4-20 mA Sensors to IoT: Reading Existing Transmitters

A pressure transmitter on the pump header, a level transmitter on the tank, a flowmeter on the main line. Many plants have dozens of these that have worked for years, yet their readings stay on a panel indicator or inside a PLC and never reach a dashboard.

They do not need replacing. Nearly all of them send an analog 4-20 mA signal, and that signal can be read. This guide explains how to connect 4-20 mA sensors to IoT: the signal basics, scaling, a 4-20 mA to Modbus RTU converter, and the wiring and noise mistakes that most often produce bad data.

How the 4-20 mA signal works

A transmitter turns a process variable, such as pressure, into a DC current: 4 mA is the bottom of the range (0%), 20 mA the top (100%), linear in between. The standard has lasted because:

• Current does not fade along the cable. In a closed loop the current at the panel equals the current leaving the transmitter, so cable resistance does not change the reading while the supply voltage is sufficient.
• It resists electrical noise better than a small voltage signal, which suits long runs.
• Live zero. With 4 mA as the minimum, a broken wire shows up as current near 0. Many transmitters can also signal a sensor fault by going below about 3.6 mA or above about 21 mA; the datasheet has the exact values.

2-wire (loop-powered) transmitters draw power from the loop, usually from a 24 VDC supply. 4-wire transmitters have their own supply and source the current themselves. Some also carry a digital HART signal on top of the current; a basic analog converter reads only the current.

Turning current into a process value

A dashboard needs bar, metres, or m³/h, not mA:

value = low limit + (current - 4) ÷ 16 × (high limit - low limit)

For a pressure transmitter ranged 0 to 10 bar:

Current	Percent of range	Pressure
4 mA	0%	0 bar
8 mA	25%	2.5 bar
12 mA	50%	5 bar
16 mA	75%	7.5 bar
20 mA	100%	10 bar
Near 0 mA	Out of range	Broken wire or dead transmitter

Take the configured range from the nameplate or the transmitter settings, not from the sensor's maximum: a 0 to 16 bar transmitter may well be ranged 0 to 10 bar.

Converters do not always report mA either. Some give raw ADC counts, some microamps (12000 for 12 mA), some can scale to engineering units. Decide where scaling happens (converter, gateway, or platform), do it in one place only, and write it down.

Three ways to bring a 4-20 mA signal into IoT

Situation on site	Route	Notes
Feeds only a panel indicator, or nothing	4-20 mA to Modbus RTU converter	The most common route; one module reads several transmitters.
Already wired into a PLC	Read the value from the PLC over Modbus	The loop stays untouched and the value is already scaled.
Used for control, but the PLC cannot be read	Signal isolator or splitter	One input, two isolated outputs: existing system and converter.

The second route is covered in our guide to connecting legacy Modbus RTU PLCs to the cloud. The third suits control or interlock loops, where adding a device in series without doing the sums can upset the process.

Connecting a 4-20 mA to Modbus RTU converter

These converters, often sold as Modbus analog input modules, read several 4-20 mA channels and expose them as Modbus RTU registers over RS-485:

1. List the transmitters: tag, variable, range, 2-wire or 4-wire, and what powers each loop.
2. Choose a converter with enough channels, the right input type (active or passive), and galvanic isolation if transmitters sit on different earthing points.
3. Mount it in the nearest panel on a stable 24 VDC supply and set the slave address, baud rate, and parity.
4. Wire it onto the gateway's RS-485 bus as one more slave beside power meters or other Modbus devices. Register basics are in Modbus RTU vs Modbus TCP.
5. Validate the dashboard against the local indicator or a calibrator at several points across the range.

An illustrative loop voltage budget

Every device in a 2-wire loop takes some voltage. Say the supply is 24 V, the transmitter needs at least 12 V (check the datasheet), and the converter input is 250 ohms. At 20 mA that input drops 5 V, leaving 7 V. Add a 250 ohm panel indicator and only 2 V remain for the cable. One more device and the transmitter runs short: near full scale the current stalls below 20 mA, so the reading stops rising while the process keeps going.

The most common wiring and noise mistakes

• The loop is powered twice, or not at all. A 2-wire transmitter needs an active input or a separate loop supply; a 4-wire one needs a passive input. Get it wrong and you read zero or damage the input.
• Reversed polarity. No current flows, which looks like a broken wire.
• Signal cable run beside motor or VFD output cables. Use shielded twisted pair on a separate route.
• Shield earthed at both ends. Earth it at one end, normally the panel, so it carries no ground loop current.
• Ground loops between a 4-wire transmitter and a converter on different earth points make the value drift. An isolated converter or signal isolator fixes it.
• Damp junction boxes or loose terminals make the value jump when it rains or nearby machines vibrate. In the tropics, check box seals and terminals.

Things to keep in mind

• Hazardous areas. Loops in explosive atmospheres usually run through intrinsically safe barriers. Any change must follow the area's certification rules and be done by an authorised technician.
• Safety loops and interlocks are best left alone. Take the value from the PLC; if an isolator is still needed, fit it only with the safety system owner's approval.
• Accuracy is a chain of transmitter, converter, and scaling. A converter cannot fix a drifted transmitter, so calibration still matters.

Frequently asked questions

How long can a 4-20 mA cable be?

As long as the loop voltage budget allows, runs of several hundred metres are common. Work out the round-trip cable resistance and check the transmitter still gets its minimum voltage at 20 mA.

How can I spot a broken wire on the dashboard?

Set the lower safe limit slightly below the 0% point of the range. A broken wire pulls the value well below it (in the 0 to 10 bar example, 0 mA reads about -2.5 bar) and raises an alarm, as long as the converter does not clamp values below 4 mA to zero.

Can a 0-10 V signal be read the same way?

Yes, with a converter that has voltage inputs. Voltage signals pick up noise more easily, so keep those runs short.

4-20 mA sensors to IoT with INCLUDE

IncludeGateways reads Modbus RTU devices over RS-485, including 4-20 mA converters, and forwards the data to the INCLUDE platform, over 4G or LoRa where a site has no internal network. To be clear, the gateway does not take a 4-20 mA signal directly; analog signals come in through a separate converter.

Each value then appears as a chart with its own safe range. A reading outside it raises an alarm that can be forwarded to Telegram, including the broken-wire alarm from the FAQ. Setting limits without false alarms is covered in IoT sensor alarm thresholds. For PLC integration or many points at once, the INCLUDE system integration team can help.