The Two Serial Transmission Modes; Rtu Transmission; Character Format; Frame Description - BONFIGLIOLI Vectron ACTIVE Manual

6.7 The two serial Transmission Modes

Two different serial transmission modes are defined: the RTU mode and the ASCII
mode. They define the bit contents of message fields transmitted serially on the
line. They determine how information is packed into the message fields and de-
coded.
The transmission mode (and serial port parameters) must be the same for all de-
vices on a MODBUS Serial Line.

6.7.1 RTU Transmission

When devices communicate on a MODBUS serial line using the RTU (Remote Termi-
nal Unit) mode, each 8–bit byte in a message contains two 4–bit hexadecimal char-
acters. 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.
6.7.1.1
One character consists of 11 bits:
Note
Character format with parity checking:
Character format without parity checking:
6.7.1.2
Address
The maximum size of a MODBUS RTU frame is 256 bytes.
05/08
05/08

Character Format

1 start bit
−
− 8 data bits, least significant bit sent first
− 1 parity bit
1 stop bit
−
If no parity is used then an extra stop bit is added.
Start B1 B2
Start B1 B2

Frame Description

Slave Function
Code
1 byte 1 byte
Modbus Instruction manual
Modbus Instruction manual
B3 B4 B5 B6
B3 B4 B5 B6
Data
0 up to 252 bytes
B7 B8 Parity Stop
B7 B8 Stop Stop
CRC
2 bytes
(CRC-Lo, CRC-Hi)
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