Advanced Energy MDX II User Manual page 130

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Advanced Energy
Data (Data Bytes)
The Data field can contain from 0 to 255 bytes of binary data, which are interpreted in various
ways, depending on the value that appears in the Command field. The Data field typically
contains data or a CSR, depending on what was requested. Since some commands don't
require accompanying data, this field is not always present.
If the value specified in the length bits of the Header field is 0 to 6, the MDXII unit expects 0
to 6 bytes of data. However, if the value in the Header field is 7, the MDXII unit looks for an
additional eight-bit byte after the Command field (the Optional field) and will expect 7 to 255
bytes of data (as specified by the Optional field).
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 XOR
(exclusive-or) of all bytes of the packet up to, but not including, the checksum byte. 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 will the MDXII unit act on the
message (which consists of the contents of the command and, if appropriate, the data fields).
Ideal Communications Transaction
Figure 7-6 on page 7-11 is a simplified graphic showing the steps in an ideal communications
transaction between a host computer and an MDXII unit.
7-10 Serial Port Connection, Controls and Indicators 5700262-R