Apex Digital 5708009-C manual Checksum, Creating AN Ideal Communications Transaction

Advanced Energy®

Checksum

This one-byte field is the last one in the packet. The content depends on the value of each of the preceding fields. The transmitting unit determines this value by accumulating the exclusive-or(XOR) of all bytes of the packet up to, but not including, the checksum value. The receiving unit accumulates the XOR of all bytes of the packet, including the checksum. If the result is zero, the packet has likely been received intact.

Only after the checksum of a message packet is validated (having no parity errors, and the address is valid) will the Apex generator act on the message (which consists of the contents of the command and, if appropriate, the data fields).

CREATING AN IDEAL COMMUNICATIONS TRANSACTION

Figure 4-6. is a simplified graphic showing the steps in an ideal communications transaction between a host computer and the Apex generator.

Host Apex generator

Figure 4-6.AE Bus communications transaction

First, the host computer sends a message packet to the Apex generator. The packet contains one of the following:

•A command that requests data or status information

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