A supplier quotes a PLC with 14 inputs and 10 outputs built into one housing, with no slots for extra cards. What type of PLC is it?

A compact (brick) PLC has a fixed number of inputs and outputs in one housing, which suits a small machine with a known I/O count. A modular PLC is the tempting answer because it is the other common form, but it is a rack of cards you choose and add to. A remote I/O rack has no processor of its own, and a safety PLC is defined by its fault detection, not its housing.

The parts list for a panel calls for a ‘16-point digital input card’. What does that mean?

On site, channels are often called points, so a 16-point card has sixteen separate input channels, each writing its own bit. A digital channel has only two states, not sixteen. The sixteen-bit word option is tempting because analogue channels do occupy a word, but this is a digital card, and the number says nothing about its voltage.

Sixteen separate input channels on one card

One input channel that can read sixteen states

An input card that runs on sixteen volts DC

A card holding one sixteen-bit analogue word

Inside a typical digital input card, each channel has its own optocoupler. What does that give the PLC?

The field current lights a small LED and the light switches a phototransistor on the processor side, so there is no copper path between the plant wiring and the electronics. That protects the processor from spikes and transients. It does not make the card match PNP or NPN sensors for you, which is why the wrong pairing never switches, and a digital channel does not read analogue signals.

A faster scan, as light switches quicker than copper

The ability to read 4–20 mA signals on any channel

Automatic matching of PNP and NPN sensors to the card

No copper path, protecting the processor from spikes

Free mock exam

PLCs and automation — Industrial Electrical Practice

Free topic-focused mock exam: all 30 questions on PLCs and automation, drawn from the wider Industrial Electrical bank. 35-minute timer, 70% to pass, worked explanation on every question. Use this when you've taken the full mock once and want to drill the topics you got wrong.

Questions: 30
Time limit: 35 min
Pass mark: 70% · 21/30
Question bank: 30

Free, no sign-up. Retake as many times as you like — the questions are reshuffled, with the answer order changed.

Sample questions

01A supplier quotes a PLC with 14 inputs and 10 outputs built into one housing, with no slots for extra cards. What type of PLC is it?
- AA compact PLC unit
- BA remote I/O rack
- CA modular PLC rack
- DA safety PLC module
Why: A compact (brick) PLC has a fixed number of inputs and outputs in one housing, which suits a small machine with a known I/O count. A modular PLC is the tempting answer because it is the other common form, but it is a rack of cards you choose and add to. A remote I/O rack has no processor of its own, and a safety PLC is defined by its fault detection, not its housing.
02The parts list for a panel calls for a ‘16-point digital input card’. What does that mean?
- AOne input channel that can read sixteen states
- BAn input card that runs on sixteen volts DC
- CA card holding one sixteen-bit analogue word
- DSixteen separate input channels on one card
Why: On site, channels are often called points, so a 16-point card has sixteen separate input channels, each writing its own bit. A digital channel has only two states, not sixteen. The sixteen-bit word option is tempting because analogue channels do occupy a word, but this is a digital card, and the number says nothing about its voltage.
03Inside a typical digital input card, each channel has its own optocoupler. What does that give the PLC?
- AAutomatic matching of PNP and NPN sensors to the card
- BA faster scan, as light switches quicker than copper
- CThe ability to read 4–20 mA signals on any channel
- DNo copper path, protecting the processor from spikes
Why: The field current lights a small LED and the light switches a phototransistor on the processor side, so there is no copper path between the plant wiring and the electronics. That protects the processor from spikes and transients. It does not make the card match PNP or NPN sensors for you, which is why the wrong pairing never switches, and a digital channel does not read analogue signals.

Common questions

Is the PLCs and automation mock exam free?

Yes — 100% free, no sign-up needed. Questions are pulled at random from a substantial bank, with a worked explanation on every question after you submit.

Can I retake the mock exam?

Yes. Every attempt picks a different random selection and reshuffles the options, so each retake gives you genuinely new questions. No limit on retakes.

Does this count towards the real qualification?

No — this is independent practice material. To gain the actual qualification you need to sit the official assessment through an approved provider.