Apex Digital 5708009-C manual Profibus-Specific Errors, Profibus Data Consistency

Advanced Energy®

Profibus-Specific Errors

In the event of a Profibus error, the Apex generator turns output power off and sets the Profibus fault status bit. All Profibus errors are treated as “explicit clear” type faults; thus you must send Command 119 (the “master reset” command) in the next download packet to clear them. Once all faults have been cleared, the Apex generator is ready to continue operation.

Profibus Data Consistency

Some PLCs using Profibus interfaces have a problem with data consistency. (We define “data consistency” as the ability to complete the message packet construction before sending the packet to the Apex generator.) This problem most often manifests as mysteriously changed values. That is, values at the Apex generator 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.

You can create a “work around” to this problem with a command sequence. (For more information about Profibus commands, see the “Host/Apex Software Commands” section later in this chapter.) Here is an example procedure:

1.Send the null command (Command 0). The Apex generator ignores 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.

5.Repeat step 1, and continue as needed.

By creating a procedure like this one, you can ensure the data for a command will not be changed before the next download packet is received.

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