AccuEnergy AcuRev 1300, User Manual

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data that terminals respond to the request. This data may be a numerical value, address or
setting. For example, Function Code tells the terminal to read one register, Data Field needs
to specify reading from which register and how many registers to read.
2.5 Error Check Field
The field allows the error check by master and slave devices. Due to electrical noise
andother interferences, a group of data may be changed while transmitting from one
location to the other. Error Check ensures master or slave devices do not respond to the
distorted data during the transmission, which enhances the system security and efficiency.
Error Check uses 16-bit Cyclic Redundancy Check (CRC 16).
3. CRC Check
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 isapplied 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 valueis
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 applying
successive 8-bitbytes of the message to the current contents of the register. Only the eight
bits of data ineach character are used for generating the CRC. Start and stop bits, and the
parity bit, donot apply to the CRC.
When generating the CRC, each 8-bit character is exclusive ORed with the register
117contents. The result is shifted towards the least signifcant bit (LSB), with a zero filled