ProSoft Technology MVI69-MCM - Manual (Page 64)

Configuring the MVI69-MCM Module MVI69-MCM ♦ CompactLogix or MicroLogix Platform
User Manual Modbus Communication Module
Page 64 of 167 ProSoft Technology, Inc.
March 22, 2011
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 will be 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 will read only half the data from the Master's
transmission. Other slaves will read all 8 bytes in this example because they will
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.