Appendix A _________________________________________________________ Modbus Reference
VAISALA ________________________________________________________________________ 77
Actual address bytes used in a Modbus Protocol Data unit.
Least significant word (bits 15 … 0).
Most significant word (bits 31 … 16).
Numeric value in range -32768 … +32767.
32 individual values, each 0 or 1.
Numeric value in range -2147483648...+2147483647.
Floating point number encoded in IEEE 754 "binary32" format.
Text up to N-1 characters with 0-byte(s) at the end.
Register value cannot be changed with Modbus functions.
Register value can be changed with Modbus functions.
Register value can be written but is always read as zero.
Modbus RTU requires 8 data bits and is supported only at bit rates 9600 b/s and above.
Modbus RTU specification recommends N82, E81, or O81 only.
A complete 32-bit value (two Modbus registers) must be read and written in a single Modbus
transaction.
A “quiet NaN” value is returned for unavailable floating-point values (e.g. in case of
measurement error). Several registers may be read in one transaction, even if there are gaps in
the register map. Registers not listed contain typically value NaN or 0.
NOTE
If incompatible settings/protocol are selected and the probe is then
restarted, it might no longer be possible to communicate with the probe
us
ing Modbus. If needed, you can override invalid communication
settings configured in Modbus mode by switching to the serial line mode
as instructed in
Accessing Serial Line Command Interface (RS-485
r Modbus Mode on page 36 and entering correct
settings with serial line commands.
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