ProSoft Technology MVI69E-MBS - Manual (Page 129)

MVI69E-MBS ♦ CompactLogix Platform Reference
Communication Module User Manual
ProSoft Technology, Inc. Page 129 of 162
January 6, 2014
Addr in Dev - same as when Float Flag: Y as long as the Slave's Float Flag = Y.
In the above example, the Master's Modbus command to transmit inside the
Modbus packet is as follows.
Slave
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 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.
Specific 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.