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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.
2.5.
Modbus Message Framing
A Modbus message is placed by the transmitting device into a frame that has a known beginning and ending point.
This allows receiving devices to begin at the start of the message, read the address portion and determine which de-
vice it is, and to know when the message is completed.
Partial messages can be detected and errors can be set as a result.
Modbus RTU Framing
In RTU mode, messages start with a silent interval of at least 3.5 character times.
This is most easily implemented as a multiple of character times at the baud rate that is being used on the network
(shown as T1–T2–T3–T4 in the figure below).
Another factor to consider is that each device has its own response time. This response time can be anywhere from a
few milliseconds to a few hundred milliseconds. The Host must be configured to allow adequate time for the slowest
device to respond.
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