4-20mA vs RS485 Modbus vs HART: Which Pressure Transmitter Output Should You Choose?

Last Updated: 2026-11-09

Quick Answer: Which Output Signal Should You Choose?

Choose 4-20mA when each pressure transmitter feeds an analog input on a PLC, DCS or controller, especially for control loops and long cable runs. Choose RS485 Modbus when you need many transmitters on one cable into a gateway, RTU, data logger or IoT platform. Choose 4-20mA + HART when the plant already runs 4-20mA but your maintenance team wants remote range changes and diagnostics without new wiring. Voltage outputs (0-10V, 0.5–4.5V) only make sense for short cables inside a machine or OEM product.

If you are still deciding between a bare sensor and a conditioned transmitter, start with our explainer on pressure sensor vs pressure transmitter, because a raw millivolt sensor has none of the outputs discussed here.

What Is a 4-20mA Pressure Transmitter?

A 4-20mA pressure transmitter converts pressure into a DC current between 4 mA (lower range value) and 20 mA (upper range value). The convention is defined in ISA-50.00.01 and IEC 60381-1, and it has been the standard analog signal in process plants for decades because every PLC and DCS analog input card accepts it.

In the common 2-wire version the same pair of wires powers the transmitter and carries the signal. The transmitter simply regulates how much current it lets through the loop. That gives three practical advantages:

• Current does not drop along the cable. Unlike a voltage signal, the receiver sees the same current that left the transmitter, so cable resistance does not create measurement error as long as the supply voltage is sufficient.
• “Live zero” detects faults. The lower range value (for a 0–16 bar unit, 0 bar) is 4 mA, not 0 mA. A broken wire reads 0 mA, which no healthy transmitter can produce. Under NAMUR NE 43 the normal measuring band is 3.8–20.5 mA, and a failed transmitter drives the loop to ≤3.6 mA or ≥21 mA so the control system can alarm.
• Low noise sensitivity. A low-impedance current loop picks up far less electrical noise from VFDs and motors than a high-impedance voltage input.

The mA-to-pressure formula

P = P_min + (I − 4) / 16 × (P_max − P_min)

Example: a 0–16 bar transmitter reading 12 mA → 0 + (12 − 4)/16 × 16 = 8 bar. A compound range of −1 to 9 bar reading 8 mA → −1 + (4/16) × 10 = 1.5 bar. If your range is in psi or feet of head, convert it first with our pressure units converter.

The loop voltage budget

The one 4-20mA calculation that matters at the design stage is the maximum loop resistance:

R_max = (V_supply − V_min,transmitter) / 0.020 A

For a transmitter with a 12 V minimum, such as the 2-wire current version of the YL4200 industrial pressure transmitter (12–30 V DC), a 24 V supply gives (24 − 12) / 0.02 = 600 Ω. That is the nominal limit at 20 mA. If the transmitter drives an alarm current above 20 mA (for example 21 mA under NAMUR NE 43), size the loop with that maximum instead: (24 − 12) / 0.021 ≈ 570 Ω. Subtract the input resistor of the PLC card (often 250 Ω, which also turns 4–20 mA into 1–5 V) and any barriers, and what is left is your cable allowance. Wiring details for 2-wire, 3-wire and 4-wire hook-ups, and how to check the loop during pressure transmitter calibration, are a separate topic; here the point is that 4-20mA distance is a voltage problem, not a fixed number of metres.

What Is an RS485 Modbus Pressure Transmitter?

An RS485 pressure transmitter sends its reading as digital data instead of a current. RS485 (TIA/EIA-485) is only the electrical layer: a balanced two-wire pair (A/B) that tolerates noise and long distances. The language spoken on top is almost always Modbus RTU, where a master (PLC, RTU, gateway or PC) polls each transmitter by address and reads the pressure value from a register.

What you gain:

• Many devices on one cable. A standard RS485 segment supports 32 unit loads, and Modbus addresses run from 1 to 247, so a tank farm or pump station can bring 10–30 transmitters back on one cable.
• No analog scaling error. The value arrives as a number (often in engineering units), so there is no 4-20mA-to-ADC conversion error at the receiver.
• Easy IoT integration. Most 4G/LoRa gateways and edge devices read Modbus RTU directly, which is why RS485 dominates remote monitoring. Our guide to level transmitters for IoT applications shows the same logic for level points.

