The VISA library AFAIK does not allow simultaneous input and
output on a single port. The difference between asynchronous and
synchronous behavior is getting a bit blurred.
Asynchronous calls to the VISA library mean that the execution
for that execution engine is not blocked. Thus other nodes
(non-visa, or VISA on a different port) can execute. However
since there are several different execution engines and now with nice
multi-threaded O/S capabilities, it is unclear whether only a single
internal thread is blocked by a synchronous node or not. Since
several threads can be allocated to a single execution engine it
becomes hard to detect the blocking of a single thead. One of
the experts may correct my terminology here of threads and execution
engines, but this is my understanding.
But a write will block until that VISA port becomes available.
hopefully this restriction wil be lifted with VISA 4. The other
thing you can do, is not to start the read until actual characters are
in the buffer. This means that you will not block the port until
it is ready to read. This can be extended to only read the
number of characters that are actually in the buffer and not use the
read timeout waiting for characters. Maybe an NI VISA guru can
tell you how to detect the difference between async and sync visa.
The question is why every use sync visa these days? It used to
be because async was buggy but that has been cleared up a long time
ago.
-Scott
At 11:44 -0700 6/4/04, Jason Dunham wrote:
Is anyone having success at running asynchronous VISA calls for serial port reads and writes?
I made a simple test VI to read one byte from COM1, and write a continuous stream of bytes out of the same port in a parallel while loop.
The write loop stalls until the read command times out. I even tried moving the read to a subvi and changing it's execution system, but to no avail.
If I read and write from different ports it the read and write are independent, even if the VISA nodes are set to run synchronously.
I'm just not getting the whole asynchronous concept, I guess. I have yet to create a VI which behaves different with sync or async VISA calls.
I'm using LV 7.1, VISA 3.1 and Windows XP.
