Siemens SITRANS LR 300, Instruction Manual, Page: 118 / 140

Page 116 SITRANS LR 300 – INSTRUCTION MANUAL 7ML19985CL02
mmmm
m
Appendix VII
When you select a module, for each of the four values there are two alternatives: a
normal version and a short version, for example, Level (short) and Level. The difference
between the two is the way each one identifies the function block used. Level uses both
the identifier and the extended identifier byte to determine which function block in the
unit to use. Level (short) uses only the identifier byte. In the current release of Profibus PA
there is no functional difference between the short and normal versions. However, the
longer identifier is the preferred way to identify the function block and you should select
the normal version in each case.
The 4 function blocks (Level, Space, Distance, Volume) 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 default setting for the three variables, level,
space, and volume, is percent. The default setting for the variable distance is meters. You
can change the settings of the variables by changing the settings of the function block.
This is typically done using PDM.
The 5th byte is the status word and the list of possible values is given in the chart on
page 117.
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.
If you select a Free Place module to fill one of the slots, this will affect the byte number.
Example 1:
Floating Point Status
Volume
Distance
Space
Level byte 1 byte 2 byte 3 byte 4 byte 5
byte 6 byte 7 byte 8 byte 9 byte10
byte 11 byte 12 byte 13 byte 14 byte 15
byte 16 byte 17 byte 18 byte 19 byte 20
byte10
Floating Point Status
Volume
Distance
Free
Place
— — — — —
— — — — byte10
byte1 byte 2 byte 3 byte 4 byte 5
byte 6 byte 7 byte 8 byte 9 byte 10
—
Free
Place
SITRANS LR 300.book Page 116 Monday, June 10, 2002 7:18 PM