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Local StationAcces restreint

The 1600 Local Stations [LS] of the south site are composed of : Our laboratory had an important responsability in the design, the realisation and the test on the electronics and the acquisition system of all the LS.

HardwareAcces restreint

The FrontEnd board receives the PMs signals, digitizes, selects and numerizes them. There are six analogic inputs (three anodes and three amplified dynodes). Data are bufferized in two FIFO memories (1024 octets by channel) in order to minimized the dead time.
Data selection is realized by several triggers (time and amplitude selection). When a set of data is selected, an interrupt signal is send by the FrontEnd to the Unified Board [UB] processor. The UB starts a DMA to transfer the data to dynamical memories located on this main board.

The station controller is the intelligence of the Unified Board. It controls all the Local Station. It is composed essentially of four main fonctions : A LED Flasher can send light impulses (108 photons per pulse) with 2 electroluminescent diodes emitting alternatively in a near ultraviolet wave length.
This LED flasher is used for tests, calibration and PMT linearity measurement.

Unified board description

The electronic fundamental contraints are the following: Unified Board principal characteristics :

The event time tagging

When an event is detected (generation of a trigger), an interrupt signal (EventClock Fast or Slow) is sent to the time tagging function which detremine the precise start time of the event using a 100 MHz clock. The GPS module (Ground Positioning System), receives signals from the satellites and generates a PPS (Pulse Per Second) whose precision is, after corrections, better than 10 nano secondes. This allows to correct the 100 MHz clock drift.
The GPS is a commercial Motorola UT+ module linked to the Unidied Board by a connector Dubox 10 points. To determine the precise time, the time tagging counts the number of clock cycles (100 MHz) between two consecutive 1PPS and detremine the correction to apply. Besides, the GPS sends, on request, the correction to apply to the 1PPS (saw tooth) which may vary from -50 to +50 nano seconds. All these corrections, associated with signal processing technics, leads to a precision of 8 nano seconds.

The first implementation of this board was based on an ALTERA FPGA (10K50EQC208-1) and was successfully tested on the Engineering Array. An ASIC version was then designed and tested by our laboratory and used on the Unified board. The 2 versions have the same functionnality. The board records the time of arrival of the two triggers generated by the front-end board, with a resolution of 10 ns:
- EVTCLKF (Fast trigger, active low): the time is recorded at the high-to-low and the low-to-high transitions of EVTCLKF.
- EVTCLKS (Slow - or muon buffer- trigger, active low): the time is recorded only at the high-to-low transition of EVTCLKS.

The time consists in two parts (naming of counters and registers refers to the block diagram figure ):
- One coarse time, handled by the 1PPS (1Hz) clock from the GPS receiver and counter C3 (24 bits ). The 1PPS signal is the time reference for the whole experiment.
- One fine time, handled by a 100MHz clock from a crystal oscillator and counter C1 (27 bits).

At each EVTCLKF transition the fine and coarse times are stored in two 4 word fifos, M2 and M3 respectively.
At each EVTCLKS high-to-low transition the fine and coarse times are stored in two 1 word registers, M4 and M5 respectively .
The 100MHz clock is liable to have medium and long term variations due to different factors , and also have skew between the different stations. So in addition , a calibration ( monitoring ) of the 100Mhz clock is locally implemented. This is done by recording the C1 (100MHz) free-running counter at each 1PPS signal , in the register M1 (27 bits).

In order to monitor the possible variations of the 40Mhz used for flash ADCs , another calibration channel has been added , identical to the one described previously. This frequency is also the one for the PPC micro controller chip of the station controller. The 40MHz ( FCPU or SYSCLKBUS) is recorded by the C2 (40 MHz) free-running counter at each 1PPS in the register M6 (27 bits) .

The analysis of the shower signal (on both anode and last dynode of each 3 photomultipliers), measured by step of 25 nanoseconds on all hit detectors, associated to the precise time measuring, leads to the direction of the incoming cosmic ray with a precision better than one degree.
The correction to apply to the measured time to take into acount all these corrections is :
         t(n)= c0*( 109+ ST( n+1)- ST(n)) / Cn+ ST( n)
With: t(n): nano second in the nth second
         C0: nth second cycle
         Cn: 100 MHz clock counts between two 1PPS
         ST(n) : nth second saw tooth (value given by the GPS).

Slowly varying parameters or Slow Control

This function is responsible of the control of the detector : command of the tension applied on the photomultipliers, LED flasher control, measure of the slowly varying parametres like PMT currents and voltages, temperatures, etc...
For the Slow Control, there are 29 analogic measures (inputs) on a range of 0 to 2.5 Volts and several numeric inputs and outputs.
A 12-bit Analogic Digital Converter (ADC) numerizes the analogic measures. 24 analogic input channels are multiplexed in order to be read by the 3 inputs of the 8 channels converter. Five other inputs allows the direct reading of the PMT voltages as well as the board temperature.
The main analogic measurements are the temperature, voltage and current of each PMT, the UB temperature, the battery and solar panels temperatures currents and voltages, as well as all the power supplies values. There remain two free inputs, reserved for the water temperature measurement.
The four analogic output voltages are delivrered by a 12-bit Digital Analogic Converter [DAC] and amplified at the output by operational amplifiers. They drive the voltages applied on the 3 PMs as well as the one on the LED Flasher. Last but not least, the numeric inputs and outputs allow, among other things, external triggering of the FrontEnd, to detectet the presence of the Ethernet Board, to generate the contraol signals of the SlowControl multiplexers, to manage the control signals of the TPS function, etc...

Real time acquisition and control softwareAcces restreint

The acquisition software which is currently running into the Local Station [LS], is started as soon as the LS reboots.
During the first years of functionning, the LS software has been strongly modified. It's likely that the part related to the LS-CS communication is now stable, but that the acquisition tasks will still need some improvements.

So, for fiability reasons, the software has been split into two parts.