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Properties of the Modbus RTU Slave
•
Client-server protocol
•
All clients have a unique address in the range from 1 to 247.
•
Packets with the address = 0 are forwarded to all clients (broadcast).
•
The individual data bytes in the messages are transmitted asynchronously with 11 bits:
1 start bit,
8 data bits,
1 parity bit and 1 stop bit or
No parity bit and 2 stop bits
•
Single messages are separated by bus silent intervals of at least 3.5 character times and end with a CRC
code for error detection.
•
RS485 is used as interface.
•
The Modbus RTU message has a maximum size of 256 bytes:
1 byte server address
253 bytes for data
2 bytes for CRC
The following parameters can be set for the Modbus RTU bus protocol:
Table 2-3
Parameter
Device address
Baud rate
Parity
Access rights
Communication supervision time
2.2.5
Numbering of Modbus Registers
2.2.5.1 Modbus Specifications
Generally, we have to differentiate between the following elements in Modbus:
•
the register number
•
the register address in Modbus messages
SICAM Q100, 7KG95xx, System Manual
E50417-C1040-C522-A8, Edition 09.2020
Modbus RTU Settings
Default Setting
1
19 200 bit/s
Even
Full
600 * 100 ms
Communication
Setting Range
1 to 247
1200 bit/s, 2400 bit/s
4800 bit/s, 9600 bit/s
19 200 bit/s, 38 400 bit/s
57 600 bit/s, 115 200 bit/s
no/1 stop bit
even
odd
no/2 stop bits
Full
Read only
0 s = none
100 ms to 6 553 400 ms
2.2 Modbus
21
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