DPMS XPQ, User Manual

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DPMS XPQ User Manual
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 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 the two values are not equal, an error will result. The CRC is started
by 66 first preloading a 16-bit register to all 1’s. Then a process begins of
applying successive 8-bit bytes of the message to the current contents of the
register. Only the eight bits of data in each character are used for generating