Apex Digital 5708009-C - Manual (Page 88)

4-42 5708009-C
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.
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
Host Apex generator