2 System description
266 MODBUS® multivariable transmitter User manual 2105216 Rev. AB | 13
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
— Data Bits = 8 (fixed)
— Parity = None
— Stop Bits = 2 (fixed)
MODBUS
®
RTU mode requires each data byte to be 11 bits (Table 8 and Table 9).
Bits per Byte:
— 1 start bit
— 8 data bits, least significant bit sent first
— 1 bit for parity completion
— 1 stop bit
Table 8: Bit sequence in RTU mode (with parity checking)
Table 9: Bit sequence in RTU mode (with no parity)
The parity bit can be set from NONE to EVEN or ODD parity. The transmitter automatically adjusts the number of stop bits.
Figure 8 depicts a typical MODBUS
®
message format.
Start End
≥ 3.5 char ≥ 3.5 char
Address
8 bits
CRC Check
16 bits
Data
N x 8 bits
Function
8 bits
MODBUS
®
message
At least 3.5 char
Frame 1 Frame 2
Frame 3
At least 3.5 char
3.5 char
4.5 char
Where:
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 Check = Indicates a data validity test of the data being sent
End = Indicates the end characters
Figure 8: Sample MODBUS
®
message frame
When no parity is selected (default for the 266) the parity completion bit is replaced by another stop bit.
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