Power Demands; Cyclic Versus Acyclic Data; Cyclic Data - Siemens sitrans LR 400 Instruction Manual And Users Manual

Power Demands

The maximum number of devices that can be connected to a bus line depends on their
current consumption and the respective application conditions. When operating in an
area where there is no risk of explosion, the couplers/links can feed up to 400 mA into the
bus.
When operating in explosion risk areas, the intrinsic safety is only guaranteed when the
maximum power fed into the bus does not exceed certain voltage and current values.
These are normally:
Current I < 128 mA, voltage U
S
WARNING: Only certified supply units (DP/PA couplers or DP/PA
links) may be used to feed the intrinsically safe PROFIBUS. See
the EC Type Examination Certificate for requirements.
The number of devices which can be connected to a bus line is determined by finding
the combined maximum current consumption of all the connected devices (10.5 mA for
the SITRANS LR 400). Plan to allow a current reserve for safety.

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 and output 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 the SITRANS LR 400 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 Level information and slot 1always Volume information. Slot2 is
always Mass information. If you do not wish to have data transmitted, then you must use
a Free Place module in that slot.
When you select a module, for the three 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.
Page 99
< 15 V
0
SITRANS LR 400 – INSTRUCTION MANUAL 7ML19985FH04