| Deciding factor | 4-20 mA analog loop | Modbus RTU |
|---|---|---|
| Payload per pair of wires | 1 measured variable | many registers |
| Power distribution | 2-wire loop powers the transmitter | usually separate supply |
| Reach without repeater | up to ~1 km | up to ~1200 m over RS-485 |
| Devices on one cable run | 1 device per pair | up to 247 addresses |
| Failure signalling | down-scale and up-scale detection | CRC, timeout, exception codes |
| Extra data channel | HART overlay on the same pair | native digital registers |
Do the 4-20 mA loop and Modbus RTU have to compete?
They often share one cabinet: hard-wired current loops carry the few variables that control equipment must never lose, while the serial bus adds the longer list of readings, alarms and device status that nobody wants to run copper for.
What does a loop-powered transmitter give up?
Because the same two wires must carry both power and the measurement, only one analog value travels, and every extra parameter goes through the HART overlay or a service tool instead of arriving at the control system continuously.
Decision rule
Keep the 4-20 mA loop where one variable must survive long cable, plain faults and mixed backgrounds, because a broken wire reads at end of scale and the loop itself can power the transmitter; pick Modbus RTU where the same twisted pair has to carry many variables and device diagnostics, and commissioning can afford to manage addresses, baud rate and termination.
All figures on this page are illustrative sample values, not measured specifications.