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• Address: address of the transmitter for command transmission.
Length = 1 byte
• Function: type of operation to be performed by the transmitter (for example, a
parameter setup).
Length = 1 byte
• Data:
Data communicated to the transmitter (for example, a parameter
value). The field can be empty in case of commands that don't involve
data transfer from PC to instrument.
Length = 0 to 252 bytes
• CRC:
Data integrity control code (Cyclic Redundancy Code).
Length = 2 bytes
Each byte (8 bits) consists of two 4-bit hexadecimal characters.
Each byte is preceded by a start bit and followed by a parity bit plus a stop bit, for a
total of 11 bits.
Fig. 8.C: Byte format with parity bit in MODBUS-RTU protocol
If parity is not used, the byte must be followed by two stop bits.
Fig. 8.D: Byte format without parity bit in MODBUS-RTU protocol
The byte is transmitted starting from the least significant bit (LSB).
All bytes in a command are transmitted in a series with no interruptions, starting from
the address byte. The last byte to be transmitted is the most significant byte in the
control code (CRC). If two consecutive bytes are separated by a pause greater than
1.5 characters, the command will be considered as invalid and will be rejected by the
receiver.
There must be an interval of at least 3.5 characters between a command and the next
one.
Frame 1
At least
3.5 char.
Fig. 8.E: Frame sequence in MODBUS-RTU protocol
After a pause in the line greater than 3.5 characters, the connected devices will
consider the command transmission as terminated. The first received character after a
pause will be considered as the start of a new command.
Frame 2
At least
3.5 char.
- 42 -
Frame 3
3.5 char.
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