Experimental Physics and
Industrial Control System



Hi all,

This is more of a conceptual question and some insight would be greatly appreciated.

In my EPICS drivers, I employed something an architecture known as "Slow Periodic Scan with Fast Change Response" seen in page 28 (Link: EPICS Re: Recommendations for StreamDevice "architecture" flow (anl.gov)). Thus far, the instruments for which I have written drivers are pretty responsive. However, as I've been using EPICS more and more, I am curious to know more about how StreamDevice handles this particular and possible situation.

Suppose I have a setter and getter for changing the voltage. Using the architecture above, the records can be linked together in the following manner (pseudo-code).

record(fanout, "slow_scan")

{

      field(SCAN, "1 second")

      field(FLNK, "get_voltage")

}

record(ao, "set_voltage")

{

      field(DTYP, "stream")

      field(SCAN, "Passive")

      field(OUT, ...)

      field(FLNK, "get_voltage")

}

record(ai, "get_voltage")

{

      field(DTYP, "stream")

      field(SCAN, "Passive")

      field(INP, ...)

}

When I process "set_voltage", a asynchronous passively-scanned record, it will start the protocol in another thread. What if it takes about 5 seconds after the first OUT command is sent to the instrument to receive a reply back. My understanding is that the protocol will lock the device, so no other records can communicate with the instrument. However, in the example above, "get_voltage" will process every 1 second, but won't be allowed access. Since "get_voltage" is also asynchronous, will it just simply not do anything until "set_voltage" completes and processes it through the forward-link? I guess my question is more about what "get_voltage" would be doing in this situation while "set_voltage" has the protocol locked. 

Thanks,

Andy