Page 136 SITRANS LR250 (PROFIBUS PA) – INSTRUCTION MANUAL 7ML19985JF02
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G: Communications PA
Cyclic versus Acyclic Data
When you request data from a device via PROFIBUS PA, you have two choices. Cyclic
data is provided at every bus scan: acyclic data is requested and provided as needed.
Input information is always requested at every bus scan and is set up as cyclic data.
Configuration information is only needed periodically and is set up as acyclic data.
Cyclic Data
When you configure SITRANS LR250 on the PROFIBUS PA bus, there are two slots
available for modules.
Slot 0 always transmits AIFB1 information
1)
; slot 1 defaults to Free Place, but can be
changed to AIFB2 information. If you do not wish to have data transmitted, then you must
use a Free Place module in that slot.
Each of the two Analog Input Function Blocks can be set up to return Level, Distance, or
Volume. Within the function blocks, the values are scaled according to the user
requirements (please see
Analog Input Function Blocks 1 and 2
on page 131 for details).
AIFB1 and AIFB2 return 5 bytes of data each:
The first 4 bytes are the floating point representation (IEEE) of the variable. The variables
are the outputs of the function block. The 5th byte is the status word and the list of
possible values is given in the chart below.
The 5 bytes must be read consistently, in a contiguous chunk: they cannot be read byte by
byte, and cannot suffer an interrupt. If you are using an S7-300 / 400, you will need to use
SFC14 DPRD_DAT: Read Consistent Data of a Standard PD Slave.
Note: Each of the slots has to have a module defined in it.
1)
For more information, please see
Analog Input Function Blocks 1 and 2
on
page 131.
Floating Point Status
AIFB2
AIFB1 byte 1 byte 2 byte 3 byte 4 byte 5
byte 6 byte 7 byte 8 byte 9 byte10byte10
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