Model S4100C
32
8.4 Modbus Read/Write - Commands
Preset multiple registers
Any of commands with Function Code 1, 2, 3, 4 allow data to be read from the
instrument. The message structure for each read command specifies a start register
address. A maximum of 5 consecutive registers can be accessed including the start
register address. Each register configures the data as 2 bytes with the most significant
byte first. If more than 5 registers are addressed or if there is an attempt to access
any register outside the valid read register address space, the Illegal Data Address
response (Exception Code 2) is returned.
Any of the commands with Function Code 5, 6 15, 16 allow write data to be written to
the instrument. The message structure for each write command specifies a register
address to which data is written. The message structure for each multiple write
command (15, 16) specifies a register address with the byte count set at 2 to allow
single register access. If more than 1 register is addressed or if there is an attempt to
access any register outside the valid write register address space, the Illegal Data
Address response (Exception Code 2) is returned. Broadcast mode uses address 0
and sends the same data to all attached slaves.
The issue of a write command to a single valid write register normally causes all of the
data specified to be overwritten. In certain situations, it is impossible to force a
condition due to the presence of an external event e.g. attempts to clear a fault while
the fault condition is still present results in the fault not being cleared. For other
situations, any attempts to assign unused, read-only or out of range values will have
no effect. It is advisable to issue a read of the same register range to verify the
true data value present subsequent to the write cycle.
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