AMPTRON Ai205, Руководство пользователя

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Data Field
The data field is constructed using sets of two hexadecimal digits, in the range
of 00 to FF hexadecimal. The data field of messages sent from a master to slave
devices contains additional information which the slave must use to take the action
defined by the function code. This can include items like discrete and register
addresses, the quantity of items to be handled, and the count of actual data bytes
in the field. For example, if the master requests a slave to read a group of holding
registers (function code 03), the data field specifies the starting register and how
many registers are to be read. If the master writes to a group of registers in the
slave (function code 10 hexadecimal), the data field specifies the starting register,
how many registers to write, the count of data bytes to follow in the data field, and
the data to be written into the registers.
If no error occurs, the data field of a response from a slave to a master contains
the data requested. If an error occurs, the field contains an exception code that
the master application can use to determine the next action to be taken. The data
field can be nonexistent (of zero length) in certain kinds of messages.
Error Check Field
Messages include an error ’ s checking field that is based on a Cyclical
Redundancy Check (CRC) method. The CRC field checks the contents of the
entire message. It is applied regardless of any parity check method used for the
individual characters of the message. The 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
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