RS485 with MODBUS-RTU Protocol
Serial interface
The following parameters are possible for data transfer via the EIA-485 interface on the ModBus sensors made by
Mela
®
:
• Baud rate: 19200 / 9600 / 4800 / 2400 / 1200 / 600
• Data bits: 8
• Parity: N / E / O
• Stop bits: 1 / 2
Modus 19200@8N2 is pre-dened.
Access to Modbus registers
To ensure compatibility with all Modbus masters, all available registers can be read both with function code 03
hex
(read
holding register), as well as with function code 04
hex
. Registers with additional write permission can be modied with
function code 06
hex
(write holding register). All registers available with Modbus sensors by Mela are listed in table 1.
Register-no. Data type Value Permission
0
FLOAT32 Temperature (°C)
readable
1
2 UINT16 Alarm code temperature
3
FLOAT32 Relative humidity (%rH)
4
5 UINT16 Alarm code humidity
6
UINT32 Serial number sensor
7
8
UINT32 Serial number sensor
9
10
FLOAT32 Dew point temperature (°C)
11
12
FLOAT32 Enthalpy (kJ/kg)
13
14
FLOAT32 Mixing ratio (g/kg)
15
16
FLOAT32 Absolute humidity (g/m³)
17
18
FLOAT32 Wet-bulb temperature (°C)
19
20 UINT16 Alarm code hx processor
205 UINT16 Modbus address read and writeable
Functioning
The measured values for relative humidity, temperature and the corresponding alarm codes are saved in registers
0…5 in a cycle time of 2s. Registers 6(8) and 7(9) contain the sensor‘s serial number. If an hx value is scanned from
registers 10…19, this is recalculated at the time of the scan, based on the current temperature and the relative humi-
dity. The hx processor emits an alarm code if the values for temperature or relative humidity are outside the permis-
sible input range. No calculation is made in this case, and the register displays the last valid value for each variable.
The memory organisation for the temperature and air humidity readings, as well as for the serial number, is Little
Endian. This means that the low byte word is in the lower register and the high byte word is in the higher register.
Table 1 - Modbus register
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