Configuring interfaces to the central BMS
Modbus RTU interface
Single room control system FSL
‑
CONTROL II
66
2
Modbus functions
The device includes the following functions:
Function 1 (0x01) Read Coils
Function 3 (0x03) Read Holding Registers
Function 4 (0x04) Read Input Registers
Function 5 (0x05) Write Single Coil
Function 6 (0x06) Write Single Register
Function 8 (0x08) Diagnostics
Function 16 (0x10) Write Multiple Registers
Function 43/14 (0x02B/0x0E) Read Device Identification
The encoding for the corresponding Modbus RTU frames is shown below (shown
without CRC). For data with 2 bytes the High-Byte comes first.
Meaning of exception codes:
1 Illegal function code Unknown code for this function or subfunction.
2 Illegal Data Address Invalid register address.
3 Illegal Data Value Inconsistent encoding in the number of registers,
number of bytes, data value.
Modbus function 3 (0x03) Read Holding Registers
Modbus function 4 (0x04) Read Input Registers
These functions are used to read several consecutive registers. Both functions
are identical.
Request:
Byte 0 Slave Address Rotary switch 1-99
Byte 1 Function Code 0x03 / 0x04
Byte 2-3 Register Address First register
Byte 4-5 Register Quantity No. of registers
Response:
Byte 0 Slave Address Rotary switch 1-99
Byte 1 Function Code 0x03 / 0x04
Byte 2 Byte Count 2 bytes per register
Byte 3-4 Register Value 0x0000-0xFFFF
Byte ... Register Value 0x0000-0xFFFF
Error:
Byte 0 Slave Address Rotary switch 1-99
Byte 1 Error Code 0x83 / 0x84
Byte 2 Exception Code 0x02,0x03