Modbus

The probe can be addressed directly using the MODBUS protocol, to request the measured humidity and temperature values. The parameters highlighted in yellow should be selected according to the table below.

Important: In order to communicate directly with the digital probe via MODBUS, please use the E2-05XX-MOD cable. The digital MODBUS protocol is delivered over RS485.

Command: (MSB first)

Probe address	Command ID		Start address		Number of records	MODBUS CRC
0x00	0x04	0x00	0x00	0x00	0x00	0x0000

Probe address

0…255, if '0' then all probes respond regardless of the address.The default probe address is 247.

Register	Name	Description	Data type
30’001	Serial Number	- Serial number of the device (part 1)	Int. 32 Bit
30’002	- Serial number of the device (part 2)

Current Values

Current values in 32-bit IEEE754 float format.

**Note! ** These values are changing every measurement cycle, so always read the two corresponding registers in one Modbus command.

Register	Name	Description	Data type
31’001	Humidity	- Current humidity value (part 1)	Float 32 Bit
31’002	- Current humidity value (part 2)
31’003	Temperature	- Current temperature value (part 1)	Float 32 Bit
31’004	- Current temperature value (part 2)
31’005	Calculation	- Current calculation value (part 1)	Float 32 Bit
31’006	- Current calculation value (part 2)
31’007	Measurement flags Humidity	- Current measurement flags humidity	Int. 16bit
31’008	Measurement flags Temperature	- Current measurement flags temperature	Int. 16bit

Read Current Float Value (Registers 31'001 to 31'008)

Example:

Transmit	01 04 79 18 00 08 68 97
Receive	01 04 10 e6 3a 33 42 bb 93 bf 41 ff ff ff 7f 00 00 00 00 97 8a

Field	Bytes	Value	Description
Transmit
RTU number	1	0x01	Modbus RTU Address
Function code	1	0x04	Read Discret Inputs
Starting address	2	0x7918	= 31'000 ( Attention! register number – 1 )
Quantity of input registers	2	0x0008	= 8, means read 8 registers
Checksum	2	0x9768	CRC Checksum
Receive
Byte count	1	0x10	= 16 x 2 * N, means numbers of returned bytes
Input registers	2 * N	0x42333AE6	= 44.81 %rh Humidity
0x41BF93BB	= 23.95 °C Temperature
0x7FFFFFFF	= NAN (calculation is disabled)
	0x0000	= 0, No measurement error
	0x0000	= 0, No measurement error
Checksum		0x8A97	CRC Checksum

Device Specific holding Registers

Via the Modbus Holding Registers you can read and write device specific data of the HCD probe.

Assisted Modbus commands are Read Holding Registers (0x03), Write Single Register (0x06)

Function 03/06 – Read/Write Single register

40001	Device address	Device address (Default 0x00F7)	Integer 16-bit
40002	Byte Order	Swap modes / Byte Order (Default 0x0000 - No swap)	Integer 16-bit

Read Modbus Address (Registers 40'001)

Example:

Note: the device answers to address 0. This can be used to get the current device address

Transmit	00 03 9c 40 00 01 aa 5f
Receive	01 03 02 00 01 79 84

Field	Bytes	Value	Description
Transmit
RTU number	1	0x00	Modbus RTU Address (00 broadcast address)
Function code	1	0x03	Read Holding Register
Starting address	2	0x9C40	= 40001 ( Attention! register number – 1 )
Quantity of Holding registers	2	0x0001	= 1, means read 1 register
Checksum	2	0x5FAA	CRC Checksum
Receive
Byte count	1	0x02	= 2 x 2 * N, means numbers of returned bytes
Holding register	N	0x0001	= 1, current address
Checksum	2	0x8479	CRC Checksum

Write Modbus Adress (Registers 40'0001)

Example:

Note: The device answers with the current Modbus RTU Address, afterwards the address is changed.

Receive	F7 06 9C 40 00 01 73 18

Field	Bytes	Value	Description
Transmit / Receive
RTU number	1	0xF7	Modbus RTU Address
Function code	1	0x06	Read Holding Register
Starting address	2	0x9C40	= 40001 ( Attention! register number – 1 )
Value	2	0x0001	= 1, new Modbus address
Checksum	2	0x1873	CRC Checksum

Byte Order / Swap modes

Selectable Swap Modes for Rotronic Devices

Selectable swap modes only for 32-bit Float and 32-bit Integer values based on Big Endian memory organisation.

Value	Swap Mode	Source Bytes	Target Bytes
0x0000	Big endian - No Swap	[ a b ] [ c d ]	[ a b c d ]
0x0001	Little endian - byte and word swap	[ a b ] [ c d ]	[ d c b a ]
0x0002	Mid-big endian - byte swap	[ a b ] [ c d ]	[ b a d c ]
0x0003	Mid-little endian - word swap	[ a b ] [ c d ]	[ c d a b ]

Read Swap mode (Registers 40'001)

Example:

Transmit	01 03 9c 41 00 01 fa 4e
Receive	01 03 02 00 02 39 85

Field	Bytes	Value	Description
Transmit
RTU number	1	0x01	Modbus RTU Address
Function code	1	0x03	Read Holding Register
Starting address	2	0x9C41	= 40002 ( Attention! register number – 1 )
Quantity of Holding registers	2	0x0001	= 1, means read 1 register
Checksum	2	0x4EFA	CRC Checksum
Receive
Byte count	1	0x02	= 2 x 2 * N, means numbers of returned bytes
Holding register	N	0x0002	= 2, Mid-big endian - byte swap
Checksum	2	0x8539	CRC Checksum

Example:

Set Swap Mode “Mid-big endian - byte swap”

Transmit	01 06 9c 41 00 02 76 4f
Receive	01 06 9c 41 00 02 76 4f

Field	Bytes	Value	Description
Transmit / Receive
RTU number	1	0x01	Modbus RTU Address
Function code	1	0x06	Write Holding Register
Starting address	2	0x9c41	= 40002 ( Attention! register number – 1 )
Value	2	0x0002	= 2, Mid-big endian - byte swap
Checksum	2	0x4F76	CRC Checksum

Response in the event of an error

0x..	0x84	0x00	0x....

0x02	Start address and number of records invalid
0x03	Number of records invalid

Sensor faults

In case of a short circuit or open circuit at the probe, a value of NAN (0x7fffffff) is output for the measured value concerned.

Additional MODBUS details can be found here: MODBUS in RMS, including Guidance for MODBUS Communication with the RMS-HCD Digital Probe.

For detailed information about Modbus protocol Read Discrete Inputs see: (http://modbus.org/docs/Modbus_Application_Protocol_V1_1b3.pdf).