What it costs you:

• Four wires. RS485 transmitters are not loop-powered; they need separate power (5–30 V DC on the YL4200 and YL0500 RS485 versions) plus the A/B data pair.
• Commissioning discipline. Every device needs a unique address, matching baud rate and framing (9,600 baud, 8 data bits, no parity, 1 stop bit is a common default), 120 Ω termination at both ends of the bus, and a daisy-chain topology rather than a star.
• A register map. Which register holds pressure, whether it is an integer or a 32-bit float, and the byte order differ between models. Ask for the Modbus register map before you write PLC code.
• Slower update per point. A master polling 30 devices at 9,600 baud refreshes each point a few times per second at best, which is fine for monitoring but not for fast pressure control loops.

What Is HART, and Is It an Output Signal?

HART (Highway Addressable Remote Transducer), maintained by the FieldComm Group, is a digital signal superimposed on the 4-20mA loop. It uses frequency-shift keying based on the Bell 202 standard, 1,200 Hz for a binary 1 and 2,200 Hz for a 0, at 1,200 bit/s. Because the tones average to zero, the analog 4-20mA value is undisturbed and any ordinary analog input still reads the transmitter normally.

So a “HART pressure transmitter” is really a 4-20mA transmitter with a digital side channel. Through that channel a handheld communicator, asset-management software or HART-enabled I/O can:

• re-range the transmitter (change LRV/URV) and adjust damping without opening the housing;
• read secondary variables such as sensor temperature;
• read device status and diagnostics, tag, serial number and calibration data;
• run loop tests by forcing a fixed output current.

The practical requirement is loop resistance: HART communication needs at least 230 Ω in the loop, so a 250 Ω resistor is standard. On long runs, total cable capacitance also limits the digital signal (the FieldComm Group application guide gives the calculation), even when the DC budget is fine. Check the voltage budget again, because that 250 Ω uses 5 V of your supply at 20 mA. HART also allows multidrop wiring with the current fixed at 4 mA, but in pressure applications it is rarely used because you lose the analog signal and update rates fall.

WirelessHART (IEC 62591) is the wireless version of the same protocol; it is a different product category from the wired HART transmitters discussed here.

Not sure which output your PLC, RTU or gateway expects? Send us the receiving device model and the pressure range through our quote form and we will match a 4-20mA pressure transmitter, an RS485 version or a HART version to it before you order.

4-20mA vs RS485 vs HART: Head-to-Head Comparison

The key decision is usually wiring architecture, not accuracy. One analog channel per transmitter suits a control cabinet with spare AI cards. One shared bus suits a site where trenching cable to every point is the expensive part.

Which SIBXSUN Pressure Transmitters Offer Which Output?

The table below is taken from the model specifications on our product pages. Output is configured per order, so a model listing several outputs ships with the one you specify. Modbus RTU is stated explicitly on the YL2088 and YLEX01 datasheets; for the other RS485 versions, confirm the protocol and register map with us before ordering.

Two patterns stand out. First, the general-purpose industrial models (YL805, YL4200, YL0500) cover analog and RS485 in the same body, so you can standardise on one model across control and monitoring points. Second, HART sits on the smart models (YL2088, YL0500, YL6200, YL6300), which also have the tightest accuracy. For remote sites with no cable at all, the YL4500-YLW5100 transmitter reports over LoRa, NB-IoT or 4G instead of a wired output.

How to Choose: Five Questions Before You Order

1. What is the receiving device? An analog input card means 4-20mA. A Modbus master, RTU or 4G gateway means RS485. A DCS with HART-enabled I/O or asset-management software means 4-20mA + HART.
2. Is the point used for control or monitoring? Fast control loops (pump speed, pressure regulation) favour 4-20mA because the signal is continuous and needs no polling. Monitoring points tolerate RS485 update rates.
3. How many points share a route? One or two points: 4-20mA is simplest. Ten points along a pipeline or tank row: one RS485 cable is usually cheaper than ten twisted pairs.
4. Is there a hazardous area? Loop-powered 4-20mA (with or without HART) is the conventional choice with barriers. Check the explosion-protection rating of the exact model, as covered in our pressure transmitter selection guide.
5. Who will maintain it? If technicians re-range transmitters often, HART saves trips to the field. If the site has no HART tools, HART electronics add cost without benefit.

Mixed answers are normal. A water treatment plant might use 4-20mA on filter differential pressure that drives backwash, and RS485 on distribution monitoring points; see pressure transmitters for water treatment plants for typical ranges per process stage.

