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docs/how-to/capture-hdf.md

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+# Capture data
+
+The `:DATA` PVs are used to capture data from the panda.
+These can be viewed from the DATA screen.
+
+```{image} /images/data_bobfile.png
+:align: center
+:alt: The data screen
+```
+
+- The file directory and name are chosen with `:DATA:HDFDirectory` and `:DATA:HDFFileName`.
+- `:DATA:NumCapture` is the number of frames to capture in the file.
+- `:DATA:NumCaptured` is the number of frames written to file.
+- `:DATA:NumReceived` is the number of frames received from the panda.
+- `:DATA:FlushPeriod` is the frequency that the data is flushed into frames in the client.
+- `:DATA:Capture` will begin capturing data.
+- `:DATA:CaptureMode` is one of the three capture modes listed below.
+
+## First N mode
+
+Begin capturing data and writing it to file as soon as it is received. Stop capturing once `NumCapture`
+frames have been written or the panda has been disarmed.
+
+## Last N mode
+
+Begin capturing data in a buffer, once capturing has finished write the last `NumCapture` frames to disk.
+
+## Forever mode
+
+Keep capturing and writing frames. Once the panda has been disarmed wait for it to be armed again and continue writing.