--- title: Connecting a Gas Analyser to PLC, DCS and SCADA url: https://www.sarveshanalytics.com/learn/connecting-an-analyser-to-plc-dcs-and-scada.html description: Bring analyser readings into a control system: scaling 4-20 mA, handling faults and calibration states, alarm set points and deadband, Modbus registers and trends. updated: 2026-11-02 source: html --- Learn · Installation and integration # Connecting an analyser to a PLC, DCS or SCADA system The analyser sends a number. The control system must decide what to do with a number that might be wrong, stale or paused. That logic is part of the instrument design. Intermediate 3 min read Lesson 4 of 6[Installation and integration](https://www.sarveshanalytics.com/learn/track-installation.html) Helpful first: [Analyser outputs: 4-20 mA, relays and Modbus explained](https://www.sarveshanalytics.com/learn/analyser-outputs-4-20-ma-relays-and-modbus.html) Quick answer Scale the 4-20 mA input to the analyser range, alarm on out-of-range currents as a fault, and use the analyser's fault and calibration-in-progress signals to hold or flag the value. Set alarms with a deadband to stop chatter, and use Modbus for diagnostics and status. Decide in advance what the process should do when the analyser is faulty or being calibrated. Updated 2 November 2026 · Figures from Sarvesh datasheets **After this lesson you can** - Scale a 4-20 mA input and detect a fault - Plan alarm set points and deadband - Handle calibration, warm-up and fault states - Use Modbus data sensibly ## Scaling and range In the control system, configure the input range to match the analyser range exactly. A mismatch gives an offset or scale error that looks like a process change. Label the engineering units, and record the range in the loop documentation: [Analyser outputs: 4-20 mA, relays and Modbus explained](https://www.sarveshanalytics.com/learn/analyser-outputs-4-20-ma-relays-and-modbus.html). ## Faults and bad data - Alarm on input current below about 3.6 mA or above about 21 mA as an instrument fault, not as a process value. - Read the analyser's fault relay or status register, and flag the value as bad when it is set. - Apply a timeout for Modbus data: if values stop updating, treat them as stale. - Decide whether control should hold the last good value, switch to manual or trip, when the analyser fails. For a safety function this belongs to the safety design. ## Calibration and warm-up states During calibration the analyser sees test gas, not the process. A control loop that keeps using that reading will react to nothing. Use the analyser's calibration-in-progress signal, or a timer on the auto-calibration command, to hold the output and the alarms for the duration, then release after a settling delay. Do the same after power-up while sensors stabilise: [Auto-Calibration and validation checks for gas analysers](https://www.sarveshanalytics.com/learn/auto-calibration-and-validation-checks.html). ## Alarm set points and deadband - Set alarm points from the process need, such as a limiting oxygen concentration with a margin: [What is limiting oxygen concentration (LOC)](https://www.sarveshanalytics.com/learn/what-is-limiting-oxygen-concentration.html). - Add a **deadband** (hysteresis) of a few percent of range so the alarm does not chatter when the reading hovers at the set point. - Use a short delay to ignore single-sample spikes, but not so long that it defeats a fast protection function. - Separate warning and trip levels. ## What Modbus adds Over Modbus you can read the concentration at full resolution, the alarm and fault states, diagnostic values and stored history, and write a command to start a calibration or change a set point. Keep a register map in the project documents, and test each register at commissioning. ## Document it Record the scaling, the fault handling and the calibration logic. Years later, the person troubleshooting an alarm will need it. **In short** - Match the control-system range to the analyser exactly. - Treat out-of-range current and fault signals as bad data, not process values. - Hold outputs during calibration and warm-up. - Add deadband and a short delay to alarms; document the logic. ## Check your understanding Answer in your head or on paper first, then open the answer. Recalling an answer teaches more than rereading. **What does input current of 2 mA indicate?** A fault or a broken loop, not a process reading, since the live zero is 4 mA. **Why use a deadband on an alarm?** To stop the alarm chattering on and off when the reading hovers at the set point. **What should the control system do during analyser calibration?** Hold the value or flag it, so it does not act on test gas readings. ## Key terms - [PLC](https://www.sarveshanalytics.com/learn/glossary.html#t-plc) - [DCS](https://www.sarveshanalytics.com/learn/glossary.html#t-dcs) - [SCADA](https://www.sarveshanalytics.com/learn/glossary.html#t-scada) - [Hysteresis](https://www.sarveshanalytics.com/learn/glossary.html#t-hysteresis) - [Alarm set point](https://www.sarveshanalytics.com/learn/glossary.html#t-alarm-set-point) - [Modbus RTU](https://www.sarveshanalytics.com/learn/glossary.html#t-modbus-rtu) ## Frequently asked questions **How do I scale a 4-20 mA input?** Set 4 mA to the low end and 20 mA to the high end of the analyser range, in the same units. **What should happen when the analyser fails?** That is a process decision: hold, switch to manual, alarm or trip. Decide it in advance. **Can the control system start an analyser calibration?** Where the analyser supports it by Modbus or a digital input. **How do I avoid nuisance alarms?** Use a deadband and a short delay, and separate warning from trip levels. ## 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 - [Nitrogen blanketing and inerting](https://www.sarveshanalytics.com/applications/nitrogen-blanketing-and-inerting.html) ## Where to go next - [Commissioning an online gas analyser: A Step-by-Step guide](https://www.sarveshanalytics.com/learn/commissioning-an-online-gas-analyser.html)