Disable data reduction when the external trigger is used

[153957]

The HiSPARC electronics support triggering on an external trigger. Tests have shown that the GPS timestamp of a signal simultaneously arriving on two sets of electronics (one at the external trigger and the other on a PMT input) will have an offset of ~125 ns. The electronics triggering on the external trigger will have a timestamp 125 ns earlier than the other.

In order to relate the GPS timestamp to the trace values we normally reconstruct the trigger of an event. However, in case of the external trigger we do not have the signal trace of the external trigger, so the reconstruction is impossible. Except when we know where the trace starts. Because if the trigger is 'only external trigger' then the timestamp is at the moment in the trace where the trigger window starts (i.e. after the pre trigger window). So data reduction should either be disabled entirely when triggering on the external trigger only, or at least the entire pre trigger part of the traces should be kept.

I'm afraid that when using a combination of external trigger and low/high thresholds it will be impossible to reconstruct the trigger in some cases.

For example, the following two events were triggered simultaneously, station 94 using the external trigger, while 95 used a one low trigger. Station 94 also had the same synchronised input signal, but it was not used for triggering.

In the traces the arrival time of the input signals is reconstructed. The difference of these neatly compensates for the difference in the timestamps (except for the expected remaining small offset). In this case it works because data reduction was disabled. So the arrival times are relative to the start of the pre-trigger window (which was 0.5 µs in this case).

Station	Timestamp	Arrival time
94	1322405880144162063	617.5
95	1322405880144162203	497.5

(I do not know if this is already 'fixed', perhaps the DAQ already disables data reduction when using the external trigger, in the tests mentioned above it was manually disabled)