ABB NMBA-01 Installation And Startup Manual page 55

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NMBA-01 Installation and Start-up Guide
Function Code Field: In a normal response, the slave echoes the
function code of the original query in the function code field of the
response. All function codes have a most-significant bit (MSB) of 0
(their values are all below 80 hexadecimal). In an exception response,
the slave sets the MSB of the function code to 1. This makes the func-
tion code value in an exception response exactly 80 hexadecimal
higher than the value would be for a normal response. With the function
code's MSB set, the master's application program can recognise the
exception response and can examine the data field for the exception
code.
Data Field: In a normal response, the slave may return data or statis-
tics in the data field (any information that was requested in the query).
In an exception response, the slave returns an exception code in the
data field. This defines the slave condition that caused the exception.
Figure B-11 'Master Query and Slave Exception Response' shows an
example of a master query and slave exception response.
The field examples are shown in hexadecimal.
QUERY
Byte
1
2
3
4
5
6
7
EXCEPTION RESPONSE
1
2
3
4
Figure B-11 Master Query and Slave Exception Response
Appendix B – Modbus Protocol
Contents Example
Slave Address 0A
Function 01
Starting Address Hi 04
Starting Address Lo A1
No. of Coils Hi 00
No. of Coils Lo 01
LRC 4F
Slave Address 0A
Function 81
Exception Code 02
LRC 73
B-17