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PLC interview questions and answers for controls engineers

Common PLC interview questions with short answers checked against makers' manuals: the scan, ladder, timers, Siemens v Rockwell, faults and safety.

Graeme, Founder9 min read

A candidate working at a laptop beside a control panel while an engineer looks on.
Expect a follow up to every answer.
In this guide
  1. Basic PLC interview questions
  2. Questions about the scan
  3. Ladder logic and Structured Text questions
  4. Timer and counter questions
  5. Siemens and Allen-Bradley PLC interview questions
  6. Fault finding questions
  7. Safety questions
  8. Practical questions for technicians
  9. Controls engineer interview questions
  10. For hiring managers: why a practical test shows more
  11. How to get ready
  12. Sources

Got an interview coming up? Here are common PLC interview questions, grouped by topic, with short answers. This isn't any company's list, and it's worth being wary of anyone who claims to sell you one.

The answers are checked against makers' documents, linked at the bottom. Some are older editions, so if the job runs on a particular model, its manual beats this page. And expect a follow up to everything: why, and what would you check next?

Basic PLC interview questions

What is a PLC, and what languages does it use?

It's an industrial computer that controls machines. It reads inputs such as buttons and sensors, runs your program and sets outputs such as motor starters and valves, over and over. IEC 61131-3 covers Structured Text, ladder, function block diagram and sequential function chart. Siemens calls them SCL, LAD, FBD and GRAPH.

What's the difference between digital and analogue signals?

Digital is on or off. Analogue is a range, such as a transmitter's 4 to 20 mA. Starting at 4 means a broken wire reads near 0 mA and can't be mistaken for an empty tank.

Does a normally closed contact mean a normally closed button?

No. A contact checks a bit in memory, not the wiring. Rockwell's XIC only asks whether its bit is set. So a stop button wired normally closed holds its input ON until pressed, and you test it with XIC, the symbol that looks normally open.

Questions about the scan

What is the scan cycle?

The scan is the loop a PLC repeats all day: read inputs, run the logic, write outputs, do some housekeeping. Siemens lists the S7-1200's starting with writing the outputs, but it's the same loop.

Can an input change part way through a scan?

On a Siemens S7-1200 or S7-1500, not by default. The CPU copies inputs into a snapshot, the process image, and your logic reads that, unless you add :P to a tag. On Rockwell Logix, yes. I/O updates separately from the logic, so Rockwell suggests copying an input to a buffer tag if it must hold still.

What happens if two rungs write the same output?

Each writes the bit when it runs, so the last one in the scan decides. That's why a stray coil further down can seem to kill a rung that's plainly true.

What does the watchdog do?

It catches a scan that runs too long. An S7-1500's maximum cycle time is 150 ms by default, and going over it stops the CPU unless OB 80 is there to handle it. On Logix, each task's watchdog is 500 ms by default, and going over it is a major fault.

Ladder logic and Structured Text questions

Write me a start and stop circuit

This seal in is one of the most useful ladder programming examples to know by heart. The pump's own bit holds it on after Start is released, and both paths go through Stop, so Stop always wins.

   |     Start         Stop     Permit    PumpOn  |
   +--+--| |--+--------|/|-------| |-------( )----+
   |  |       |                                   |
   |  | PumpOn|                                   |
   |  +--| |--+                                   |
#PumpOn := (#Start OR #PumpOn) AND NOT #Stop AND #Permit;
PumpOn := (Start OR PumpOn) AND NOT Stop AND Permit;

Here, Stop is ON when pressed. Wire the button normally closed and the test flips. Then try the fault where Stop doesn't stop it.

What's the difference between a seal in and a latch?

A seal in uses a normal coil, so it drops out when the rung goes false. A latch stays set until something clears it: OTL and OTU on Rockwell, S and R on Siemens. Rockwell's prescan, a special scan going into Run mode, clears a normal coil but leaves a latched bit alone. So could a latched run command restart something at power up?

What is a one shot?

A one shot is a pulse lasting one scan when a signal turns on, so one event isn't acted on every scan. Rockwell has ONS and OSR, Siemens has P_TRIG. Practise with a counter.

When would you use Structured Text rather than ladder?

Give reasons, because Siemens' own guideline says each language has its advantages. Structured Text tends to suit maths, arrays and loops. Ladder is easy to follow online when someone's tracing a fault. And ask who has to maintain it.

Timer and counter questions

What's the difference between TON, TOF and a retentive timer?

An on delay, TON, turns on once the input has been on for the preset, and resets when the input drops. An off delay, TOF, stays on for the preset after the input goes off. A retentive timer keeps its time until you reset it: RTO and RES on Rockwell, TONR and R on Siemens. Watch the names, because Rockwell's TONR, used in Structured Text, is a plain on delay.

How would you write a five second on delay?

// Delay is a TON_TIME instance in Static
#Delay(IN := #Start, PT := T#5s);
#PumpOn := #Delay.Q;
// Delay is an FBD_TIMER tag
Delay.PRE := 5000;
Delay.TimerEnable := Start;
TONR(Delay);
PumpOn := Delay.DN;

Logix timers always count in milliseconds, so 5000 is five seconds. Practise with the on delay mission.

