KROHNE OPTISONIC V6 Modbus - Manual (Page 5)

2011-07-19
OPTISONIC_V6_Modbus_6.doc 5/29
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ts 3 SERIAL TRANSMISSION FORMAT
Two transmission modes are defined for a Modbus data communication link:
• Modbus ASCII
• Modbus RTU.
Both transmission modes are supported, the user can select the desired mode along with the serial
communication parameters (baud rate, parity).
The default configuration of the OPTISONIC V6 is Modbus RTU communication mode with “standard”
Modbus settings.
Check chapter 7 for a list of programmable parameters and the default settings of these parameters. Except
for the device addresses all these parameters must be the same for all controllers in the network.
3.1 ASCII mode
In the Modbus message each byte of data is coded as 2 ASCII characters; one to represent the upper 4 bits
and another to represent the lower 4 bits. Each group of 4 bits is represented by a hexadecimal number,
transmitted as an ASCII character from the range 0-9, A-F.
Standard serial communication parameters:
Start bits:
1
Data bits: 7
Parity: odd/even/none
Stop bits: 1 stop bit if parity is used, or
2 stop bits if no parity is used.
Error check field: Longitudinal Redundancy Check (LRC).
An advantage of ASCII mode is that it allows for a time interval up to 1 second between characters without
causing a timeout. A disadvantage of ASCII mode is the larger message length.
3.2 RTU mode
Each byte of data is represented in the message by an equivalent number of bits (8).
The number of bits transmitted in the process of communicating one byte of information is sometimes also
referred to as a “character”. Note that this is not the same as an ASCII character.
Default serial communication parameters:
Baud rate:
19200
Data bits: 8
Parity: even
Stop bits: 1
Error check field: Cyclic Redundancy Check (CRC).