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03 ILLEGAL DATA VALUE
04 SLAVE DEVICE ABORT
05 ACKNOWLEDGE
06 SLAVE DEVICE BUSY
07 NEGATIVE ACKNOWLEDGE
08 MEMORY PARITY ERROR
An application program in the master is responsible for handling exception responses.
Typical processes include successive attempts to send a query, sending diagnostic
messages to the slave, and notifying the operators.
3.9 CRC generation
The Cyclical Redundancy Check (CRC) field is two bytes, containing a 16–bit binary value.
The CRC value is calculated by the transmitting device, which appends the CRC to the
message. The receiving device recalculates a CRC during receipt of the message, and
compares the calculated value to the actual value it received in the CRC field. If the two
values are not equal, an error results.
Placing the CRC into the Message:
When the 16–bit CRC (two 8–bit bytes) is transmitted in the message, the low-order byte
will be transmitted first, followed by the high-order byte.
Example: here is an example of calculating directly the CRC.
/*-----------------------------------------------------------------
FUNCTION
-----------------------------------------------------------------
INPUT PARAMETERS :
controller
-----------------------------------------------------------------
OUTPUT
-----------------------------------------------------------------
*/
A value contained in the query data field is out of
range. The contents of the register or the status of
the coil has not changed (see paragraph 4.3).
An unrecoverable error occurred while the slave was
attempting to perform the requested action. This
may happen when the access level for changing a
parameter is not reached (see paragraph 4.2.)
Not supported
Not supported
Not supported
Not supported
: This routine calculates the crc high and low byte of a message.
buf -> Array containing message to be sent to controller
start -> Start of loop in crc counter, usually 0.
cnt
->
Amount of bytes in message being sent to
:
temp -> Returns crc byte for message.
RVT communication ç Modbus function codes 21
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