GE EPM 9650/9800 manual CRC Error Checksum Algorithm

Table 1.4: Function Code 10 Example
Master Packet		Slave Packet
Slave Address	01H	Slave Address	01H
Function Code	10H	Function Code	10H
Data Starting Address-Hi	E0H	Data Starting Address-Hi	E0H
Data Starting Address-Lo	01H	Data Starting Address-Lo	01H
Number of Setpoints-Hi	00H	Number of Setpoints-Hi	00H
Number of Setpoints-Lo	03H	Number of Setpoints-Lo	03H
Byte Count	06H	CRC-Lo	E6H
Data #1-Hi	00H	CRC-Hi	08H
Data #1-Lo	01H
Data #2-Lo	00H
Data #2-Hi	01H
Data #3-Lo	00H
Data #3-Hi	01H
CRC-Lo	4DH
CRC-Hi	46H

1.5: CRC (Error Checksum) Algorithm

The Cyclic Redundancy Check (CRC) field is an error checksum calculation that enables a Slave device to determine if a request packet has been corrupted during transmission.

Every request packet transmitted from Master to Slave includes a special 16-bit value derived from a CRC-16 algorithm performed on the packet’s contents. When an EPM Slave receives a packet, it performs a CRC-16 calculation and compares the value with the one included in the request packet. If the two values do not match, the Slave will ignore the packet.

The following is the pseudocode for calculating the 16-bit CRC:

Initialize a 16-bit register to FFFFH.

Initialize the generator polynomial to A001H.

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