How does a counter work, and why might it miss counts?

It adds one when its input goes from false to true, not while it stays true. At the preset, DN on Rockwell or Q on Siemens comes on. A normal counter only sees what the scan sees, so very short pulses can be missed. Siemens points you to high speed counters for those.

Siemens and Allen-Bradley PLC interview questions

If the job's on one brand and you know the other, expect to be asked how you'd get on. Here's the S7-1200 and S7-1500 against Logix controllers.

SiemensRockwell (Allen-Bradley)
SoftwareTIA Portal with STEP 7Studio 5000 Logix Designer
LanguagesSCL, LAD, FBD, GRAPH on S7-1500Structured Text, ladder, FBD, SFC
StructureOBs call FCs and FBsTasks hold programs, programs hold routines
Reusable codeFB with instance data blockAOI
Timer presetT#5s5000, in milliseconds

What's the difference between an FC and an FB?

An FC keeps nothing between calls. An FB keeps its values in an instance data block, so it suits anything that has to remember, such as a pump's state.

What's the difference between controller and program scoped tags?

Every program can read and write a controller scoped tag. A program scoped tag is hidden from other programs, so you can reuse its name.

Fault finding questions

A motor won't start. How do you find the fault?

Say your steps in order. Check the alarms and that the controller is running, then go online. If the output is off, read the rung backwards from the coil to find what's false. If it's on, the problem is likely outside the program: the card's LED, a fuse, wiring, the contactor, the overload. There's more in the PLC troubleshooting guide.

The PLC has stopped. Where do you look?

On Siemens, the CPU's diagnostics buffer, which lists events in order. On Logix, the Major Faults tab in Controller Properties. Note the fault down before you clear it.

What is forcing, and what are the risks?

A force overrides a real value, which helps with testing and checking wiring. Rockwell warns it can cause unexpected machine motion that could injure people, and says to use forces only as a temporary measure. So work out what will move, keep people clear and remove it afterwards.

A pump keeps starting and stopping. Why?

Likely a reading wobbling around one switching point. The fix is hysteresis: start low, stop high, remember what you're doing in between. Try the pump chatters.

Safety questions

Can an emergency stop run through a normal PLC program?

Expect the answer to be no. A standard program can know a stop has happened, but the safety function belongs in safety rated equipment, such as a safety relay or fail safe controller, assessed to standards such as ISO 13849-1 or IEC 62061. And releasing the stop shouldn't restart anything. Siemens' example needs an acknowledgement and then a press of Start, the logic in stop means stop.

What are stop categories?

They come from IEC 60204-1. Category 0 removes power at once. Category 1 brakes under power, then removes it. Category 2 stops under control and keeps the power on. The risk assessment decides which you need, and an emergency stop has to be category 0 or 1, not 2.

What do dual channel and feedback monitoring mean?

Dual channel means two separate signal paths, and the safety hardware checks they agree. Feedback monitoring, sometimes called EDM, reads a contact on each contactor to prove it dropped out, so a welded one blocks the restart. Try contactor feedback.

Practical questions for technicians

For a PLC technician role, expect more about the machine than the code.

How do you make a machine safe before working on it?

Follow the site's procedure: isolate every energy source, lock it off, release stored energy and test that it's dead. In the US, OSHA's lockout/tagout standard is 29 CFR 1910.147. In the UK, look to the HSE and the Electricity at Work Regulations 1989.

A sensor's light is on but the PLC input is off. What do you check?

Work along the signal. Is the input card's LED on? If not, check the wiring, fuse and sensor. If it is, check the tag mapping, the module's status and whether anything's forced.

Controls engineer interview questions

What's the difference between an interlock and a permissive?

The words vary by site. Commonly, a permissive must be true before something may run, and an interlock blocks or stops it when a condition goes bad. Say that, then ask what they mean by it here.

How would you program ten identical pumps?

One reusable block, called once per pump: an FB with instance data on Siemens, an AOI on Rockwell, with a UDT for the data. Then a fix goes in one place. See one block for two pumps.

For hiring managers: why a practical test shows more

If you're on the other side of the table, you'll have spotted the problem. Everything above is on a public web page. Reciting it tells you what someone has read, not what they do when a machine misbehaves and they don't know why yet.

So keep your questions, but add a PLC programming test built around doing the work. PLC Gym's is held in person with one of your engineers, who watches and then talks it through with the candidate. What's in it stays off this page, for obvious reasons. It runs on PLC Gym's simulator, not your controllers, so it won't show whether someone knows your plant.

How to get ready

Reading only goes so far. Test yourself with the PLC quiz, then work through the PLC programming exercises until you can explain why each fix works. For the wider picture, start with PLC programming. The first few missions and the first module's questions are free, and the rest is on Pro.

PLC Gym is a simulator in a browser, not TIA Portal or Studio 5000, and it isn't made by or affiliated with Siemens or Rockwell. It doesn't do hardware configuration, networks, real I/O or RSLogix 500. If the job needs those, get time on the real software and a real controller.

And in the interview, if you don't know, say "I haven't done that, but here's how I'd find out" rather than bluff.

Sources

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