Manual

ALTOSONIC V
ModBus Manual 0300 rev07 E 7.30855.35.00 Page 8 of 64
In ASCII mode this byte is made of 2 ASCII characters.
The data field of messages contains information which both master and slave use to perform an
action. This includes the register address, quantity of registers, and the necessary data.
3.4 The Error Checking Field
The error checking field contents depend on the transmission mode.
Two kinds of error methods are used.
Error check with ASCII-mode
When the ASCII mode is used, the error-checking field contains two ASCII characters.
The error check characters are the result of a Longitudinal Redundancy Check calculation. This is
performed on the message contents with exception of the beginning colon, the carriage return and line
feed characters.
The LRC characters are appended to the message as the last field preceding the
CR-LF characters.
 See 9.2 Appendix B for more information about the Longitudinal Redundancy Check.
Error check with RTU-mode
When RTU mode is used, the error-checking field contains a 16-bit value implemented as two bytes.
The error check value is the result of a Cyclic Redundancy Check calculation performed on the
message contents.
The CRC field is appended to the message as the last field.
 See 9.3 Appendix C for more information about the Cyclic Redundancy Check.
3.5 Other Error Checking Methods
Standard Modbus uses two kinds of error checking methods:
1. Character based check
an additional parity bit for each character (even or odd parity).
2. Message based check
an additional error check calculated over the entire message.
Both character check and message check are generated in the transmitting device and applied to the
message before transmission.
The slave checks each character and the entire message frame during receipt.
The master has a predetermined timeout interval before aborting the transaction. This interval is set
long enough for any slave to respond normally.
The timeout interval is set by the parameter 7.2 REQUEST_TO_RESPONSE_TIMEOUT.
ASCII mode
In ASCII mode the maximum time between 2 characters is one second. If a longer interval occurs, the
message will be rejected and the search for a starting character (colon) is resumed.
RTU mode
In RTU mode the entire message frame must be transmitted as a continuous stream. If a silent interval
of more than 3.5 character times occurs before completion of the frame, the receiving device flushes
the incoming message and assumes that the next byte will be the address field for the new message.