Send single command to a device

[cosw]

I want to send a single command to a device about the com1-port. I
made a case-structure with a visa-write vi in the true case. In the
false case is another command. But if I start the vi with a binary
switch, the vi send continously data to the com port. How can I
inhibit this? I need only one execution of the vi in both cases.