A.3.1 32-Bit Floating Point Format
Least-significant 16 bits of floating point or integer numbers are placed at the smaller Modbus
address as specified in Open Modbus TCP Specification, Release 1.0. This is also known as
"little-endian" or "Modicon" word order. The CDAB byte order is used. Floating point values are
represented in standard IEEE 32-bit floating point format.
Despite the specification, some Modbus masters may expect "big-endian" word
order (most-significant word first). In such case, you must select "word-swapped"
floating point format in your Modbus master for the Modbus registers of the
device.
A "quiet NaN" value is returned for unavailable values. A quiet NaN is, for example,
7FC00000
hex
; however, the master should understand any NaN value.
A complete 32-bit floating point or integer value should be read and written in a
single Modbus transaction.
A.3.2 Measurement Data
Table 23 Measurement Data Registers (Read-Only)
Register Number
(Decimal)
Address
(Hexadecimal)
Register
Description
Data Format
Unit and Notes
Measurement Status
3
0002
hex
Measurement
reliability
16-bit field
Only last bit is
currently used. It
is set as follows:
0 = measurement
is unreliable
because of high
ambient
temperature
1 = Measurement
is reliable
Present Values
21
0014
hex
Methane
32-bit float
ppm
v
23
0016
hex
Acetylene
32-bit float
ppm
v
25
0018
hex
Ethylene
32-bit float
ppm
v
27
001A
hex
Ethane
32-bit float
ppm
v
29
001C
hex
Carbon monoxide 32-bit float
ppm
v
Appendix A – Modbus Reference
61
Summary of Contents for Optimus OPT100
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