SIGNUX THG01

The THG01 measures air temperature and humidity and delivers them over RS485 in Modbus RTU. The transmitter is always the same; what is chosen when ordering is the head and how the cable ends.

This device must not be buried; it is to be used to measure air.

It measures from −40 to +80 °C and from 0 to 100 % relative humidity, with an accuracy of ±0.5 °C and ±4 % RH, and refreshes the reading every two seconds.

It is chosen in two steps: first the head and then how the cable ends.

Both heads measure the same, with the same accuracy and the same register map. The electronics work from −20 to +60 °C in both cases. What changes, besides the material, is the size:

The head measures the air around it, so wherever it is placed is exactly what will be read. Four things that change the reading more than any setting:

• •Away from heat sources and direct sun. A sunlit wall reads several degrees above the air in the room.
• •Clear of cold surfaces. If the head condenses, the humidity goes to the top of the scale and is slow to come down.
• •With the filter clear: no paint, no tape, no partition a centimetre away. The air has to get in.
• •At the height where the reading matters, not on the ceiling if what is being controlled is down below.

The PVC head has a mounting plate and screws to a wall or a bracket. The PE one does not: it is held by the cable gland or by the cable itself.

The unit comes out with four wires. Two are the supply and two are the bus:

On the version with a connector the four wires come already fitted into the M10, and it is simply plugged into a four-contact lead.

The supply is protected against reverse polarity: crossing the brown and the black does not break the unit. The bus wires have no such protection, in the sense that, crossed, the unit simply does not answer; if it answers nothing at all, the first thing to try is swapping the green and the blue.

With several units on the same bus a terminating resistor is needed at both ends.

Modbus RTU, from the factory at 4800 8N1 and at address 1. It takes 2400, 4800 and 9600.

Functions 03 and 04 return the same thing, so either will do. The measurement refreshes every two seconds; asking for it more often does not give a new figure.

For writing there are two more: 06 writes one register and 16 writes several in the same frame. Only the four read-and-write registers in the table below accept them; temperature and humidity are read only.

Temperature comes in two’s complement: a value above 32767 is below zero. Both corrections leave the factory at zero and are added to the measurement.

All the frames are for a unit at address 1 and already carry their CRC. There are only two shapes: function 03 to read and function 06 to write.

1 · Read the temperature and humidity

Two registers (00 02) are asked for from 0x0000. The unit returns four bytes (04): humidity first, 01 02, which is 258, that is 25.8 % RH; temperature after, FF 33, which is −205, that is −20.5 °C. Negatives come in two’s complement: a value above 32767 is below zero.

2 · Read the temperature correction

A single register (00 01), 0x0050. It answers two bytes (02): FF F6 is −10, that is a correction of −1.0 °C. From the factory that register is zero.

3 · Write the temperature correction

Function 06 writes one register: FF F6, which is −1.0 °C, goes into 0x0050. The unit answers by repeating the whole frame; if it comes back different, something has failed. The humidity correction is written the same way, changing 0x0050 for 0x0051.

4 · Change the address to 2

The address number goes in 0x07D0. We have not tried this one on the bench: the frame is the one the manufacturer documents.

5 · Set the baud rate to 4800

The baud-rate code goes in 0x07D1: 0 is 2400, 1 is 4800 and 2 is 9600. This one has not been tried on the bench either.

6 · Ask what address it is at

The FF is a wildcard, for when the address of the unit is not known. The reply starts with 01, which is its own address, and the data 00 01 repeats it. We have not checked this on the bench; what is measured is that this unit does not answer at address 0.

7 · Read the baud rate

The reply is identical byte for byte to the one in the previous example and means something else: here the 00 01 is the baud-rate code, that is 4800 bit/s. What gives it meaning is the register that was asked for, not the reply.