FCM 300 PROFIBUS / VLT 2800 PROFIBUS
MG.90.A2.02 – VLT is a registered Danfoss trademark
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System layout
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Bus topology
Single master operation with DP
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Single master
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PLC communicates with telegrams of constant
length
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Fits to time critical requirements
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No need for equidistant transmissions of
set points
Cyclical transmission
1. Set point transmission
2. Actual value feed back
3. New setpoints computed
4. New setpoint transfer
Featur
Featur
Featur
Featur
Features of DP
es of DP
es of DP
es of DP
es of DP (Distributed Periphery)
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Is used by several PLC manufacturers for remote
peripheral I/O communication.
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Supports cyclical communication.
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SRD service gives fast cyclical exchange of
process data between master and slaves.
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Freeze and synchronize function is supported
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Fixed data structure.
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Fixed telegramme size.
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Occupies I/O memory space in PLC proportional
to the number of slaves employed, which may limit
the number of participants. Additional data
require additional I/O memory space.
Transferring large amount of data where rapid transfer
of process data does not have highest priority
DP should be used when fast cyclical process control
is needed. Such a concept would typically call for
single master operation with a limited number of slave
stations. (A high number of slaves will reduce the
system response).
This could also be the case where control loops are
closed over the bus.
As a very fast alternative it is of course possible to
close the control loop outside the bus.
