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What is a thermocouple module

A thermocouple module (temperature transmitter) is a device that:

Accepts thermocouple inputs (temperature sensors)
Measures their voltage (mV)
Converts it to temperature with cold-junction compensation
Exposes the results through a communication interface — most often Modbus RTU over RS-485

Such modules are made by F&F Filipowski (MB-TC series), ADAM (Advantech), Wago, Phoenix Contact and others. They differ in channel count, supported thermocouple types and measurement resolution.

Thermocouple types

A thermocouple is a pair of wires made from different alloys. The hot junction measures the temperature; the cold junction sits inside the module and requires compensation.

Type	Materials	Range	Sensitivity
K	NiCr / NiAl	−200 to +1350°C	~41 μV/°C
J	Fe / CuNi	−210 to +750°C	~52 μV/°C
T	Cu / CuNi	−250 to +400°C	~43 μV/°C
N	NiCrSi / NiSi	−270 to +1300°C	~39 μV/°C
E	NiCr / CuNi	−270 to +1000°C	~68 μV/°C
R, S, B	Pt/Rh alloys	0 to +1700°C	~6–13 μV/°C

Type K is the most popular in industry — wide range, decent sensitivity, cheap. Laboratories often use T (low temperatures) or R/S (high, precise).

Multi-channel module — how it works

[Thermocouple ch.1] ──┐
[Thermocouple ch.2] ──┤  [ADC]  [Cold junction] → [MCU] → [RS-485 / Modbus]
[Thermocouple ch.3] ──┤
[...             ] ──┘

Each channel has its own ADC input. The module simultaneously measures the thermocouple voltage and the cold-junction temperature (usually an NTC or PT100 sensor inside the enclosure), then applies correction curves for the selected thermocouple type.

Modbus registers — typical 8-channel module

Input registers (FC 04 or FC 03 — depends on the manufacturer):

Register	Content
0–1	Channel 1 temperature (float 32-bit, 2 registers)
2–3	Channel 2 temperature
4–5	Channel 3 temperature
...	...
14–15	Channel 8 temperature
16	Status — error bits per channel
17	Cold-junction temperature

Values may be stored as:

IEEE 754 float — 2 registers, directly in degrees Celsius
Integer × 10 — 1 register, e.g. 2356 = 235.6°C
Integer × 100 — 1 register, e.g. 23560 = 235.60°C

Check the module documentation — manufacturers use different representations.

Reading in Python

from pymodbus.client import ModbusSerialClient
import struct

client = ModbusSerialClient(port="COM8", baudrate=9600, timeout=1)
client.connect()

# FC 04 — Read Input Registers, 16 registers (8 channels × 2)
result = client.read_input_registers(address=0, count=16, slave=1)

if not result.isError():
    temps = []
    for i in range(0, 16, 2):
        # Compose float from two registers (big-endian)
        raw = struct.pack(">HH", result.registers[i], result.registers[i+1])
        temp = struct.unpack(">f", raw)[0]
        temps.append(round(temp, 1))

    for ch, t in enumerate(temps, 1):
        print(f"Channel {ch}: {t}°C")

client.close()

Channel error — how to detect it

If a thermocouple is disconnected, damaged or out of range — the module returns an error value. Typical signals:

Value -32768 or 0x7FFF (integer) — channel in error
Value NaN or ±Inf (float) — channel in error
Bit in the status register — check the documentation

Always check these values before displaying or writing to a database — a disconnected thermocouple shouldn't write -32768°C to the measurement history.

Pitfalls

Byte order (endianness) for float — manufacturers use various combinations: big-endian, little-endian, mixed-endian (swap words). If the temperature comes out nonsensical, try other combinations.
Thermocouple type — the module must be configured for the same type as the connected thermocouple. Wrong configuration gives results with no error flag but a completely wrong value.
Cold-junction compensation — for precise measurements in changing ambient conditions, check where the cold junction is physically measured and whether the module isn't in a location with large temperature swings.
Thermocouple cable — extending a thermocouple with ordinary copper wire introduces an error at the joint. Use compensation cable matched to the thermocouple type.