Inputs and outputs

What inputs and outputs are

• An input carries information from the field to the PLC: a pressed button, a sensor detecting a part, the temperature of an oven.
• An output carries an order from the PLC to the field: start a motor, open a valve, turn on a beacon.

Each input and output has an address that identifies it in the program. On this platform we use a generic style by default: I0.0 is the first digital input and Q0.0 the first digital output. Each brand names its addresses its own way; we will see this in the memory and addressing topic.

Digital and analog

Digital signals

They have only two states: 0 or 1, off or on. They are the most common: push buttons, selector switches, limit switches, proximity sensors, auxiliary contacts.

The most common voltage is 24 VDC, although 110 or 230 VAC inputs also exist. The module decides what is a 0 and what is a 1 using thresholds defined in the IEC 61131-2 standard. For example, a typical 24 VDC input reads 1 from about 15 V (11 V for type 3 inputs) and 0 below 5 V. Between those values the state is not guaranteed, so a loose connection can cause erratic readings.

Analog signals

They represent a continuous value: level, pressure, temperature, speed. The most common are:

• Voltage: 0–10 V (also ±10 V).
• Current: 4–20 mA (also 0–20 mA).
• Temperature sensors wired directly to special modules (RTDs such as Pt100, or thermocouples).

The analog module turns the signal into an integer using an analog-to-digital converter. The program then scales it to engineering units: for example, from 0–27648 to 0–100 % level. The range of that integer depends on the module and the manufacturer.

Why 4–20 mA and not 0–20 mA?

The 4–20 mA current signal is the standard in industrial instrumentation for three reasons:

1. Current is not lost along the cable: in a series loop, the current is the same everywhere, even if the cable is hundreds of metres long.
2. It is more immune to electrical noise than a voltage signal.
3. It has a “live zero”: 0 % of the range is 4 mA, not 0 mA. If the PLC measures 0 mA, it knows something is wrong: a broken wire or an unpowered instrument. Following the NAMUR NE 43 recommendation, values below 3.6 mA or above 21 mA indicate a fault.

Move the level and then simulate a broken wire. Compare what the PLC sees with each signal type:

Tank level: 60%

4–20 mA signal

13.6 mA

Valid signal

0–10 V signal

6.0 V

Valid signal

Simulate a broken wire

PNP and NPN sensors

Inductive, capacitive and photoelectric proximity sensors are usually three-wire devices: two wires for power and one for the signal. According to IEC 60947-5-2, the colours are:

• Brown (BN): +24 V
• Blue (BU): 0 V
• Black (BK): output or signal

The sensor’s output is a transistor, and here is the key difference:

• A PNP sensor connects the signal wire to +24 V when it detects. It supplies current (sourcing).
• An NPN sensor connects the signal wire to 0 V when it detects. It sinks current (sinking).

Sinking and sourcing: how to wire the common

For current to flow, the PLC input has to “close the circuit” on the other side. That is set by the input group’s common (COM):

Sensor	Signal goes to…	Module common	Input type
PNP	+24 V	0 V	Sinking
NPN	0 V	+24 V	Sourcing

Change the sensor type, put an object in front of it and follow the current:

Sensor type

pnpnpn

Object in front of the sensor

Types of digital outputs

Digital outputs are built with three different technologies, and choosing the right one avoids a lot of trouble:

Type	Load type	Switching speed	Service life	When to use it
Relay	AC or DC	Slow (milliseconds)	Limited by contact wear	Mixed loads, isolation, infrequent switching
Transistor	DC only	Very fast	Very long (no moving parts)	Frequent switching, pulses, 24 VDC loads
Triac	AC only	Fast	Long	AC loads that switch often

Like sensors, transistor outputs can be sourcing (they supply +24 V) or sinking (they switch to 0 V).

Each output also has a maximum current per point and per group. Large loads, such as a motor, are never connected directly: they are driven through a contactor or an interposing relay.

Typical actuators

• Contactors: the PLC energises the coil and the contactor switches the motor’s power.
• Solenoid valves: control the air or oil of pneumatic and hydraulic cylinders.
• Pilot lights and beacons: show states and alarms.
• Variable frequency drives: control a motor’s speed. The PLC gives them the run command (digital), the speed (analog) or both over a communication network.
• Interposing relays: adapt a PLC output to another voltage or a bigger load.

Normally open and normally closed buttons

• A normally open (NO) contact closes when the button is pressed. Used for start.
• A normally closed (NC) contact opens when the button is pressed. Used for stop.

Why is the stop button wired NC?

For fail-safe behaviour. Imagine the stop button’s wire breaks:

• If the stop were NO, the machine would keep running and could no longer be stopped with that button. Nobody would notice until they needed it.
• With an NC stop, a broken wire looks exactly like a pressed button: the input drops to 0 and the machine stops. The fault becomes obvious immediately.

This has a consequence in the program that confuses many beginners. With an NC stop button, the input is 1 when nobody touches it, so in Ladder it is examined with an NO contact: the contact conducts while the input is 1 and opens when someone presses the button or the wire breaks.

Physical button	Input at rest	Input when pressed	Contact in the program
Start (NO)	0	1	NO ┤ ├
Stop (NC)	1	0	NO ┤ ├

Try it in the simulatorOpen a preloaded example and experiment yourself.

Summary

• Digital signals have two states; analog signals represent a continuous value.
• 4–20 mA is the instrumentation standard because it resists noise and reveals a broken wire.
• A PNP sensor outputs +24 V and needs the common at 0 V; an NPN outputs 0 V and needs the common at +24 V.
• Relay outputs for mixed loads, transistor for fast DC, triac for AC.
• The stop button is wired NC so that a broken wire stops the machine.