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01 Introduction

This is the fifth episode in an 8 part series.

02

In the previous two episodes we looked at the Allen Bradley PLC 2 and the Siemens S5 series of PLCs.

In those episodes we mentioned I/O modules, but didn't really go into any detail on them.

Instead we looked at the CPU units and focused on things from the programmer's perspective.

03

In this episode we will look at the I/O modules, what they are, how they work, and the purpose that they serve in the system.

In addition to this, I will discuss machine safety systems and also claims made by certain people about cyber security and industrial safety.

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04 What is I/O?

First though, we must address the question of what I/O is.

While the CPU module may run the program, in order for the program to do anything useful it must interact with the real world outside the PLC.

It must sense the state of the machine it is controlling through inputs, and it must affect the operation of the machine through outputs.

05 A Simple Example

Let's take a very simple example to try to make the above a bit more tangible.

Assume that we wish to press two parts together, such as pressing a bearing into a housing.

This is a very common industrial process.

06

Let us assume we have the following items which connect to the inputs.

* An emergency stop circuit.

* A safety system to protect the operator.

* A part presence sensor used to detect that the housing was correctly placed in the tooling.

* Another part presence sensor used to detect that the bearing was correctly placed in the tooling.

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* A sensor which indicates the hydraulic cylinder is fully retracted.

* Another sensor which indicates the hydraulic cylinder is fully extended and the part was fully pressed home.

* A button which the operator uses to initiate the press cycle.

* Another button which the operator can use to abort the press cycle in the event of a fault.

08

We also have the following items which connect to the outputs

* A hydraulic valve with two solenoids (down and up).

* A pilot light which tells the operator that the cycle was completed successfully and the part is good.

* A pilot light which tells the operator that the cycle was not completed successfully and the part is bad.

This gives us a total of 8 inputs and 4 outputs.

09

A typical very small PLC would be suitable for this application.

The PLC program would look at the state of the inputs, and apply the necessary logic to turn the outputs on or off at the appropriate time.

As you can see, what makes a PLC useful is its ability to control I/O.

10

Small PLCs typically have a limited amount of I/O built into the same package as the CPU and can be used as is right out of the box.

Medium to large PLCs will interface to racks of I/O, and the number of I/O points can be in the dozens to hundreds and even the thousands.

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11 Types of I/O

The above example is very simple.

There are in fact many types of I/O.

I will outline the most common ones here.

12 Digital I/O

The first is "digital" I/O.

This is overwhelmingly the most common type.

Digital I/O are basic on-off signals.

A digital input will detect if a switch is on or off.

A digital output will turn a relay or valve on or off.

13

Digital I/O comes in three main types categorized by the type of electronic device used.

These are

* AC

* Relay

* DC

14 AC

AC I/O was the earliest type, but has largely faded away by now.

This works with 100 to120 VAC or 230 VAC.

Inputs detect the presence or absence of these voltages, and outputs output these voltages when turned on.

The relay logic that PLCs replaced was largely driven by these voltages, so being able to interface with existing systems was an important design consideration for early PLCs.

15 Relay

Relay outputs were an alternative to triac devices used for solid state AC outputs.

These were very small relays built into the output modules.

Their main attraction was they were usually somewhat cheaper than solid state outputs.

However, they suffered from being much less durable and had a shorter life.

16 DC

By the 1980s and 1990s, AC and relay I/O had been largely replaced in new applications by 24VDC.

This allowed for more compact and less expensive control wiring, and fewer electrical safety concerns.

Both inputs and outputs worked with 24VDC.

All of the standard sensors, solenoid valves, relays, push buttons, lights, and pretty much all other devices were designed to be compatible with 24VDC.

In cases where you had large motor contactors or relays, or large solenoid valves which required higher voltages for practical reasons, an additional interposing relay could be used to amplify the control signal.

17

I/O modules or cards came in 8, 16, 32 point sizes, and there were modules which combined both inputs and outputs in a single module.

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18 Analogue I/O

As well as simple on-off voltages, PLCs could also work with variable voltages, where the actual voltage or current represents a measurable value, such as temperature or dimension.

These are known as analogue values.

These can be inputs or outputs and are known as analogue inputs or analogue outputs.

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The simplest is voltage.

The most common standard voltage ranges are

* 0 to 10V.

* Plus or minus 10V.

