Electronics and DAQ

The trigger was provided by the time coincidence of 4 scintillators ($S_{1}$, $S_{2}$, $S_{3}$ and $S_{4}$ with overlap area of about $40~ cm^{2}$) suitably arranged along the beam, as shown in Fig.4.

Two Cerenkov detectors ($C_{1}$ and $C_{2}$) allowed the discrimination of electrons by pions and muons up to 10 GeV/c.

All 129 PMT's were readout by using LeCroy 2249A ADC modules. A fast ADC's gate ($170 ~ns$ for CAL and $700~ ns$ for TRD) was generated by $S_{1} \cdot S_{2}$, then a veto was set and (about $100~ ns$ after particle crossing the TRD and CAL) a delayed coincidence on $S_{3} \cdot S_{4}$, defined the event to be accepted. In the absence of the coincidence with $S_{3} \cdot S_{4}$ a CLEAR signal was sent to all the electronics and the veto was reset after $3~\mu s$, in order to take into account the ADC's recovering time.

In order to avoid noise pick-up during ADC conversion (the 2249A model takes about $60~ \mu s$) the readout of the CAEN STAS C187, performing streamer tubes acquisition, was suitably delayed.

The data acquisition system (DAQ) was based on a LabView package running on a Power-PC Macintosh. It used a VME system for the connections to two CAMAC crates where all the readout modules were hosted. The DAQ system was optimized in order to maximize the number of events/spill written on disk. This resulted in a processing time per event of $2~ ms$ (about 340 $\times$ 16 bit words read out), and $\sim 130$ events/spill recorded.