ProSoft Technology MVI69-MCM - Manual (Page 63)

MVI69-MCM ♦ CompactLogix or MicroLogix Platform Configuring the MVI69-MCM Module
Modbus Communication Module User Manual
ProSoft Technology, Inc. Page 63 of 167
March 22, 2011
(Modbus Command parameters)
DB Addr - Tells the Master where in its data memory is the beginning of data to
obtain and write out to the Slave (slave) device.
Reg Count - Tells the Master how many data points to send to the Slave. Two
counts will mean two floating points with Float Flag: Y and the "Addr in Dev" =>
the "Float Start" Parameter.
Swap Code - Tells the Master how to orient the Byte and Word structure of the
data value. This is device dependent. Check Command Entry formats Section.
Func Code - Tells the Master to write the float values to the Slave. FC16.
Addr in Dev - Tells the Master where in the Slave's database to locate the data.
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 8 85.37 22.86
HEX 01 10 1B BC 00 02 08 BD 71 42 AA E1 48 41 B6
In this example, the Master's Modbus packet contains the data byte and data
word counts that have been doubled from the amount specified by Reg Count
due to the Float flag set to Y. Some Slaves look for the byte count in the data
packet to know the length of the data to read from the wire. Other slaves know at
which byte the data begins and read from the wire the remaining bytes in the
packet as the data the Master is sending.
Specific Example#2: Master is issuing Modbus command with FC 16 (with Float Flag: No) to
transfer Float data.