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Page 26 • MNFL003 ║ Issue/Rev. 0.1 (9/13)
microFlow.net Liquid Modbus Communications Manual
Appendix
4.51
Field Contents In Modbus Messages
Examples of a Modbus query message and normal response are shown in the tables on
the following page. The field contents in both examples are displayed in hexadecimal.
In this example, the master sends a Read Holding Registers request to microFlow.net
Liquid address 06. The microFlow.net Liquid is specifically directed to return data from
three holding registers, starting with address 0107 (006B hex).
As is the case in any normal response, the microFlow.net Liquid first echoes the func-
tion code sent by the master. The microFlow.net Liquid then transmits the byte count
field, indicating the number of 8-bit data items being returned. Finally, the microFlow.
net Liquid returns the 8-bit bytes containing the requested data.
How to Use the Byte Count Field:
When constructing responses in buffers, use a
byte count value that equals the count of 8-bit bytes in the message data. The value
is exclusive of all other field contents, including the byte count field. The microFlow.
net Liquid response example illustrates a typical byte count field in a normal response.
Master Query
Field Name
Example (Hex)
RTU 8-Bit Field
Header
None
microFlow.net Liquid
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)
Total Bytes:
8
microFlow.net Liquid Response
Field Name
Example (Hex)
RTU 8-Bit Field
Header
None
microFlow.net Liquid
Address
06
0000 0110
Function
03
0000 0011
Byte Count
06
0000 0110
Data Hi
02
0000 0010
Data Lo
2B
0010 1011
Data Hi
00
0000 0000
Data Lo
00
0000 0000
Data Hi
00
0000 0000
Data Lo
63
0110 0011
Error Check
CRC (16 bits)
Total Bytes:
11
The microFlow.net Liquid monitors the amount of time between the receipt of characters.
If three and one-half character times elapse without the microFlow.net Liquid seeing a
new character or the end of a frame, the message is flushed and the next characters
received will be viewed as an address. If the address is for that microFlow.net Liquid,
it will respond. If the address is not for that microFlow.net Liquid, the message will be
flushed and it will look for the next message.