On Mon, 2007-09-10 at 17:21 -0500, Mark Rivers wrote:
> Emmanuel,
>
> > I am a little confused, and cannot find a clear way to define
> > synchronous devices. Are PCI devices (ADC/DAC, frame grabber) all
> > synchronous devices? I would think so, but after all maybe not...
>
> According to the EPICS Application Developers Guide synchronous devices
> those for which the processing can be done "without waiting". This
> obviously has room for interpretation, but a rule of thumb might be that
> it takes no more than 1 msec or so to complete. Not all PCI devices
> will fall in this category. Consider a frame grabber: if this were
> implemented using the asyn Int32Array or asynOctet interface, and the
> driver implemented the "read" operation as trigger a frame, wait for
> acquisition to complete, and return the data, this might take 1/30
> second or longer. This would definitely need to be asynchronous.
> However, if the driver were constantly caching frames and just returned
> the most recent one immediately, it could be synchronous. Thus, what
> model to use depends on how the driver is written.
I'm not sure but I think there is a bit of confusion around the
sync/async stuff here :)
It seem that Emmanual is thinking about asynchronous/asynchronous in
terms of hardware communication method. For example you can define a
UART interface as asyn and an SPI interface as sync.
But sync/async inside ASYN driver doesn't have anything todo with the
actual hardware communication method. The option simply decides if the
ASYN processing is done in a seperate thread for each port or not.
(Hope I got that right :)
So you can use ASYN sync/asyn for any device.
But if you use ASYN sync and the processing blocks, it will block all
other ports also and all processing of records also I think.
So if processing is going to take longer than a few msecs then it is
usually better to use asyn processing :)
I hope I didn't confuse any more :D
If I did hopefully somebody can help me also.
-H
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