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Reference Manual
00809-0100-4832, Rev AA
October 2004
3-41
Rosemount 3095FC
• Remote Terminal Unit (RTU)– Allows for greater character density
and better data throughput than ASCII for the same baud rate.
Each message is transmitted in a continuous stream. Data is sent
in 8-bit binary characters. In the ASCII mode, each RTU character
is divided into two 4-bit parts that are represented by their
hexadecimal equivalent. RTU mode uses Cyclic Redundancy
Check (CRC) error checking. By default, RTU is enabled.
In either mode, ASCII or RTU, a Modbus message is placed by the
transmitting device into a frame that has a known beginning and
ending point.
Table 3-4. Message Framing
3. Select Log Modbus Events Enabled to log all Modbus parameter
changes to the Event Log or select Disabled to allow Modbus
parameter changes to occur without being logged. By default, Log
Modbus Events is Enabled.
4. Select the Byte Order of data bytes in a transmission or request can
be reversed by the selection made in these check boxes. This only
affects the Data field of a Modbus message, has no effect on the data
for Function Codes 01, 02, and 05, and only applies to floating points.
• Least Significant Byte First - Selecting this check box places the
least significant byte first. This is the default value.
• Most Significant Byte First - Selecting this check box places the
most significant byte first.
5. Select EFM Modbus Enable to use an implementation of Modbus
protocol with EFM extensions. This causes the Hourly (Periodic) and
Daily indices to be returned as floating point values instead of the
standard integer and character values. In addition, the history index is
adjusted to accommodate a one-based index (1 to 840), and the date
stamp (MMDDYY) is returned before the time stamp (HHMM), which
does not include seconds.
6. The “status” field displays the status codes returned from the modem.
ASCII Message Framing
Begin of
Frame
Address Function Data
LRC Error
Check
End
: 2 characters 2 characters N characters 2 Characters CRLF
RTU Message Framing
Begin of
Frame
Address Function Data
CRC Error
Check
End
T1-T2-T3-T4 1 Byte 1 Byte N * 1 Byte 2 Bytes T1-T2-T3-T4