Many instruments, gas detectors among them, offer a 4-20 mA loop and an RS485 Modbus port at the same time. They are not rivals; they suit different receivers. The choice is about what the reading has to reach and how many devices share the cable.
| 4-20 mA | RS485 Modbus RTU | |
|---|---|---|
| Carries | One value, as a current | Many values per device, as numbers |
| Devices per cable | One per loop | Several on one bus, each with an address |
| Fault visibility | A broken wire reads 0 mA, below the live zero | A device that stops answering is reported as missing |
| Fits | An existing PLC, DCS or alarm panel with analogue inputs | A controller reading several devices on one cable |
When an installed PLC or alarm panel has to see the value, the loop drops into it without changes.
When one cable has to carry several detectors or sensors, or more than one value from each.
A gas detector can feed its loop to the existing panel and its Modbus port to a controller for the record and the remote alert.
Genesis reads RS485 Modbus devices on a shared bus on any of its four ports, as well as analogue, I2C and 1-Wire sensors.
Our gas detectors offer 4-20 mA and RS485 Modbus at the same time, plus two relays, so the existing panel and the controller can both be fed.
Linearly between the value at 4 mA and the value at 20 mA. The 4-20 mA scaling calculator does it both ways.
An RS485 bus runs to about 1200 m with shielded twisted pair and termination. A 4-20 mA loop runs long distances as long as the loop supply covers the cable resistance.
Tell us what has to be measured or switched, where, and what it has to report into. We say which approach fits, including when it is not ours.
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