ABB NMBA-01 Installation And Startup Manual page 48

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Appendix B – Modbus Protocol
B-10
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.
QUERY
Field Name
Header
Slave Address
Function
Starting Address Hi
Starting Address Lo
No. of Registers Hi
No. of Registers Lo
Error Check
Trailer
Figure B-3 Master Query with RTU Framing
Example RTU
(Hex) 8-Bit Field
None
06 0000 0110
03 0000 0011
00 0000 0000
6B 0110 1011
00 0000 0000
03 0000 0011
CRC (16 bits)
None
Total Bytes: 8
NMBA-01 Installation and Start-up Guide