ProSoft Technology CompactLogix MVI69E-MBS User Manual page 130

MVI69E-MBS ♦ CompactLogix™ Platform
Modbus Serial Enhanced Communication Module
Example #2: Master is issuing Modbus command with FC 16 (with Float Flag: No) to transfer Float
data.
Float Flag: "N" tells the master to ignore the floating values and treat each register data
as a data point composed of 1 word, 2 bytes or 16 bits.
Float Start: Ignored.
DB Addr - same as when Float Flag: Y.
Reg Count - Tells the master how many data points to send to the slave.
Swap Code - same as when Float Flag: Y.
Func Code - same as when Float Flag: Y.
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
01
DEC
01
HEX
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.
ProSoft Technology, Inc.
Function Address in Reg
Code Device Count
16 7100 2
10 1B BC 00 02
Byte Data
Count
4 85.37
04 BD 71 42 AA
Reference
User Manual
Page 130 of 159