Functions
5.11 Operation in fieldbus systems
Frequency inverter
Operating Instructions, 08.10. 2008, t.b.d.
157
Parameter value (PWE)
When communication takes place via the USS, the number of PWEs can vary. One PWE is
required for 16 bit values. If 32 bit values are exchanged, two PWEs are required.
Note
U8 data types are transferred as U16, whereby the upper byte is zero. U8 fields, therefore,
require one PWE for each index.
A parameter channel for 3 words is a typical data message frame for exchanging 16 bit data
or alarm messages. The mode with a fixed word length of 3 is used when P2013 = 3.
A parameter channel for 4 words is a typical data message frame for exchanging 32 bit data
variables and requires P2013 = 4.
A parameter channel allowing a flexible word length is used when P2013 = 127. The
message frame length between the master and slave can have a different number of PWEs.
When the length of the parameter channel is fixed (p2013 = 3 or 4), the master must always
transmit either 3 or 4 words in the parameter channel, otherwise the slave will not respond to
the message frame. The response from the slave will also contain either 3 or 4 words. When
the length is fixed, 4 should be used because 3 is insufficient for many parameters (i.e.
double words). When the parameter channel is of a variable length (P2013 = 127), the
master transmits only the amount of words required for the task in the parameter channel.
The length of the response message frame is also restricted to the required size.
Rules for processing requests/responses
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A request or response can only refer to one parameter.
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The master must repeat a request until it receives a suitable response.
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The master recognizes the response to a request by:
–
Evaluating the response ID
–
Evaluating the parameter number (PNU)
–
Evaluating the parameter index IND (if required)
–
Evaluating the parameter value PWE (if required)
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The request must be transmitted in full in a message frame. Request message frames
cannot be split up. The same applies to responses.
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If response message frames contain parameter values, the drive always returns the
current parameter value when it repeats response message frames.