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Checking Data Link Status Section 5-4
5-4-5 Error Detection Program Example
When the source node's data link is participating, the AND condition of the
data link status for each node can be taken using the Data Link Participation
Flag and the Communications Error Flag, and the error output.
The following example shows an error output program for cases where the
local node data link stops.
The local node's Communications Error Flag (in the data link status area) will
be turned ON if the local node is isolated from the network while the data link
is active. In this case, the data link remains active, so the node's Data Link
Participation Flag will remain ON even though the node is isolated from the
network.
5-4-6 Programming Examples for Processing Data when Errors Occur
If a communications error occurs in a data link and valid data link data cannot
be received, the previous data is stored in the data link area. To stop process-
ing using the data link data when an error occurs, refer to the following pro-
gram example.
Processing Data Only when Data Links Are Operating Normally
Perform processing only when the data links are operating normally by includ-
ing an AND condition for the Data Link Normal Operation Flag in the part of
the program that handles data link data for each node. This is an example of
programming locks in the appropriate places.
“Data Link Normal Operating Flags” can be created using the programming
described in
5-4-5 Error Detection Program Example.
Local node
Data Link
Participation Flag
Node 1
Data Link Participating Flag
Node 1
Communications Error Flag
Node 1
Error output
Node 2
Error output
Node 32
Error output
Node 2
Data Link Participating Flag
Node 32
Data Link Participating Flag
Node 2
Communications Error Flag
Node 32
Communications Error Flag
Local node
Communications Error Flag
Release Flag
Local node
Error output
Local node
Error output
DIFD 10000
10000
Local node
Data Link Participation Flag
DIFD 10000
The parts of the ladder program
that use the data link area for
the relevant node are processed
only when the corresponding
Normal Operation Flag is ON.
Normal Operation
Flag
Additional part
Normal Operation
Flag