J. Lewis Muir
2010-05-21
Revision 2

External Software Trigger Control API

The External Software Trigger Control API provides an EPICS PV interface for controlling an areaDetector acquisition of images in the same way regardless of the areaDetector driver being used.

Supported areaDetector Drivers

• ADSC
• MarCCD
• Pilatus

PV Interface

$(P)$(R)ExSwTrAcq

Set to Acquire to begin an acquisition of images; set to Done when the acquisition is finished.

$(P)$(R)ExSwTrOkToExp

Indicates whether it is OK to start an image exposure.

$(P)$(R)ExSwTrCtl

Set to 1 to start an exposure, and set to 0 to stop it.

$(P)$(R)ExSwTrCtlRsp

Will be Start, Stop, OK, or Again. Start indicates a request to start the exposure has been received (i.e. via $(P)$(R)ExSwTrCtl). Stop indicates a request to stop the exposure has been received (i.e. via $(P)$(R)ExSwTrCtl). OK indicates the exposure was successful. Again indicates the exposure was not successful and should be tried again.

Protocol for Acquiring Images

1. Set $(P)$(R)ExSwTrAcq to Acquire
2. Wait for $(P)$(R)ExSwTrOkToExp to be Yes
3. Set $(P)$(R)ExSwTrCtl to Start to start the exposure
4. Set $(P)$(R)ExSwTrCtl to Stop to stop the exposure (unless the exposure automatically stops)
5. Wait for $(P)$(R)ExSwTrCtlRsp to be OK or Again
6. If $(P)$(R)ExSwTrCtlRsp is Again, the exposure did not work and should be tried again
7. Repeat from step 2 to acquire additional images
8. Set $(P)$(R)ExSwTrAcq to Done

Note that care must be taken when waiting for $(P)$(R)ExSwTrCtlRsp to be OK or Again to ensure the PV value is not stale (i.e. from the previous exposure). There are at least two methods to ensure this:

• After starting the exposure, wait for $(P)$(R)ExSwTrCtlRsp to be Start, OR
• Use a CA monitor on $(P)$(R)ExSwTrCtlRsp; before waiting for the OK or Again values, wait for the Stop value; (a CA monitor is used to receive the value changes since the PV will have the Stop value for just a short time); (note that this method will not work for an API implementation where the exposure stops automatically since $(P)$(R)ExSwTrCtl will not have been set to Stop)

API Implementation and Behavior Differences

• ADSC

  Before stopping the exposure for the last image, the ADSC control software requires that it be notified that the last image is about to finish being exposed. Set the $(P)$(R)ADSCLastImage PV to 1 to signal this before setting $(P)$(R)ExSwTrCtl to Stop.

• PILATUS

  • The areaDetector Pilatus driver will always be used with $(P)$(R)TriggerMode equal to Ext. Trigger. So even though an exposure is started by setting $(P)$(R)ExSwTrCtl to 1, the trigger pulse must be externally generated to actually start the detector exposure sequence.
  • The exposure will end automatically, so normally it should not be stopped by setting $(P)$(R)ExSwTrCtl to 0. Only in the case where the exposure should be stopped prematurely (e.g. aborted due to an error or user request) should $(P)$(R)ExSwTrCtl be set to 0.
  • When the exposure ends automatically, $(P)$(R)ExSwTrCtlRsp will be set to OK.