Appendix B – Modbus Protocol
B-10
NMBA-01 Installation and Start-up Guide
mal response. The ‘Byte Count’ field specifies how many
8-Bit data
items are being returned. It shows the count of 8-bit bytes to follow in
the data for RTU.
For example, the value 63 hex is sent as one 8-bit byte in RTU mode
(01100011). The 'Byte Count' field counts this data as one 8-bit item,
regardless of the character framing method (ASCII or RTU).
How to Use the Byte Count Field: When you construct responses in
buffers, use a Byte Count value that equals the count of 8-bit bytes in
your message data. The value is exclusive of all other field contents,
including the Byte Count field.
Figure B-4 ‘Slave Response with RTU
Framing’ shows how the byte count field is implemented in a typical
response.
Figure B-3 Master Query with RTU Framing
QUERY
Field Name
Example
(Hex)
RTU
8-Bit Field
Header
None
Slave Address
06
0000 0110
Function
03
0000 0011
Starting Address Hi
00
0000 0000
Starting Address Lo
6B
0110 1011
No. of Registers Hi
00
0000 0000
No. of Registers Lo
03
0000 0011
Error Check
CRC (16 bits)
Trailer
None
Total Bytes:
8
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