Common Ordering Mistakes with Output Signals

These are the mismatches we see most often when buyers send RFQs:

• “4-20mA” with a 3-wire or 4-wire hookup assumed. Most industrial 4-20mA transmitters are 2-wire loop-powered. If your panel expects a 3-wire sourcing transmitter, say so on the order.
• HART ordered, no 250 Ω resistor in the loop. Many PLC cards have low input impedance; without ≥230 Ω total, the communicator cannot talk to the transmitter even though the analog value is fine.
• RS485 ordered without the register map. The pressure value may be scaled (for example ×100 with a decimal-point register) or stored as a float. Get the map with the quotation, not after delivery.
• Voltage output on a long cable. A 0-10V signal across 100 m of thin cable next to a VFD will drift and pick up noise. Use 4-20mA beyond a few tens of metres.
• Wrong supply for the output. On multi-output models, supply range depends on configuration. On the YL4200, for example: 12–30 V for 4-20mA (2-wire), 6–24 V for voltage (3-wire), 3.3–5 V for I²C and 5–30 V for RS485.

Sourcing Notes: MOQ, Lead Time and What to Send

SIBXSUN supplies pressure transmitters from our partner production lines in China with MOQ from 1 piece for most products, so you can test an RS485 or HART version on one point before rolling it out. In-stock configurations ship in 2–3 days; custom ranges, outputs or connections typically take 15–30 days. Range, output signal, process connection and cable length can be specified per order. Pricing is FOB Ningbo, with CIF available.

For a fast, accurate quotation, include:

• pressure type and range (for example 0–16 bar gauge), medium and temperature;
• output (4-20mA, 4-20mA + HART, RS485 Modbus, 0-10V) and the receiving device;
• supply voltage available on site;
• process connection and electrical connection;
• quantity and destination.

Related Guides

• How to choose a pressure transmitter: 8-step selection guide
• Types of pressure transmitters: gauge, absolute, differential, hydrostatic and smart HART
• Pressure sensor vs pressure transmitter: what’s the real difference?
• How to choose a level transmitter for IoT applications

Frequently Asked Questions

Is 4-20mA better than RS485 for a pressure transmitter?

Neither is better in general. 4-20mA is better when each transmitter feeds one analog input on a PLC or DCS, when the signal must survive long, noisy cable runs, and when you want a 2-wire loop-powered device. RS485 Modbus is better when many transmitters share one cable to a gateway, RTU or IoT logger, because up to 32 standard unit loads can sit on one bus and each reading arrives as a digital value with no analog scaling error. Many plants use 4-20mA for control loops and RS485 for monitoring points.

How do I convert a 4-20mA signal to pressure?

Use P = P_min + (I − 4) / 16 × (P_max − P_min), with I in milliamps. For a 0–16 bar transmitter, 12 mA gives 8 bar and 6 mA gives 2 bar. For a compound range of −1 to 9 bar, 4 mA is −1 bar and 20 mA is 9 bar. A reading below about 3.8 mA or above about 20.5 mA usually means a wiring fault or a sensor out of range rather than a real pressure.

Does a HART pressure transmitter still work with a normal 4-20mA input?

Yes. HART superimposes a 1,200 bit/s FSK signal (1,200 Hz and 2,200 Hz tones) on the 4-20mA current, and its average value is zero, so a standard analog input reads the HART transmitter exactly like a plain 4-20mA device. You only need a HART modem, handheld communicator or HART-enabled input card when you want diagnostics, secondary variables or remote re-ranging. The loop needs at least 230 Ω for HART communication; a 250 Ω resistor is the usual choice.

How far can I run RS485 and 4-20mA cable?

RS485 under TIA/EIA-485 is commonly rated to about 1,200 m (4,000 ft) at baud rates up to about 100 kbit/s, with 120 Ω termination at both ends of the bus. 4-20mA has no fixed length limit; the limit is the voltage budget. The supply minus the transmitter’s minimum voltage, divided by 0.02 A, gives the maximum total loop resistance. With a 24 V supply and a transmitter needing 12 V, that is 600 Ω, which leaves margin for over 1 km of normal instrument cable after a 250 Ω input resistor.

Can I change the output signal after ordering?

Usually not in the field. The output stage is built into the transmitter electronics, so 4-20mA, 0-10V, RS485 and I²C versions are different configurations of the same model. Confirm the output, supply voltage, wiring (2-wire, 3-wire or 4-wire) and, for RS485, the Modbus register map before the order is placed. Through SIBXSUN, output signal is one of the parameters that can be specified per order, together with range, process connection and cable length.

Ready to specify your transmitter? Browse our pressure sensors and transmitters or send your range, output signal and quantity and we will reply with a matched model, datasheet and FOB Ningbo quotation.