W&T 57630, Руководство пользователя

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135
W&T
OPC
Subject to errors and modifications:
Enable I/O devices:
Enable in this context means breaking the TCP connection to
the devices so that other clients can again have access. The
effect is comparable to manually deactivating individual
devices.
The OPC variables of enabled devices continue to exist but no
longer provide valid values.
Watchdog
(VT_R4, R/W) is a global OPC variable which is not associated
with any particular I/O device. It contains a seconds value which is
continually counted down as long as the corresponding option is
activated here.
The I/O devices are enabled as soon as the value 0 is reached.
The status of the OPC server then changes to
OPC_STATUS_SUSPENDED
and reading individual variables only returns
OPC_QUALITY_NOT_CONNECTED.
The client can prevent this by repeatedly (at regular intervals)
assigning a value other than zero, for example the value 15 every
10 seconds.
The other enable mechanism, which only checks whether there are
still OPC clients connected, is clearly more elegant, since it
functions without any additional effort on the client side.
It does depend however on all clients correctly logging in and out,
which is not necessarily the case. Conceivable problems would be
for example incorrectly programmed or unexpectedly crashing
clients as well as interrupted network connections to clients on
another computer. The watchdog variable procedure is immune
to such problems.