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Acuvim-L
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
Every message includes an error checking field which is based on the 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 long, containing
a 16-bit binary value. The CRC value is calculated by the transmitting device, and
is appended to the message.
The receiving device recalculates the CRC value during reception of the message,
and compares the calculated value to the actual value it received in the CRC
field. An error will be reported if the two values are not equal. CRC calculation is
first started by preloading the whole 16-bit register to 1’s. The process begins by
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