66 266CRH / 266CRT, 266CSH / 266CST MULTIVARIABLE PRESSURE TRANSMITTER | OI/266CXX/MODBUS-EN REV. A
… 8 Operation
Interface description
Communication parameters
For communication to work properly, the transmission mode
(and serial port parameters) must be the same for all devices on
a Modbus serial line.
The default communications parameters are set to:
• Baud Rate: 9600 baud
• Data Bits: 8 (fixed)
• Parity: None
• Stop Bits: 2 (fixed)
Modbus RTU mode requires each data byte to be 11 bits (refer to
Figure 1).
Bits per Byte:
• 1 start bit
• 8 data bits, least significant bit sent first
• 1 bit for parity completion
• 1 stop bit
RTU mode (with parity checking)
M11192
Start
Parity
Stop
1
2
3
4
5
678
Start
Stop*
Stop
1
2
3
4
5
678
RTU mode (with no parity)
Figure 1: Modbus RTU bit sequence
* When no parity is selected (default for the 266) the parity completion bit is
replaced by another stop bit.
The parity bit can be set from NONE to EVEN or ODD parity. The
transmitter automatically adjusts the number of stop bits.
Figure 2 depicts a typical Modbus message format.
M11193
Start
End
≥ 3.5 char
≥
3.5 char
Address
8bits
CRC
16 bits
Data
Nx8bits
Function
8bits
Modbus message
At least 3.5 char
Frame1 Frame2
Frame 3
3.5 char
4.5 char
At least 3.5 char
Figure 2: Sample Modbus message frame
Message part Description
Start Indicates the quiet time before a data transfer
Address In the request frame, this is the device being
addressed.
In the response frame, this is the device answering the
request.
Function In the request frame, this is the command byte. It
presents the device with a read or write sequence.
In the response frame, this could be echoed or could
be returned as an error code.
Data In the request frame, the data is held in the beginning
register and read along with the number of
consecutive registers.
In the response frame, this is the data being read.
CRC Indicates a data validity test of the data being sent.
End Indicates the end characters
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