Field communication: 4-20 mA, HART, Modbus, Profibus and Fieldbus

Why it matters

Every measurement and every valve command has to travel between the field and the control system, often hundreds of metres through noisy, hazardous plant areas. Whether that link is a 4–20 mA pair, HART, Modbus or a fieldbus decides wiring cost, diagnostics, accuracy and how faults show up. Loop-checking, loop power budgets and protocol choices are daily work for instrumentation engineers.

Key ideas

4–20 mA current loop. A transmitter regulates the loop current between 4 and 20 mA in proportion to the measured value; the receiver reads it as a voltage across a resistor (250 Ω gives 1–5 V).

• Why current: the same current flows everywhere in a series loop, so wire resistance and moderate noise pick-up do not change the reading, and long cable runs are fine.
• Live zero: 0 % is 4 mA, not 0 mA, so a broken wire (0 mA) is distinguishable from a true zero, and the 4 mA is enough to power a two-wire transmitter from the loop itself.
• Fault signalling (NAMUR NE 43): normal measurement roughly 3.8–20.5 mA; a current below about 3.6 mA or above about 21 mA indicates a transmitter failure, so the control system can tell "bad value" from "low value".
• Loop power budget: the supply voltage must cover the transmitter's minimum operating voltage plus the drops across the receiver resistor, cable and any safety barrier at the highest current.

HART. Highway Addressable Remote Transducer superimposes a digital signal on the 4–20 mA loop using Bell 202 frequency-shift keying: 1200 Hz for a binary 1 and 2200 Hz for a 0, at 1200 bit/s, about ±0.5 mA peak. The sine waves average to zero, so the analog value is undisturbed. HART gives configuration, calibration (trim), diagnostics and extra variables while the 4–20 mA signal still does the control. The loop needs at least about 230 Ω (normally 250 Ω) for the HART signal to be detected. In multidrop mode the current is fixed at 4 mA and several devices share a pair with digital addresses. WirelessHART carries the same data over a mesh radio network.

Modbus. An open, simple master/slave (client/server) protocol. Modbus RTU sends binary frames with a CRC over RS-485 (multidrop, up to 32 unit loads per segment without repeaters, about 1200 m at low baud rates, addresses 1–247); Modbus ASCII uses readable characters with an LRC; Modbus TCP runs over Ethernet on port 502. Data is organised as coils, discrete inputs, input registers and holding registers, accessed by function codes such as 01 (read coils), 03 (read holding registers), 06 (write single register) and 16 (write multiple registers). There is no built-in time-stamping, data typing or device description, and only one master on a serial line.

PROFIBUS.

• PROFIBUS DP (decentralised periphery): RS-485 at 9.6 kbit/s to 12 Mbit/s; maximum segment length falls as speed rises (about 1200 m at 93.75 kbit/s, 100 m at 12 Mbit/s); token passing between masters and cyclic polling of slaves; widely used for remote I/O and drives.
• PROFIBUS PA (process automation): uses the IEC 61158-2 physical layer (Manchester-coded, bus-powered, 31.25 kbit/s), suitable for intrinsically safe areas, linked to DP through a coupler.

FOUNDATION Fieldbus. H1 uses the same 31.25 kbit/s bus-powered physical layer, up to 1900 m per segment including spurs, with terminators at both ends. Devices contain standard function blocks (AI, PID, AO), so control can run in the field devices, scheduled by the Link Active Scheduler. HSE (High Speed Ethernet) links H1 segments to the host.

Comparison. 4–20 mA: one variable per pair, simple, universal. HART: adds digital data on existing wiring. Fieldbuses: many devices and variables on one pair, full diagnostics, less wiring, but need careful design (power, segment length, termination) and skilled maintenance. Industrial Ethernet (PROFINET, EtherNet/IP, Modbus TCP) dominates at higher levels and increasingly at the field level (Ethernet-APL).

Formulas

I = 4 + 16·(x − LRV)/(URV − LRV)

• I: loop current (mA); x: measured value; LRV, URV: lower and upper range values (same units as x).

V_R = I·R

• Voltage across the receiver resistor (V), I in A, R in Ω.

V_supply ≥ V_tx,min + I_max·(R_receiver + R_cable + R_barrier)

• Loop power budget; I_max: highest loop current to be supported (20 mA, or about 22 mA to allow fault signalling).

R_loop,max = (V_supply − V_tx,min) / I_max

t_bit = 1 / bit rate

• HART: 1/1200 s ≈ 0.833 ms per bit.

Worked examples

Example 1 (standard) — scaling a temperature loop. A transmitter is ranged 0–250 °C and the receiver uses a 250 Ω resistor. (a) What current and receiver voltage correspond to 150 °C? (b) The DCS later reads 2.1 mA. What does that mean?

1. I = 4 + 16 × (150 − 0)/(250 − 0) = 4 + 9.6 = 13.6 mA.
2. V_R = 0.0136 × 250 = 3.4 V.
3. (b) 2.1 mA is below 3.6 mA, outside the live range: it is a fault (open circuit, failed transmitter or loss of power), not a temperature near 0 °C. The DCS should flag the value as bad.

Example 2 (GATE level) — loop power budget. A two-wire transmitter needs at least 10.5 V at its terminals. The loop has a 24 V supply, a 250 Ω receiver, an intrinsic-safety barrier of 300 Ω and a cable 1.5 km long with 18 Ω/km per conductor. (a) Check the transmitter voltage at 20 mA and at 22 mA. (b) Find the maximum total loop resistance at 22 mA. (c) Is HART communication possible?

1. Cable resistance (two conductors) = 2 × 1.5 × 18 = 54 Ω.
2. Total external resistance = 250 + 300 + 54 = 604 Ω.
3. At 20 mA: drop = 0.020 × 604 = 12.08 V; transmitter gets 24 − 12.08 = 11.92 V ≥ 10.5 V, OK.
4. At 22 mA: drop = 0.022 × 604 = 13.29 V; transmitter gets 10.71 V, still OK but with little margin.
5. (b) R_loop,max = (24 − 10.5)/0.022 = 614 Ω.
6. (c) The loop includes 250 Ω (more than about 230 Ω), so HART can be used.

Common mistakes

• Converting current to percentage with I/20 instead of (I − 4)/16.
• Forgetting the cable has two conductors when computing loop resistance.
• Checking the power budget only at 4 mA or only at 20 mA; the worst case is the highest current.
• Removing or shorting the 250 Ω resistor and then wondering why the HART communicator cannot talk to the transmitter.
• Missing or extra terminators on RS-485 and fieldbus segments, causing reflections and intermittent errors.
• Assuming "Modbus" means one thing — RTU, ASCII and TCP frames are not interchangeable.

For GATE IN

Most questions are 4–20 mA conversions (current ↔ engineering value ↔ receiver voltage), loop resistance and supply-voltage budgets, and basic facts about HART (FSK frequencies, bit rate, superimposed signal), Modbus (master/slave, RS-485, function codes) and fieldbus physical layers (31.25 kbit/s, bus-powered). Practise conversions with non-zero lower range values, e.g. −50 to 150 °C.

Quick check

1. A level transmitter ranged 0–5 m outputs 10 mA. What is the level?
2. What frequencies represent 1 and 0 in HART?
3. With a 24 V supply and a transmitter minimum of 12 V, what is the maximum loop resistance at 20 mA?
4. What is the bit rate of FOUNDATION Fieldbus H1 and PROFIBUS PA?

Answers: 1. 1.875 m. 2. 1200 Hz = 1, 2200 Hz = 0. 3. 600 Ω. 4. 31.25 kbit/s.