Figure 7-5. Representation Of A Message Packet - Advanced Energy MDX II User Manual

Figure 7-5 on page 7-9 shows the organization of these data fields in the AE Bus message
packet. The subsequent paragraphs describe each data field.
7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0
Header
5-bit address
3-bit length

Figure 7-5. Representation of a message packet

Header
This field contains two pieces of information: the first five bits contain the address, and the last
three bits contain the length of the Data field. If the message packet originates with the host
computer, the address specifies the packet's destination (to an MDXII unit, for example). If the
packet is going to the host, the address specifies the packet's origin (from the MDXII unit).
The address section of the Header field is 5 bits long, which allows a total of 32 distinct
addresses. Address 0 (zero) is reserved for the network broadcast address; when this address is
used in a host-originated packet, all units receive the packet (but will not respond).
The remaining 3 bits tell the receiving unit how long the Data field is so that the receiving unit
can determine when the entire message has been received.
Note: The value in these bits should refer only to the number of actual data bytes. Do not
include the checksum byte when calculating the value for these bits.
The Header field (address and length) must be at the beginning of the message packet so that
the receiving unit can compute the length of the packet after receiving the first byte.
Optional
This field supplements the Header field. The Optional field contains a value only when the
length bits in the Header field contain a value of 7. (A value of 7 indicates that the Data field
contains more than 6 bytes of data.) Under those circumstances, the Optional field contains a
one-byte value (between 7 and 255) indicating the actual length of the Data field.
Command
This field contains a one-byte value: 00h to FFh (0 to 255). If the message packet originates
with the host computer, this value specifies the purpose of the message packet. If the message
originates with the MDXII unit, the value specifies the command to which it is responding.
See Table 7-9 on page 7-20 for a complete list of serial port commands.
5700262-R
7 6 5 4 3
Optional
Serial Port Connection, Controls and Indicators
MDXII Power Supplies
1 0 7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0
Checksum
7-9