AccuEnergy AcuRev 2000, User Manual

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Data Field
Data field contains the data that terminals need to complete the request and the 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.
Error Check Field
The field allows the error check by master and slave devices. Due to electrical noise and
other 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 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 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