Siemens MICROMASTER 440 Operating Instructions Manual page 156

3 Functions
PKW area
With the help of the PKW mechanism, the following tasks can be performed via any
serial interface with the USS protocol:
Reading and writing parameters
Reading the description of a parameter
The PKW area can be varied. Depending on the requirements, the following
lengths can be parameterized using parameter P2013:
3 words
4 words
variable word length
The following is an example of a structure when access (write/read) is made to
single-word (16 bit) parameter values. The PKW area must be permanently set to
3 words at the master and the slave. This setting is made during start-up and
should not be altered any more during bus operation.
Parameter ID
The following is an example of a structure when access (write/read) is made to
double-word (32 bit) parameter values. Parameterization to a fixed length of 4
words applies to telegrams from the master to the slave and from the slave to the
master.
1st word
PKE
Parameter ID
Telegram data transfer with variable telegram length (refer to the following
example) means that the slave responds to a telegram from the master with a
telegram whose length does not have to be the same length as the telegram from
the master to the slave.
1st word
PKE
With:
1 word
in the net data block.
1 word ≤ m ≤ 126 words (maximum) when there is no PZD.
The length of elements PEW1 to PWEm in the reply telegram and what is
contained in them depends on the task issued by the master. Variable length
means that only the number of words necessary to pass on the appropriate
information is transferred. The minimum length, however. is always 3 words. If a
slave, for example, transfers a parameter value which is a 16-bit quantity (e.g. the
status word in parameter r0052; data format: U16), then only 3 words of the PKW
area are sent in the telegram from the slave to the master. If, for example, for
MICROMASTER the actual frequency is to be read (parameter r0021), then the
PKW area in the telegram from the slave to the master is 4 words large; the reason
for this is that the speed is represented as 32-bit quantity (data format: Float). It is
mandatory to parameterize for a variable word length, if e.g. all values are to be
156
P2013 = 3
P2013 = 4
P2013 = 127
1st word
PKE
2nd word
IND
Index
2nd word 3rd word
IND
words (maximum) when 8 PZD words (maximum) are contained
≤  m ≤  1 18
2nd word
IND
Index
3rd word
PWE1
Parameter value (double word)
4th word
PWE1 PWE2
MICROMASTER 440
Issue 10/06
3rd word
PWE1
Parameter value
1
4th word
PWE2
(m+2). word
. . . . .
Operating Instructions
6SE6400-5AW00-0BP0