Experimental Physics and
Industrial Control System

John Sinclair <[email protected]> · Tue, 11 Sep 2007 09:52:53 -0400

My definition of a synchronous device transaction is one that must be
completed without

interrupts or one that will complete faster than the overhead required
to service the

interrupt, including the context switch back to the requesting process.
John Sinclair
Heinrich du Toit wrote:

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

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System