Cyclic Versus Acyclic Data; Cyclic Data - Siemens Sitrans LR250 Operating Instructions Manual

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.
Note:
Each of the slots has to have a module defined in it.
Slot 0 always transmits AIFB1 information
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
AIFB1 and AIFB2 return 5 bytes of data each:
AIFB1 byte 1
AIFB2 byte 6
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.
1)
For more information, please see
page 142.
Page 148 SITRANS LR250 (PROFIBUS PA) – OPERATING INSTRUCTIONS
1)
; slot 1 defaults to Free Place, but can be
Analog Input Function Blocks 1 and 2
Floating Point
byte 2 byte 3
byte 7 byte 8
Analog Input Function Blocks 1 and 2
on page 142 for details).
Status
byte 4 byte 5
byte 9 byte10 byte10
on
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