ProSoft MVI69E-MBS User Manual page 122

Reference
User Manual
In this example, the master'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 slave 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 slaves read only half the data from the master's
transmission. Other slaves 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: master is issuing Modbus command with FC 3 to transfer Float data from
slave.
Float Flag: Not applicable with Modbus Function Code 3.
Float Start: Not applicable with Modbus Function Code 3.
DB Addr - Tells the master where in its data memory to store the data obtained
from the slave.
Reg Count - Tells the master how many registers to request from the slave.
Swap Code - same as above.
Func Code - Tells the master to read the register values from the slave. FC3.
Addr in Dev - Tells the master where in the slave's database to obtain the data.
Page 122 of 154
MVI69E-MBS ♦ CompactLogix Platform
Modbus Serial Enhanced Communication Module
ProSoft Technology, Inc.
June 28, 2017