* 0 to 5V.

* 1 to 5V.

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Another type is current.

This is where the amount of current rather than the level of the voltage is what is significant.

Current has an advantage over voltage in that it is less affected by resistance losses in long wiring runs and is also less prone to being influenced by electrical noise due to the lower impedance.

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The most common standard current ranges are

4 to 20 mA.

0 to 20 mA.

22

Another common type is thermocouple inputs.

These have special circuitry built into them to interface with thermocouple devices, which are a common device used to measure temperature by measuring the voltage generated when two dissimilar metals are heated in contact with each other.

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23 Serial Communications

A number of PLCs have serial communications ports, mainly used to interface with things like label printers or bar code readers.

Some of these have even had basic interpreters built into them, mainly to allow for better string formatting or parsing.

24 Servo and Stepper

Another category of I/O are servo and stepper motor controllers.

25 High Speed Counter Timer

Another type are counter/timer modules, which allow for high speed counting of events, or very precise timing of events in hardware, or interfacing with rotary or linear encoders.

Encoders are measuring devices which have a series of very small but precise marks which interrupt beams of light when they move, allowing measurements to be made.

26 Networking

Many PLCs also support some sort of networking.

I won't go into any detail on that here, but will instead address this area in another episode.

However, network ports may be built into the CPU, or may reside in separate network I/O modules.

27 Specialty

As well as the preceding there are many other specialty modules which I won't bother to describe here.

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28 Operator Panels

The original operator panels were arrays of push buttons, pilot lights, and other basic operator controls mounted in boxes for an operator to interact with the machine.

Later on these were replaced by specialty electronic devices which were also known as operator panels as they served the same function.

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Modern operator panels are screens and keypads which are used to display information to an operator or to allow data to be entered into the PLC.

While not strictly speaking a type of I/O, operator panels typically look like I/O to the PLC CPU.

Buttons on a touch screen may be effectively the same thing as actual hardware buttons from the PLC's perspective, and analogue values output to the operator panel for display are like hardware analogue devices.

30

All of these data are mapped to memory locations in the PLC, with the details of how this is done varying according to the make and model of PLC.

31

I won't go into any detail on this as this could be a separate subject on its own.

Generally though, you would use software to create screens to display on the operator panel.

The indicators and buttons defined in your screens would be mapped to addresses in the PLC, either directly or indirectly.

This configuration would then be downloaded into the operator panel.

Your PLC program could then interact with the operator panel using ordinary ladder logic.

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32 Safety Systems

Up to now I have mentioned only PLCs in any detail.

There is however more to a complete machine control system than just a PLC.

One of these is safety systems.

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I will leave out historical safety systems and just focus on modern ones.

I will also leave out large process industries involving things like boilers or chemical reactors and just focus on assembly operations in industries such as automobiles, appliances, and other similar things.

This won't be a tutorial on safety design, just a very brief overview.

34

An industrial machine must be designed with safety as one of its fundamental design principles.

If the machine cannot be operated safely, then it cannot be put into use.

Safety must take into account the operators, bystanders, and maintainers.

You must assume that people will be inattentive or make mistakes.

35

You must also assume that the PLC program will have bugs and that the electronics will fail in a state which could cause a hazard if it is permitted to.

In other words, the PLC and your PLC program are not part of the safety system.

36

The safety system will decide when the PLC will be permitted to act and when it will not.

The PLC in turn must monitor the safety system in order to understand when it can and cannot act.

Your PLC program must take this into account so that it can differentiate between not being able to act because the safety system has decided it is not safe to cycle, versus there being something wrong with the machine which prevents it from operating.

37 A Few Basic Safety Devices

Here are a few basic safety devices.

I will not attempt to be comprehensive.

Rather, I am just giving a few common examples so that you will understand the environment in which the PLC can operate.

38 Emergency Stop Circuit

A basic element of nearly all industrial machines is some sort of emergency stop circuit.

When the "on" button is pressed, a relay will close and hold itself on.

This will enable power to the machine, including the compressed air supply if there is one.

If the "stop" or "e-stop" button is pressed, the relay will open and remove energy from the machine, rendering it safe.

If the electrical power supply to the machine goes off, due to for example the building losing electrical power, the emergency stop circuit will also "drop out" or open, and remain off until the start button is pressed again.

