How to wire 2, 3, and 4-wire analog transmitters
Wiring 2-wire, 3-wire, and 4-wire transmitters to a PLC comes down to how the device gets power vs. how it sends the 4-20 mA signal. The main gotcha is mixing them up — you’ll either get no reading or potentially damage the analog input card.
2-Wire Transmitter
Power and signal share the same two wires. This is the go-to for most field instruments because it’s simple and loop-powered.
24V DC (+) from power supply → Transmitter (+)
Transmitter (-) → PLC analog input (+)
PLC analog input (-) → 24V DC (-) from power supply
Think of it as one continuous loop: power supply → transmitter → PLC → back to power supply.
3-Wire Transmitter
These have separate wires for power+ and signal+, but share a common ground. Common on devices that need more current than a 2-wire loop can provide.
24V DC (+) → Transmitter V+
24V DC (-) / Ground → Transmitter GND
Transmitter Signal Out → PLC analog input (+)
PLC analog input (-) ties to the same ground
4-Wire Transmitter
Fully isolated — power and signal are completely separate. Used for higher-power devices like analyzers or flowmeters.
Separate 24V DC supply → Transmitter power terminals V+ and V-
Transmitter signal terminals (+ and -) → PLC analog input (+ and -)
Quick tips so you don’t fry anything
Match your PLC card: Some input cards source power for 2-wire loops, others are passive. Check if you need an external supply.
4-20 mA advantage: 4 mA = live zero, so 0 mA means broken wire or dead sensor — instant fault detection. It’s also noise immune and runs long distances without voltage drop.
Cabling: Always use shielded twisted pair to kill EMI from motors/VFDs nearby.
Scale it: Verify your PLC analog module is set for 4-20 mA in hardware jumpers AND software scaling before powering up.
Mixing up 2-wire and 4-wire is the #1 way to blow an input channel. If you’re unsure, meter it out with the transmitter disconnected first.
Need help figuring out which type your specific sensor is?<!--MD-->How do I tell if my transmitter is 2-wire, 3-wire, or 4-wire by looking at it?<!--/MD-->