Experimental Physics and
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This means that you cannot have a single button on a display screen which both sets the output value and gets updated whenever the output gets modified by some other means. If you manage to create such a button that appears to work, there will always be some time window where either your button does not show the actual status of the output, or a user's button press will be ignored. Neither of these outcomes is desirable, in both cases the user is likely to complain about flaky software. I can also imagine situations where you could even cause the output to oscillate, although those are slightly harder to cause. Here at APS the accepted solution to this problem is for our MEDM screens to have buttons or a menu to command the output value, and a separate indicator for the current status coming back from the device. You can use menu or radio buttons for the demand, but you must still have the separate status feedback as well. Eric Norum pointed out that command buttons should use verbs and the status displays adjectives (Enable -> Enabled), and this significantly reduces the confusion between the two. That people are trying to develop bidirectional I/O support is IMHO partly a reflection on the capabilities of our current record types - the output record types really should have a separate field for the readback value in engineering units, but they generally don't. The ao and mbboDirect records both have an RBV field which may be set by the device support and can be monitored by CA clients, but the record support code doesn't convert the long integer value back into engineering units, and neither has the fields to store the back-converted engineering values anyway. It would make sense to add a field called say IVAL to most of our current output record types to hold the readback channel's engineering value - this would make it much easier for applications developers than requiring them to create a separate input record for the readback channel. - Andrew -- Dear God, I didn't think orange went with purple until I saw the sunset you made last night. That was really cool. - Caro
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ANJ, 10 Aug 2010 |
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