AccuEnergy Acuvim II, User Manual

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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. If the two values are not equal, an error will be reported. CRC
calculation is first started by preloading the whole 16-bit register to 1’s. The
process begins by 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 the CRC. Start and stop bits, and the parity bit, do not apply
to the CRC. When generating the CRC, each 8-bit character is exclusive ORed
with the register contents. The result is shifted towards the least significant bit
(LSB), with a zero filled into the most significant bit (MSB) position. The LSB is
extracted and examined, if the LSB equals to 1, the register is exclusive ORed
with a preset, fixed value; if the LSB equals to 0, no action will be taken. This
process is repeated until eight shifts have been performed. After the last (eighth)
shift, the next 8-bit byte is exclusive ORed with the register's current value, and
the process repeats for eight more shifts as described above. The final contents
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