Advanced Energy MDX II User Manual page 137

In the first status byte 0, bit 5 (the active toggle bit) indicates, in general, the status of the
Profibus interface. After the unit has powered up, this bit's continuous change indicates that
the Profibus interface is ready. During operation, a cessation of this change indicates that a
communication problem exists.
In the upload packet, bytes 8, 9, 10, 11, and 12 comprise the "data field" and contain
information defined by byte 13, the data field definition byte. Note that when the data extends
over more than one byte, the Profibus sends the least significant byte (LSB) before the most
significant byte (MSB). Byte 13 references the requesting command (>127).
The AE Profibus does not support the following functions: address changing, freeze/unfreeze
modes, or sync modes via the Master. For a complete listing of available supply functions, see
Table 7-9 on page 7-20.
Profibus Data Consistency
Some PLCs using Profibus interfaces have a problem with data consistency. (We define "data
consistency" here as the ability to complete the message packet construction before sending
the packet to the unit.) This problem most often manifests as mysteriously changed values,
that is, values at the unit seem to have changed automatically even though no command has
executed, or they appear changed because one data byte contained the data that another data
byte should have contained.
The root of the problem is a shared memory block. Most PLCs share a memory block with the
Profibus interface. The PLC places data/packet information in the memory block, and the
Profibus interface reads the memory block for the next data/packet to transmit. The problem
occurs when the PLC updates the data from High to Low memory locations and does not
signal the Profibus interface when the update is complete. (If the PLC were to notify the
Profibus interface, then there would be "data consistency".) As a result, the Profibus interface
sends the memory block regardless of where the PLC is in its update of that memory block.
If your PLC supports data consistency, select data consistency only for outbytes. Inbyte data
consistency is always enabled by the MDXII unit and does not need to be enabled at the PLC.
If your PLC does not support data consistency, you can create a "work around" to this problem
with a command sequence. (For more information about Profibus commands, see Table 7-9 on
page 7-20.) Here is an example procedure:
1. Send the null command (Command 0). The unit will ignore this command.
2. Update the download packet with data for the desired command.
3. Update the packet with the desired command.
4. Send the download packet.
5700262-R Serial Port Connection, Controls and Indicators
MDXII Power Supplies
7-17