Allen-Bradley 857-VPA3CG, Руководство пользователя, Страница: 3 / 14

Rockwell Automation Publication 857-UM003B-EN-P - December 2017 3
857-VPA3CG Profibus DP Option Module
The device calculates the needed buffer length. For example, if the device's
display shows:
InBuf 32 byte
OutBuf 0 byte,
select the corresponding module "INPUT: 32 Byte" from the .gsd file (for
example, VPA_00F7.gsd) when configuring the PROFIBUS-DP master for this
device.
The following example reveals the algorithm that is used by the supplier of this
device to calculate the needed PROFIBUS-DP buffer length:
Example:
•
The parameter Eimp (imported active energy) has the biggest offset, 30, of
all activated PROFIBUS-DP data items in InBuf.
•
The size of Eimp item is 4 bytes.
•
The parameter Direct05C (Direct Operate Object 5 Close) has the
biggest offset, 13, of all activated PROFIBUS data items in OutBuf.
•
The size of Direct05C is 1 byte.
•
Both buffers are forced to the same length.
Input:
Offset of the last occupied byte:
Round up to next 2
n
(n is an integer):
Round up to next integer ----* 6:
Output:
Offset of the last occupied byte:
Round up to next 2n (n is an integer):
Round up to next integer ~ 4:
After restarting, the device displays these buffer sizes:
InBuf 32 byte
OutBuf 0 byte,
If both buffers are in use in the continuous mode, the length of the smaller buffer
is forced to match the longer buffer. In the example: 2
4
= 16 is forced to 2
6
= 64.
30 4 1 –
 
+ 33 =
33 log log
2
5.044 =

2
6
64 =
13 1 1 –
 
+ 13 =
13 log log
2
3.700 =

2
4
16 =