--- title: Analyser Outputs: 4-20 mA, Relays and Modbus Explained url: https://www.sarveshanalytics.com/learn/analyser-outputs-4-20-ma-relays-and-modbus.html description: How a gas analyser sends its reading to a control system: 4-20 mA loops, live zero, fault levels, relay alarms and failsafe logic, and Modbus RTU over RS-485. updated: 2026-11-03 source: html --- Learn · Installation and integration # Analyser outputs: 4-20 mA, relays and Modbus RTU Three kinds of output cover nearly every plant: a continuous analogue signal, discrete alarm contacts and a digital data link. Use each for what it is good at. Beginner 3 min read Lesson 2 of 6[Installation and integration](https://www.sarveshanalytics.com/learn/track-installation.html) Helpful first: [Where to mount a gas analyser: panel, shelter or field](https://www.sarveshanalytics.com/learn/where-to-mount-a-gas-analyser.html) Quick answer A 4-20 mA output sends the reading as a current, with 4 mA for the bottom of the range and 20 mA for the top; the live zero lets the system detect a broken wire. Relays signal alarms and faults, wired so they fail safe. Modbus RTU sends digital values and status over RS-485. ACCURRA-S offers configurable 4-20 mA, relay alarms and Modbus RTU. Updated 3 November 2026 · Figures from Sarvesh datasheets **After this lesson you can** - Convert a 4-20 mA signal to a reading and back - Explain live zero and fault levels - Wire relays so they fail safe - Describe what Modbus RTU adds ## 4-20 mA A current loop carries the reading as a current between 4 and 20 mA, which is linear with the configured range. Current does not droop along long cables the way voltage does, so it travels hundreds of metres reliably. reading = low + (current - 4) / 16 x (high - low) **Worked example** An oxygen analyser ranged 0 to 25 % outputs 12 mA. The reading is 0 + (12 - 4) / 16 x 25 = 12.5 %. **Live zero.** The lowest value is 4 mA, not 0, so a reading of 0 mA means the loop is broken. Many systems treat below about 3.6 mA or above about 21 mA as a fault, in line with the NAMUR recommendation. Configure the control system to alarm on those values rather than treat them as readings. ## Relays A relay output is a dry contact that opens or closes on an alarm: a low alarm, a high alarm, or a fault. ACCURRA-S has relay alarms with configurable set points. Wire them so that loss of power or a wire break puts the system in a safe state, for example normally energised relays that de-energise on alarm. This is called failsafe wiring and is explained further in the glossary: [Failsafe relay](https://www.sarveshanalytics.com/learn/glossary.html#t-failsafe-relay). **Common mistake** Wiring an alarm contact normally open so that a broken wire looks the same as no alarm. Prefer a circuit where a break signals an alarm. ## Modbus RTU Modbus RTU is a simple serial protocol, usually over a two-wire RS-485 bus, in which a master asks slaves for values held in numbered registers. It lets the control system read the concentration, alarm state, fault codes and diagnostics, and even start a calibration, in digital form without conversion error. Each device has an address; multiple analysers share one bus. Use shielded twisted pair, terminate the bus at both ends and keep to one master. ## Which to use | Need | Output | |---|---| | Continuous value to a controller or recorder | 4-20 mA | | Alarm that trips an interlock or horn | Relay, failsafe | | Several values, diagnostics, remote calibration | Modbus RTU | | Redundancy for a critical measurement | 4-20 mA plus relay plus Modbus | Fault handling matters as much as the reading: decide what each output should do when the analyser is in calibration, warm-up or fault. See [Connecting a gas analyser to PLC, DCS and SCADA](https://www.sarveshanalytics.com/learn/connecting-an-analyser-to-plc-dcs-and-scada.html). The ACCURRA-S dual-range 4-20 mA output with its range ID relay. The ranges shown are an example; each range is set over the full measuring range. **In short** - 4-20 mA carries the reading; 4 mA is the live zero. - Treat below 3.6 mA or above 21 mA as a fault. - Relays should fail safe. - Modbus RTU adds digital values and diagnostics over RS-485. ## Check your understanding Answer in your head or on paper first, then open the answer. Recalling an answer teaches more than rereading. **An analyser ranged 0 to 100 ppm outputs 8 mA. What is the reading?** 25 ppm: (8 - 4) / 16 x 100. **Why does a 4-20 mA loop use 4 mA for zero?** So a broken wire (0 mA) can be told apart from a zero reading. **What is failsafe relay wiring?** A circuit where loss of power or a broken wire produces the alarm state, not the healthy state. ## Key terms - [4-20 mA](https://www.sarveshanalytics.com/learn/glossary.html#t-4-20-ma) - [Modbus RTU](https://www.sarveshanalytics.com/learn/glossary.html#t-modbus-rtu) - [Failsafe relay](https://www.sarveshanalytics.com/learn/glossary.html#t-failsafe-relay) - [RS-485](https://www.sarveshanalytics.com/learn/glossary.html#t-rs-485) - [Live zero](https://www.sarveshanalytics.com/learn/glossary.html#t-live-zero) ## Frequently asked questions **What is a 4-20 mA signal?** A current that represents the reading, 4 mA at the bottom of the range and 20 mA at the top. **Which outputs does ACCURRA-S have?** Configurable 4-20 mA, relay alarms and Modbus RTU, according to the catalogue. **What is the maximum cable length for 4-20 mA?** Hundreds of metres, limited by the loop's compliance voltage and wire resistance. **Can Modbus start a calibration?** On models that support it; ask for the register map. ## Related products [ ### ACCURRA-S Online analyser for trace, percent and purity O2, CO, CO2, CH4 and dew point; can measure two gases from one sample. View ACCURRA-S ](https://www.sarveshanalytics.com/safe-gas-analyser.html) ## Where this applies - [Gas purity certification and cylinder filling](https://www.sarveshanalytics.com/applications/gas-purity-certification-and-cylinder-filling.html) ## Where to go next - [Connecting a gas analyser to PLC, DCS and SCADA](https://www.sarveshanalytics.com/learn/connecting-an-analyser-to-plc-dcs-and-scada.html)