Experiment Proposal - opera - Infn

The dead time is completely defined by the front end chip. The rest of the acquisition can stand the
data rate without extra dead time. If only fired channels are read out, present estimates give a dead time
of about 0.4%.
Front-end Unit
Optical fibers
FEU
FEU
Tracker
Wall
FEU
FEU
Data readout
by a serial link
FEU
FEU
To VME Readout
Interface
FEU
FEU
Figure 69: Data readout scheme of the scintillator strips.
4.7.4 Readout interface
The VME board controls two tracker planes (X and Y , 1024 channels). This interface receives the data
(serial links) and the discriminator signals from the front end unit (Fig. 70). The first task of the module
is to assign a common time stamp to each group of 64 channels.
In order to create the time stamp information from the GPS is needed. One needs a PPS (Pulse Per
Second) signal which allows the precise synchronisation of a local counter for all cards and the complete
absolute time received by a RS232 serial link. The combination of both information provides an absolute
time stamp for each event, as shown in Fig. 71. The time stamp accuracy is defined by the frequency
of a local oscillator (200 MHz for 5 ns). The duration of the PPS signal is divided by the period of
the local counter oscillator. The counter value gives the precise time between the last PPS edge and the
arrival of the discriminator signal.
A local trigger could be implemented on the VME board to handle the external trigger signal (TRIG
OUT) for the front end unit. The X and Y correlation or correlations between planes can be performed
by this hardware logic, since all the discriminators from the X and Y planes are connected to this module.
A TRIG IN signal is also handled to allow a global hardware trigger for the experiment (if necessary).
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