Communication
2.2 Modbus
21SICAM Q100, 7KG95xx, System Manual
E50417-C1040-C522-A8, Edition 09.2020
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:
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
Table 2-3 Modbus RTU Settings
Parameter Default Setting Setting Range
Device address 1 1 to 247
Baud rate 19 200 bit/s 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
Parity Even no/1 stop bit
even
odd
no/2 stop bits
Access rights Full Full
Read only
Communication supervision time 600 * 100 ms 0 s = none
100 ms to 6 553 400 ms
SYS_SICAM_Q100_US.book Page 21 Thursday, September 24, 2020 1:17 PM
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