из Modicon Modbus Protocol Reference Guide
PI–MBUS–300 Rev. J
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RTU Mode
When controllers are setup to communicate on a Modbus network using 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 for the same baud rate.
Each message must be transmitted in a continuous stream.
The format for each byte in RTU mode is:
Coding System: 8–bit binary, hexadecimal 0–9, A–F
Two hexadecimal characters contained in each
8–bit field of the message
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)