1MDU07205-YN rev. F
REF601 CEI
User’s manual
75
7.2.2 RS-485 Serial Communication
When connecting the REF 601 to an RS485 communication link, you must use
termination resistors at each end of link.
As a reference, cable number 9842 from Belden (4 wire cable) or any equivalent
cable can be used to for the RS485 connection.
7.3 Modbus RTU message format
7.3.1 Modbus message RTU framing:
For Modbus serial line using the RTU (Remote Terminal Unit) mode, each 8–bit byte
in a message contains two 4–bit hexadecimal characters. The main advantage of this
mode is that its greater character density allows better data throughput than ASCII
mode for the same baud rate. Each message must be transmitted in a continuous
stream of characters.
The format for each byte (11 bits) in RTU mode is:
Coding System: 8–bit binary
Bits per Byte: 1 start bit
8 data bits, least signifi cant bit sent fi rst
1 bit for parity completion
1 stop bit
Parity: Even, other modes (odd parity, no parity). The default parity
mode must be even parity.
Baud rate: default 19200, Baud rates supported are
2400,4800,9600,19200,38400
Remark: The use of no parity requires 2 stop bits.
A Modbus message is placed by the transmitting device into a frame that has a
known beginning and ending point. This allows devices that receive a new frame
to begin at the start of the message, and to know when the message is completed.
Partial messages must be detected and errors must be set as a result.
In RTU mode, message frames are separated by a silent interval of at least 3.5
character times this time interval is called t3,5.
Serial Communication
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