Manual

ALTOSONIC V
ModBus Manual 0300 rev07 E 7.30855.35.00 Page 6 of 64
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).