Appendix B – Modbus Protocol
NMBA-01 Installation and Start-up Guide
B-13
Response
The register data in the response message are packed as two bytes
per register, with the binary contents right justified within each byte. For
each register, the first byte contains the high order bits and the second
contains the low order bits.
Data is scanned in the slave at the rate of 125 registers per scan for
984-X8X controllers (984–685, etc.), and at the rate of 32 registers per
scan for all other controllers. The response is returned when the data is
completely assembled.
Here is an example of a response to the previous query:
Figure B-6 Read Holding Registers - Response
The contents of register 40108 are shown as the two byte values of
02 2B hex, or 555 decimal. The contents of registers 40109–40110 are
00 00 and 00 64 hex, or 0 and 100 decimal.
RESPONSE
Field Name
Example
(Hex)
Slave Address
11
Function
03
Byte Count
06
Data Hi (Register 40108)
02
Data Lo (Register 40108)
2B
Data Hi (Register 40109)
00
Data Lo (Register 40109)
00
Data Hi (Register 40110)
00
Data Lo (Register 40110)
64
Error Check CRC
CRC (16-Bits)
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