39

While every, or nearly every, machine will have an emergency stop circuit, is it often not sufficient on its own.

In other words, it is necessary, but not always sufficient.

40 Guard Switches

Equipment hazards must be enclosed by protective guarding.

Movable guards, such as access doors, must be interlocked with the safety circuit such that when the guard is opened, the machine is rendered safe.

To achieve this, there will be safety rated switches on each access door which are connected to the safety circuits of the machine.

41 Two Hand Control

Another common safety circuit is two hand control, often called "anti tie down", although that latter phrase actually describes one of the design principles rather than being the proper name.

With two hand control, the operator station will have two large buttons which the operator must press simultaneously and hold to enable the machine to operate.

42

With both hands on the buttons, you know where the operator's hands are.

The operator must remove both hands from the buttons in order to enable the circuit again.

If one button remains closed and the operator attempts to activate the machine again with the other button, then the safety system will detect this and refuse to enable the machine again until both buttons are de-activated.

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This is the "anti tie down" feature.

The operator cannot tie one button down and cycle the machine repeatedly with just the other button.

The buttons will be connected to a special relay which monitors and self tests to ensure it operates correctly.

44

The PLC will monitor an auxiliary contact on this relay so that it knows when to start its sequence of operation, but the redundant safety contacts on the relay lie in between the PLC and whatever hazards are being guarded against.

The safety device has ultimate control over whether the machine moves or not.

45 Light Curtain

Another basic safety device is the light curtain.

This has a series of closely spaced light beams across the opening which forms the operator's access to the working area of the machine.

46

If any of these light beams is interrupted, the light curtain controller will not permit the machine to move.

Light curtains can usually be used in places where two hand control systems can be used.

They have an advantage over two hand control in that the operator can be performing other tasks while the machine is cycling instead of being occupied with holding the two hand control buttons down.

This allows for productivity improvements which can usually more than pay for the higher cost of the light curtain compared to simple two hand control.

47 Other Devices

There are many other safety devices, many of which address niche applications.

I won't bother listing any more than I have so far however, as you should understand the basic principles by now.

48 Safety PLCs

There are special safety PLCs which have redundant CPUs which monitor one another, and which have redundant mutually interlocked I/O, and which run safety reviewed software.

Anything connected to a safety PLC must itself be safety rated if it could affect the safety of the machine.

However, these are quite rare and are only used in certain special industries.

49

These are normally used in places where some programmed sequence of operations is required to render a system safe.

This is more commonly found in places such as oil refineries, chemical plants, or electric power generating plants than in typical factories.

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50 Safety and Cyber Security

This is a good place to bring up the question of safety and cyber security.

There have been a number of articles in the IT press recently about how PLCs could be indirectly affected by computer viruses and how this could cause dangerous safety problems in manufacturing equipment.

51

The premise is that they found security holes in software which runs on personal computers running Windows operating systems which are used to do things like record production data.

There are a number of software systems which are designed for this purpose, although SCADA systems are usually the ones mentioned.

52

I will briefly describe SCADA systems in a later episode.

For our purposes here however, just understand it as specialized industrial software that runs on a PC and conducts monitoring and display using data from a PLC that it is networked to.

53

According to these security researchers, a virus could exploit these security holes to send commands to the PLC and cause it to do arbitrary things which could cause injury or death.

The PLCs being referred to in this instance are ordinary PLCs, not safety PLCs.

54

I find these claims to be highly implausible for the reasons that I have stated above when describing safety systems.

An ordinary PLC is not a safety device, and so the safety of the machine should not be affected by a malfunctioning PLC, regardless of whether any viruses were involved or not.

55

I have seen PLCs malfunction due to memory corruption errors, but these posed loss of production problems, not safety problems.

A malfunctioning PLC may cause property damage to the machine and loss of money, but the control system, of which the PLC is only a part, should not permit it to pose a hazard to personnel.

56

I have not seen any of these security researchers offer any plausible mechanism by which their claims of safety problems may be realized.

As a result of this, I take all such claims with a very, very, very, large grain of salt.

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57 Conclusion

In this episode we looked at the I/O modules, what they are, how they work, and the purpose that they serve in the system.

In addition to this, we discussed machine safety systems and also claims made by certain people about cyber security and industrial safety.

58

In the next episode I will cover networking, including a detailed look at one industrial network protocol, called Modbus.

59

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

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