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The Query:
The function code in the query tells the addressed slave device what kind of action to
perform. The data bytes contain any additional information that the slave will need to
perform the function.
The data field must contain the information telling the slave which register to start at
and how many registers to read.
The error check field provides a method for the slave to validate the integrity of the
message contents.
The Response:
If the slave makes a normal response, the function code in the response is an echo of the
function code in the query. The data bytes contain the data collected by the slave, such
as register values or status. If an error occurs, the function code is modified to indicate
that the response is an error response, and the data bytes contain a code that describes
the error.
The error check field allows the master to confirm that the message contents are valid.
2.4 Serial Transmission Mode
The transmission mode defines the bit contents of message fields transmitted serially
on the networks. It determines how information will be packed into the message fields
and decoded.
Modbus defines two transmission modes: ASCII or RTU.
Only RTU mode will be used here. The mode and serial parameters must be the same for
all devices on a Modbus network.
RTU Mode
The main advantage of this mode is that its greater character density allows better data
throughput than ASCII for the same baud rate.
Each message must be transmitted in a continuous stream.
The format for each byte in RTU mode is:
Bits per Byte:
1 start bit
8 data bits, least significant bit sent first
1 bit for even/odd parity; no bit for no parity
1 stop bit if parity is used; 2 bits if no parity
Error Check Field: Cyclical Redundancy Check (CRC)
The messages are transmitted in the network from left to right, i.e. the Least
Significant Bit (LSB) first and the Most Significant Bit (MSB) last.
RVT communication ç Modbus protocol overview 9
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