RS485 Modbus RTU and 4–20mA solve different integration problems. A 4–20mA loop sends a continuously scaled analogue value to a compatible input. RS485 Modbus RTU lets a digital host read supported registers over a serial network. Select from the PLC, DCS, SCADA or recorder requirements.
Quick decision table
| Requirement | 4–20mA | RS485 Modbus RTU |
|---|---|---|
| One primary value | Often straightforward | Possible, but may add setup |
| Several supported values | May need several analogue channels | Can carry supported registers over one link |
| Receiving hardware | Compatible analogue input card | Compatible serial port or gateway |
| Commissioning focus | Scaling, load, isolation and wiring | Address, baud rate, polarity, register map and scaling |
Choose 4–20mA when simplicity matters
Confirm which variable the output represents, its engineering range, isolation, maximum load and channel count. Label the same scaling in the controller, PLC and commissioning record. Do not assume a second channel is present or that it carries temperature.
Choose RS485 Modbus RTU for supported digital data
RS485 is useful when the plant already uses serial communication or needs more supported data than available analogue channels. Slave address, baud rate, parity, stop bits, A/B polarity, termination, register addresses and scaling must match the host. Having RS485 terminals does not make two devices automatically compatible.
Current Tenber examples
| Model | Documented interface | Selection direction |
|---|---|---|
| PH-303 | Two isolated 4–20mA outputs | pH/ORP projects needing two analogue channels without currently documented RS485 |
| PH-302 | One isolated 4–20mA output, RS485 Modbus RTU and two relays | Panels needing analogue, serial communication and relay functions |
| EC-702 | Two isolated 4–20mA outputs and RS485 Modbus RTU | Conductivity projects needing two analogue channels or digital communication |
Signal-selection checklist
- List the values the control system must receive.
- Confirm analogue input channels and serial interfaces.
- Define update rate, cable route, network size and panel standard.
- Match the requirement to a documented model.
- Confirm channel mapping or current Modbus register information.
- Define relay logic separately from measurement transmission.
- Plan invalid-signal and communication-loss behavior.
- Test display, transmitted value and alarm action.
Common integration mistakes
- Assuming every family model has the same communication functions.
- Connecting RS485 without confirming A/B, address, baud rate and scaling.
- Using 4–20mA without documenting its value and range.
- Building automatic control before defining sensor-failure response.
Commissioning record
For 4–20mA, record displayed value, expected current, measured current and PLC value. For Modbus, record address, settings, register, raw response, scaling and host value. Test alarm direction and failure behavior separately.
Information to send before quotation
Send the measured parameter, liquid, range, temperature, pressure, sensor, mounting, required 4–20mA channels, RS485 host settings, relay logic, power supply, quantity and destination country.
Send your control-system requirements for model selection
For a controller with 4–20mA output, see our industrial pH controller selection guide.
Current model examples and commissioning record
| Model | Documented interface | Selection direction |
|---|---|---|
| PH-303 | Two isolated 4–20 mA outputs. | pH/ORP projects needing two analogue channels. |
| PH-302 | One isolated 4–20 mA output, RS485 Modbus RTU and two relays. | Compact pH/ORP panels needing analogue, serial and relay functions. |
| EC-702 | Two isolated 4–20 mA outputs and RS485 Modbus RTU. | Conductivity projects needing analogue channels or digital communication. |
- List the values the PLC, DCS or SCADA must receive.
- Confirm analogue channels, serial settings, cable route and network size.
- Match the requirement to the exact documented model.
- Record displayed value, transmitted value, register/scaling and alarm behavior during commissioning.
Using both interfaces is appropriate only when the selected controller documents both functions and the project has a clear reason for each. Obtain the current register information; never infer addresses from a generic Modbus template.
Compare the PH-302 RS485 Modbus RTU option, the PH-303 dual 4–20 mA option and the EC-703 conductivity controller against the PLC input, distance, relay logic and required measurement variables.
Read the pH wiring checklist or send your control-system requirements for model selection.