ProSoft Technology MVI69E-MBTCP - Manual (Page 125)

MVI69E-MBTCP ♦ CompactLogix Platform Reference
Modbus TCP/IP Enhanced Communication Module User Manual
ProSoft Technology, Inc. Page 125 of 150
May 20, 2015
In the above example, the client's Modbus command to transmit inside the
Modbus packet is as follows.
Server
Address Function
Code Address in
Device Reg
Count Byte
Count Data
DEC 01 16 7100 2 4 85.37
HEX 01 10 1B BC 00 02 04 BD 71 42 AA
In this example, the client's Modbus packet contains the data byte and data word
counts that have NOT been doubled from the amount specified by Reg Count
due to the Float Flag set to N. The server looks for the byte count in the data
packet to know the length of the data to read from the wire. Because of
insufficient byte count, some servers read only half the data from the client's
transmission. Other servers read all 8 bytes in this example because they know
where in the packet the data starts and ignore the byte count parameter inside
the Modbus packet.
Example #3: client is issuing Modbus command with FC 3 to transfer Float data from server.
Float Flag: Not applicable with Modbus Function Code 3.
Float Start: Not applicable with Modbus Function Code 3.
DB Addr - Tells the client where in its data memory to store the data obtained
from the server.
Reg Count - Tells the client how many registers to request from the server.
Swap Code - same as above.
Func Code - Tells the client to read the register values from the server. FC3.
Addr in Dev - Tells the client where in the server's database to obtain the data.