Appendix A _________________________________________________________ Modbus Reference
VAISALA ________________________________________________________________________ 77
PDU address
Actual address bytes used in a Modbus Protocol Data unit.
LSW
Least significant word (bits 15 … 0).
MSW
Most significant word (bits 31 … 16).
16-bit integer
Numeric value in range -32768 … +32767.
32-bit bit field
32 individual values, each 0 or 1.
32-bit integer
Numeric value in range -2147483648...+2147483647.
32-bit float
Floating point number encoded in IEEE 754 "binary32" format.
N-byte string
Text up to N-1 characters with 0-byte(s) at the end.
read-only
Register value cannot be changed with Modbus functions.
read/write
Register value can be changed with Modbus functions.
write-only
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
using 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
Mode) from Analog or Modbus Mode on page 36 and entering correct
settings with serial line commands.
