User's Manual

Appendix C: Communications via PROFIBUS
C.7 Cyclic versus acyclic data
SITRANS LR250 (PROFIBUS PA)
Operating Instructions, 01/2014, A5E32221386-AB
233
C.7
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.
C.7.1
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; 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
[see
Analog Input Function Blocks 1 and 2 (Page 225) for details].
AIFB1
and
AIFB2
return 5 bytes of data each:
Floating Point
Status
AIFB1
byte 1
byte 2
byte 3
byte 4
byte 5
AIFB2
byte 6
byte 7
byte 8
byte 9
byte 10
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.