Operation
13
Peculiarities of "User_Prm_Data"
The "User_Prm_Data" already mentioned in the examples for
reading and writing values contain 4 predefined bytes at the
beginning of the file. -
These four bytes have the following meaning:
The 1st, 2nd and 3rd byte must be preset to 0x00.
The 4th. byte defines the order of bytes for cyclic data transfer, i.e.
the order in which the bytes are to be transmitted on the bus. -
Refer to section "Numerical Formats and Displays" for details.It is
important that the setting is made for all dates, not specifically for
a special date.This byte should be set to 0 or 0x00 or 255 or
0xFF. If other values are entered, a parameter data error will be
indicated.
Some DP configuration tools require that the maximum number of
data bytes is set in the "User_Prm_Data".---- Usually, less than
110 variables have to be read or written. To avoid a complicated
parameter definition for all unused variables in this case, the
procedure can be simplified by entering the integer value 0x0000
(or 2 bytes 0x00, 0x00) after the last used index. - When this
value is recognized as and index by the PROFIBUS-DP module,
the following part of "User_Prm_ Data" will be ignored.
This means that the two examples shown below have the same
result in terms of cyclic data transfer for "User_Prm_Data":
User_Prm_Data = .., .., .., .., 5, 7, 9, 11 End_User_Prm_Data
User_Prm_Data = .., .., .., .., 5, 7, 9, 11, 0, 47, End_User_Prm_Data
Configuration tools of this kind always transfer the max. possible
data quantity at the time when parameters are defined. However,
while cyclic data transfer is in progress only the absolutely
required data is transmitted to keep the bus load as low as
possible for the respective station. -
