Allen-Bradley 857-VPA3CG, Руководство пользователя, Страница: 4 / 14

4 Rockwell Automation Publication 857-UM003B-EN-P - December 2017
857-VPA3CG Profibus DP Option Module
Select the corresponding module "INPUT/OUTPUT: 64 Byte" from the .gsd
file (for example, VPA_00F7.gsd) when configuring the PROFIBUS DP master
for this device.
Request Mode
Using the request mode, it is possible to read all available data from the 857
protection relay and still use only a short buffer for PROFIBUS data transfer.
The drawback is the slower overall speed of the data transfer and the need of
increased data processing at the PROFIBUS master as every data item must be
separately requested by the master from the slave.
After setting the PROFIBUS driver of the device into request mode, the device
will restart (wait approximately 30 seconds). After restarting the device will
ignore parameter offsets and will display the following fixed buffer sizes:
InBuf 8byte
OutBuf 8byte,
Select the corresponding module "INPUT/OUTPUT:8 Byte" from the .gsd file
(for example, VPA_00F7.gsd) when configuring the PROFIBUS-DP master for
this device.
Data Request Message from the PROFIBUS Master
The request is 8 bytes long consisting of three fields.
The Identifier word, ID, contained in the Data Request Message, consists of 3
fields with a bit configuration is shown below.
IMPORTANT
The recommended maximum size for output buffer in continuous mode is
64 bytes.
IMPORTANT
It is not possible to read continuously only one single data item. In request
mode, request at least two items in turn to get updated data from the device.
ID
(1)
(1) Type and number of the requested data.
IND
(2)
(2) Index 0…65,535 of the parameter member to be requested or 0 if the parameter is scalar.
VALUE
(3)
(3) Not used.
Byte 0 Byte 1 Byte 2 Byte 3 Byte 4 Byte 5 Byte 6 Byte 7
Request Type
(1)
(1) 0001 Read (0010 = Write).
SM
(2)
(2) 0 Spontaneous bit (not used).
Parameter Number
(3)
(3) 0... 2047 Each available data item in the device has a fixed parameter number.
b15 b14 b13 b12 b11 b10 b9 b8 b7 b6 b5 b4 b3 b2 b1 b0