Technical data

87
W&T
Subject to errors and modifications
Release I/O devices: In this context „release“ means
disconnecting the network connections to the devices so that
other applications can again have access to them.
Watchdog (
VT_R8, R/W) is a global OPC variable, i.e. not
associated with any particular I/O device. It contains a seconds
value which is continually counted down if this option is
enabled. As soon as a value of 0 is reached, the I/O devices are
released. Please note: Even if obviously only an OPC-DA client
can prevent the watchdog from being turned off (by writing
over and over a watchdog value other than zero, e.g. sending
the value 15 every 10 seconds), both DA and A&E clients are
also affected.
If no OPC clients are still connected: Depending on the
device type it may take a while (even several seconds!) until a
closed connection can be opened again and the OPC server
resumes providing valid values.
Limit update rate: An attempt to read values from a device at a
faster rate than it can actually provide those values results in
the OPC interface always finding itself in timeout situations.
The affected DA variables then continually swing back and forth
between OPC_QUALITY_GOOD and OPC_QUALITY_BAD, which
makes them effectively useless. To avoid such a situation OPC
clients are prevented by this parameter from setting too fast an
update rate.
We consider the standard default value of 800ms to be a
practical compromise between reliability and speed. Enter a
higher value if the problems described still occur, or a lower
value if you want to try out the highest possible update rate
(for your special application case). The latter depends mainly
on the device types used: Web-IO Analog-In/Out and any
devices to which a permanent network connection is opened
can be polled much faster than for example a Web-Thermome-
ter.