Modbus Holding Registers; Modbus Addressing Model; Modbus Function Codes - Siemens SITRANS FC410 Operating Instructions Manual

Modbus holding registers

A.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.
A.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.
FC410 with Modbus
Operating Instructions, 02/2016, A5E33120874-AC
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