the concepts are indeed analogous.

However these electromechanical conventions predate the existence of solid state.

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The drawings also follow completely different principles.

Whereas solid state logic gate drawings show inputs and outputs on a single device grouped together in a single block with complex interconnections, electrical ladder drawings separate them, which has a number of very significant consequences.

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First it greatly simplifies drawing the interconnections.

Second, it allows inputs and outputs to be grouped together by function rather than by physical packaging.

Thirdly it allows drawings to be spread over many smaller standard size sheets which in turn allows a functional collection of related rungs to be viewed in a single glance.

These three factors were to be of great significance when physical relays were replaced by software.

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45 Historical Origins and Development

Origins

Nobody seems to know when or where industrial relay logic was first introduced.

Best guesses seem to say some time after the widespread electrification of factories in the early 20th century.

Automation in the era of steam power seemed to rely mainly on the profile of mechanical cams mounted on shafts to time machine operations.

Most sources seem pretty confident that relay logic was widespread by the 1940s or 50s.

46 Solid State

The introduction of transistors did not immediately displace relays for control purposes.

Relays had the ability to handle significant amounts of power directly, allowing them to interface with inputs and outputs directly.

Transistors would have still required interposing relays anyway, increasing the component count and complicating the system for little benefit.

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There were solid state logic modules developed specifically to replace relays in control logic, but these were not widely used.

These solid state logic modules still required wiring connections between them, limiting their advantages over electromechanical relays.

A number of families of electromechanical control relays were developed by a number of companies, and these tended to be very robust and reliable, although rather expensive.

48 Conclusion

In this episode I have covered the following.

What is a relay.

Common input and output devices.

How relays are used to create boolean logic systems.

Designing relay logic systems and the electrical drawings used to document them.

The brief history of relay logic systems.

49 Terms to Remember

Here are some terms you may wish to remember.

Programmable Logic Controller, also known as "PLC"

Control relay

Coils

Contacts

Normally open

Normally closed

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Latch

Unlatch

Set

Reset

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On delay timers

Off delay timers

Counters

Push buttons

Pilot lights

Limit switches

Proximity switches, also known as "proxies"

Ladder diagrams

Ladder rails

Ladder rungs

52 Next Episode

In the next episode I will discuss the following

The origins of PLCs.

How they evolved from relay logic.

How and why physical relays and wiring became virtual relays in software.

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Did you think that visual or graphical programming was something new?

Listen to the next episode and find out that people were doing it in factories half a century ago back when you were still trying to get your first Fortran program to run on a mainframe.

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This has been the first episode in an 8 part series.

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