Data taking and unpacking
-------------------------
Data is transferred between SRU and laptop with UDP packets.
To each UDP packet is an event header added by the laptop and stored
on disk in raw data format.

When a run is finished the data file is unpacked and converted into
ROOT ntuples.

A cluster is data from a given channel with pulseheights above a threshold.
There can be many clusters per channel when running with zerosuppression.
With a threshold of zero there is no zerosuppression and there is only
one cluster.

ntuple content for each cluster:
iEventnb : SRU event number (inserted by SRU)
iMcm: MCM number, same as input on SRU 0-39. On smalltpc normally 0 and 1.
(inserted by SRU)
iChip: Chip number 0-7
iChan: Channel number in chip 0-15
iClLength: Length of cluster data from SALTRO chip.
iClTime: Time stamp of cluster
iSamples: cluster data (with length iClLength)


There is more information in the raw data file (UDP packet numbering etc...)
which is not in the ROOT file.


Files:
mapping-1.csv	mapping of pad numbering and mcm,chip,channel
		(; separated fields)
plotevent.cpp	example of very simple event display, which
		reads the mappingfile and rootfile and makes simple plots.
		If the argument 'thr' is > 0 then it uses the last (20) data samples
		(assuming no hardware zerosuppression)
		to calculate a pedestal for 'offline' zero suppression.
		plot: XY (pad column,row) - colour coded number of data samples above threshold per pad
		plot: Time - (pad column,row,time) -
		time sample of data above threshold for a pad,
		box size is pulse height.

datafile1.root  example of root file (one mcm, every second channel pulsed)
		NOTE: not data from the smalltpc but from test of one mcm.
datafile1-evt1-test.pdf  event display event 1
datafile1-ped-evt1-test.pdf (data-pedestal) for last 20 samples.