Anybus Communicator PROFINET IRT User Manual page 62

Anybus Configuration Manager
6.12.3 Data Consistency
The Data Valid bits in the Control and Status registers are used to ensure data consistency dur-
ing start-up and fieldbus offline/online transitions.
If the Control/Status Word parameter in Anybus Configuration Manager is set to Enabled, the
gateway will wait for the fieldbus control system to set the Data Valid bit in the Control register
before it starts exchanging data on the sub-network.
If the same parameter is set to Disabled or Enabled but no startup lock, communication will start
as soon as the fieldbus goes online.
State Machine
Fieldbus network participation can be described using a state machine:
A: Offline (no data exchange)
1. Clear the Data Valid bit in the Control Register.
2. Write initial data to the Output Area according to
the subnetwork configuration.
3. Wait until the fieldbus control system and the
gateway are online on the fieldbus network, then
shift to state B.
B: Online (not yet exchanging data)
4. Wait until the Data Valid bit in the Status Register
is cleared by the gateway.
5. Set the Data Valid bit in the Control Register.
6. When the Data Valid bit in the Status Register is
set by the gateway, shift to state C.
7. If the fieldbus goes offline, shift to state A.
C: Online (exchanging data)
• Exchanging valid data in both directions.
If the fieldbus goes offline, shift to state A.
The gateway cannot spontaneously clear the Data Valid bit in the Status Register.
Note on Latency
The Data Valid bit in the Status Register may in some cases be delayed. This latency can be
caused by a missing node, or by a bad connection to a node with a long timeout value assigned
to it. The fieldbus control system should therefore never wait for this bit to be set before commu-
nicating with the subnetwork devices. The Data Valid bit should only be considered as an aid
for the fieldbus control system to know when all data has been updated.
®
Anybus Communicator ™ PROFINET
®
IRT (2.32) User Manual
A
Start
Offline
(no data exchange)
B
Online
(not yet exchanging data)
C
Online
(exchanging data)
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