How to cancel syncronous I/O DLL-call?

We have an application with timeout set to 300seconds. If the application is terminated whilst a Send of Receive is in progress, the application is killed, but the NI driver remains active until the timeout is reached (and therefore blocks any further GPIB attempts).
Is there any way to cancel a syncronous I/O operation other than timeout?
Thanks.

It's not the case that ibln doesn't work with our instruments.
The reason we use a 300sec timeout is because we occasionally do a read from an instrument before the instrument is ready to respond (some instruments don't support SRQ or polling status byte). In which case, if we did a ibln prior to doing the receive, it'd fail, since it is currently processing and not listening to the bus. In these circumstances we do a receive and wait around for a while. Problem is that sometimes the response never arrives.
A lot of this is to do with the way we are programming the GPIB driver. We're using the low-level send/receives, so we can't set individual timeouts for each instrument. One instrument make take 4 minutes to respond, which is o
k, but another may take 30 seconds. If we don't receive a response after this period, we wish to abort the read.
It's a bit of a dodgy arrangement, but we are stuck with these instruments and their method of responding. What we are trying to achieve is a "controlled" failure, rather than the NI driver blocking for the full length of time.

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