Operating instructions

PTQ-MCM ♦ Quantum / Unity Platform Modifying the Configuration File
Modbus Communication Module
Page 74 of 139 ProSoft Technology, Inc.
April 29, 2008
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 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 conclusion
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.