FMC Technologies Smith Meter microFlow.net Liquid - Exception Responses

Issue/Rev. 0.1 (9/13) ║ MNFL003 • Page 35
microFlow.net Liquid Modbus Communications Manual Appendix
Query
Field Name Example (Hex)
microFlow.net Liquid Address 11
Function 10
Starting Address Hi 00
Starting Address Lo 01
No. of Registers Hi 00
No. of Registers Lo 02
Byte Count 04
Data Hi 00
Data Lo 0A
Data Hi 01
Data Lo 02
Error Check (CRC) --
Response
A normal response consists of the slave address, function code, starting address, and
quantity of registerspreset.
An example of a response to the preceding query appears below:
Response
Field Name Example (Hex)
microFlow.net Liquid Address 11
Function 10
Starting Address Hi 00
Starting Address Lo 01
No. of Registers Hi 00
No. of Registers Lo 02
Error Check (CRC) --
5.5 Exception Responses
When a master device sends a query to an microFlow.net Liquid device, there are three
possible outcomes:
1. The microFlow.net Liquid receives the query with no communication errors,
handles the query normally, and returns a normal response.
2. A communication error bars the microFlow.net Liquid from receiving the query,
so no response is returned. The master program eventually processes a timeout
condition for the query.
3. The microFlow.net Liquid receives the query without error, but returns no
response. The master program eventually processes a timeout condition for the
query.
Two fields in the exception response message differentiate it from a normal response:
Function Code Field: A microFlow.net Liquid normally echoes the function code of
the original query in the function code field of the response. Because the values of all
function codes are below 80 hexadecimal, all function codes have a most significant bit
(MSB) of 0. In an exception response, however, the slave sets the MSB of the function
code to 1. The value of the function code in an exception response is therefore 0x80
(hex) higher than the value for a normal response.