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The '''SiPM digital control board''' is the communication block for controlling the SiPMs.  It provides the interface through which an external system can control or monitor the SiPMs. Additionally, it serves as an outlet of amplified SiPM signals.

The '''SiPM digital control board''' provides remote control and monitoring of the SiPM (silicon photomultiplier) electronics. Variation in performance from one SiPM to the next, their temperature sensitivity, variation from one optical channel to the next as well as varying output due to scintillator degradation from radiation calls for active control of individual SiPMs to compensate for adverse effects.  

== Responsibilities of the control board ==

== Responsibilities of the control board ==

The control board is responsible for providing a layer of interaction between the researchers running the experiments and the tagger microscope.  The foremost responsibility of the control board is to allow the users to program the bias voltage (which controls the gain) of the SiPMs.  It receives signals from an external PC and communicates that information to a DAC which controls the bias voltage.  It also monitors itself and reports back to the PC certain statistics, such as voltage of the power lines (to ensure the chips and SiPMs are receiving the required voltages) and temperature of the control board and its immediate vicinity (to ensure that the electronics are not overheating).

The control board is responsible for providing a layer of interaction between the researchers running the experiments and the tagger microscope's SiPM electronics.  The foremost responsibility of the control board is to allow the users to program the SiPM bias voltage, which controls the gain and photo-detection efficiency.  It receives signals from an external PC and communicates that information to a [http://en.wikipedia.org/wiki/Digital-to-analog_converter DAC] which controls the bias voltage.  It also monitors itself and reports back to the PC certain statistics, such as voltage of the power lines (to ensure the chips and SiPMs are receiving the required voltages) and temperature of the control and amplifier boards (to ensure that the electronics are not overheating). Additionally, voltage information read back from the DAC allows for feedback that assists calibration.  

The hub of the control board, its "central nervous system", is an [http://en.wikipedia.org/wiki/FPGA FPGA].  All components on the board connect to the FPGA and it coordinates their interactions.  Communication with the outside world (more specifically an external PC) occurs over [http://en.wikipedia.org/wiki/Ethernet Ethernet].  Towards that end an Ethernet chip is included on the board and connected to the FPGA.  The main purpose of the board is to control bias voltages, so a [http://en.wikipedia.org/wiki/Digital-to-analog_converter DAC] is attached to the board and connected to the FPGA.  There are two monitoring devices so that the board can ensure that it is running properly: a temperature sensor and an [http://en.wikipedia.org/wiki/Analog-to-digital_converter ADC], both of which are connected to the FPGA.  The functional block diagram of the board is shown to the right.  Note that communication on the left (to the analog sensor board) is outgoing simplex and communication to the right (to the Ethernet hub) is duplex.

[[Image:ControlBoard.png|thumb|300px]]

To better understand the internal flow of information, a netlist of the connections between the components shown in functional block diagram is available here: [[SiPM digital control board netlist]].

The hub of the control board, its "central nervous system", is an [http://en.wikipedia.org/wiki/FPGA FPGA].  All components on the board connect to the FPGA and it coordinates their interactions.  Communication with the outside world (more specifically an external PC) occurs over [http://en.wikipedia.org/wiki/Ethernet Ethernet]. This was a natural choice, being a robust, long distance, inexpensive communication bus. To implement this, an Ethernet controller chip is included on the board and connected to the FPGA.  There are two monitoring devices so that the board can ensure that it is running properly: a temperature sensor and an [http://en.wikipedia.org/wiki/Analog-to-digital_converter ADC].  

== The components ==

The adjacent diagram shows the component connection scheme. The dashed horizontal line represents the "backplane" board which interfaces the digital control board with the SiPM amplifier board and seals the latter inside the light-tight microscope enclosure while keeping the control board outside for easy cable connection. As shown, up to 32 voltage lines on the amplifier board may be controlled by the DAC chip chosen of the control board design. In practice, the natural segmentation of the optical channels makes 25 or 30 channels the maximum. Conveniently, the remaining 2 DAC channels are used for [[SiPM_Amplifier_Optimization#The_Gain_Switch|amplifier gain selection]] and for calibration. It is important to note that the Ethernet based control board is not addressed only by its MAC address, but is automatically cross-referenced to its location in the microscope electronics board array. An "Location Stamp" number, jumper-coded into the backplane slot to which the control board is connected clarifies its identity in terms of the microscope energy bins it controls. This significantly simplifies setup, since a once time slot-coding fixes the board address space no matter which generic copy of the control board is inserted.

 

Additionally, more detailed information on the [[SiPM digital control board supporting components|supporting components is available]].

=== The FPGA ===

=== The FPGA ===