Wiring 6 — 4-20 mA Analog Input Loop
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Briefing
The 4-20 mA current loop is the industrial standard for analog signals over long cable runs. **Why current, not voltage?** Cable resistance doesn't drop the signal — only the current matters. **Loop-powered transmitter circuit:** `24 V (+) → TX+ → internal electronics → TX- → 250 Ω burden → 24 V (−)` The 250 Ω burden resistor converts 4-20 mA into 1-5 V for a voltage-input analog card. Many modern PLC analog cards have the burden built in — check your spec sheet. Wire the pressure transmitter to analog input **%IW0**.
Objectives
- Connect PSU +24 V to transmitter TX+ terminal
- Connect transmitter TX− to the 250 Ω burden resistor
- Connect burden resistor output to PLC %IW0 analog input
Hints
- 4 mA = live-zero (sensor connected but measuring zero). 0 mA = broken wire.
- With a 250 Ω burden: 4 mA → 1.0 V, 20 mA → 5.0 V.
I/O Table
Inputs
TX_PLUSTransmitter + terminal (PSU +24 V side)
INT · %IW0
BURDEN_OUTBurden resistor output → PLC analog input
INT · %IW1
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