SMAX MODBUS®
6.3.1 RTU transmission mode
When devices communicate on a Modbus serial line using the RTU (Remote
Terminal Unit) mode, each 8-bit byte in a message contains two 4-bit
hexadecimal characters. Each message must be transmitted in a continuous
stream of characters. Synchronization between the messages exchanged by the
transmitting device and the receiving device is achieved by placing an interval
of at least 3.5 character times between successive messages, this is called “silent
interval”. In this way a Modbus message is placed by the transmitting device into
a frame that has a known beginning and ending point. This allows devices that
receive a new frame to begin at the start of the message and to know when the
message is completed. So when the receiving device does not receive a message
for an interval of 4 character times, it considers the previous message as
completed and the next byte will be the first of a new message, i.e. an address.
When baud rate = 9,600 bit/s the “silent interval” is 4 ms.
When baud rate = 19,200 bit/s the “silent interval” is 2 ms.
When baud rate = 115,200 bit/s the “silent interval” is 3.5 ms.
The format (11 bits) for each byte in RTU mode is as follows:
Coding system:
8-bit binary
Bits per Byte:
1 start bit;
8 data bits, least significant bit (lsb) sent first;
1 bit for parity completion (= Even);
1 stop bit.
Modbus protocol uses a “big-Endian” representation for addresses and data
items. This means that when a numerical quantity greater than a single byte is
transmitted, the most significant byte (MSB) is sent first.
Each character or byte is sent in this order (left to right):
lsb (Least Significant Bit) … msb (Most Significant Bit)
Start
1
2
3
4
5
6
7
8
Parity* Stop
* When “No parity” is activated, the parity bit is replaced by a stop bit.
The default parity mode must be Even parity.
The maximum size of the Modbus RTU frame is 256 bytes, its structure is as
follows:
Slave
Address
Function code
Data
CRC
1 byte
1 byte
0 up to 252 byte(s)
2 bytes
CRC Low
CRC Hi
MAN SMAX MB E 1.2
Modbus® interface
47 of 84
