Technical data

Communication
9.5 Communication using USS
Function Manual
Function Manual, 06/2012, 6SL3097-4AB10-0BP4
687
Parameter value (PWE)
With communication over USS, the number of PWEs can vary. One PWE is required for 16-
bit values. Two PWEs are required if 32-bit values are exchanged.
Note
U8 data types are transferred as U16 with the upper byte set to zero. U8 fields therefore
require one PWE per index.
A parameter channel of 3 words represents a typical data telegram for exchanging 16-bit
data or alarms. The mode with a fixed word length of 3 is used when p2023 = 3.
A parameter channel for 4 words is a typical data telegram for exchanging 32-bit data
variables and requires p2023 = 4.
A parameter channel with a variable length is used with p2023 = 127. The telegram length
between the master and slave can vary in terms of the number of PWEs.
When the length of the parameter channel is fixed (p2023 = 3 or 4), the master must always
send either 3 or 4 words in the parameter channel accordingly. Otherwise the slave will not
respond to the telegram. The response of the slave will also comprise 3 or 4 words. For a
fixed length, 4 should be used because 3 is insufficient for many parameters (i.e. double
words). For a variable length of parameter channel (p2023 = 127), the master will only send
the number of words necessary for the task in the parameter channel. The response
telegram is also no longer than necessary.
Rules for editing requests/responses
● A request or a response can only be referred to one parameter.
● The master must constantly repeat a request until it receives a suitable response.
● The master recognizes the response to a request that it sent by:
– Evaluating the response identifier
– Evaluating the parameter number (PNU)
– Evaluating the parameter index (IND), if necessary, or
– Evaluating the parameter value PWE, if necessary.
● The complete request must be sent in a telegram. Request telegrams cannot be
subdivided. The same applies to responses.
● If response telegrams contain parameter values, the drive always returns the current
parameter value when it repeats response telegrams.