Serial Transmission Format; Ascii-Mode; Rtu-Mode - KROHNE ALTOSONIC V Reference Manual

ALTOSONIC V

2 SERIAL TRANSMISSION FORMAT

The two transmission modes used are called:
1.
ASCII, and
2.
RTU.
The user has to select the desired mode along with the serial communication parameters (baud rate,
parity-type).
Note that all these parameters must be the same for all controllers in the network.
2.1

ASCII-mode

Each byte of the message is sent as two ASCII characters.
•
This means only the ASCII characters 0-9, A-F are transmitted.
Serial communication parameters:
•
1 start byte, 7 data bits, even/odd/no parity, 1 stop bit if parity is used and two stop bits if no parity
is used.
Error check field:
•
Longitudinal Redundancy Check (LRC).
The 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.
2.2

RTU-mode

Each byte of the message is sent as 8 bits.
•
Serial communication parameters:
•
1 start byte, 8 data bits, even/odd/no parity, 1 stop bit if parity is used, and two stop bits if no parity
is used.
Error check field:
•
Cyclic Redundancy Check (CRC).
ModBus Manual 0300 rev07 E 7.30855.35.00 Page 6 of 64