Modbus communication FCT010
C
C.1
Modbus addressing model
The device allows read/write access to the following standard Modbus RTU data holding register
blocks:
• Holding registers (ref. 4x address range)
The minimum value of a writable holding register can be read by adding 10000 to the Modbus
address of the register.
The maximum value of a writable holding register can be read by adding 20000 to the Modbus
address of the register.
The default value of a writable holding register can be read by adding 30000 to the Modbus
address of the register.
C.2
Modbus function codes
This device supports following function codes: 3, 8 and 16.
Function codes 3 and 16 are used for accessing registers, max. 16 registers per read/write
request is accepted.
Function code 8 is used for reading Modbus communication diagnostic information.
Below the various function code are described.
Function code 3 (Read holding registers)
General exceptions:
• Requesting less than 1 or more than 16 registers => Exception 3 (Illegal data value)
• Requesting invalid start address or start address with invalid quantity => Exception 2 (Illegal
data address)
Application exceptions:
• Application errors; min/max limit of parameter exceeded; or parameter write-protected =>
Exception 4 (Slave device error)
Holes/register alignment:
• The read command always returns data if no exception is given.
• Holes in the holding register map return value zero in all bytes. E.g. reading 2 registers
starting at 4:0004 above will result in 2 bytes of "float B" followed by 2 zeroes.
SITRANS MASS 2100 & FC300 (FCT010/DSL)
Operating Instructions, 06/2020, A5E39788682-AB
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