3in1 System Failure Analysis
This discussion will be separated into 2 sections dealing first with the individual 3in1 cards, and then with the drawer card control system.
First it must be remembered that each cell of the calorimeter is viewed by 2 phototubes. Loss of function of one 3in1 card does not mean that information from a cell is lost.
Total Failure Modes
The EPLD on the 3in1 card is set up to configure to a prefered default state on power up. This means that even if all control communications to a 3in1 card is lost, the core functions of shaping and transmittion pulses to the digitization system still function. The principle event that would cause the total loss of a 3in1 card is loss of power. In addition, if the EPLD failed with either of the lines controlling the calibration switches open ( output lines stuck high ) the output pulse from the shaper would be distorted. The failure of an inductor or capacitor in the shaper network would likely make the card unusable.
Partial Failure modes
Failure of the clamping opamp or line driving opamp networks would cause the loss of one of the outputs. The trigger output requires the functioning of the low gain clamping opamp and gated trigger output driver opamp networks.
There are 4 lines that are required to execute any function in the 3in1 cards. These are DATA_IN, CLOCK, CARD_SEL, and ENABLE. These are required to clock data into the EPLD and initiate execution of the function loaded. In addition for charge injection, the TP line must function. This means that U1 and U2 (DS26c32 differential recievers) must function. Other control lines (BACK_LOAD, RXW, MULTI_SEL, and DATA_OUT) are not strictly required to operate all functions of the card.
At power up, the integrator gain switches are set for the lowest gain setting. The output driver switch control in the EPLD is set to close the switches when CARD_SEL goes high. Special care is taken with the CARD_SEL line so that any loss in continuity of the signal results in the CARD_SEL line going low. The system is designed so that only one card can be selected at one time. This is required so that the integrator output bus is not multiply driven.
Clearly, the ALTERA EPM7064 EPLD is a critical component. Many different failure modes are conceivable, but the most likely ( loss of communication) does not disable data through put but does compromise calibration. This chip has been tested to 50KRAD with no loss in exposure expected in the worst position in the drawer. Most cards see and exposure nearer to 0.5Krad. In addition, the radiation calculations were made without shielding at the end of the drawers (fingers). We expect the true radiation levels to be well below the 2Krad figure.
The drawer cards supply power and control signals to the 3in1 cards. With one exception, all control functions can be originated via the TTC system or via CANbus routed through the integrator readout card which is plugged into the drawer card. The one exception is the TP signal which originates charge injection. The TP signal can be controlled via CANbus, but to do so with precise timing with respect to the digitization clock, the TTC system is required.
The drawer recieves +15v, +5v, and -5v feeds from supplies external to the drawer. These lines will be safety fused at the drawer card input and must be fused at smaller values external to the drawer. Loss of any of the three voltages will render the drawer unusable.
… Something about power supply redundancy …..
Crucial to the functioning of the drawer is the TTC system. This is probably true of all ATLAS subsystems. Although CANbus can initiate commands, the TTC 40Mhz clock must function. It is used to clock the microprocessor on the integrator ADC system which operates the CAN node. It also clocks the state machines in the drawer card FPGA. These state machines decode commands and send them to the 3in1 cards. The optical fiber coming into the drawer is split to driver receivers -- TTCrx chips on the 3in1 drawer card and digitizer card. These optical connections and associated TTCrx circuitry must function for the drawer to be operable.
The CANbus system is required for the slow integrator readout function. In addition it provides a diagnostic readout path for the 3in1 system. CANbus is not required for charge injection calibration or normal readout operations of the 3in1 card system.
TTC or CANbus commands are received by an ALTERA FKEX10K10-144 FPGA on the first drawer card. This FPGA controls the bus lines down the drawer. TTL signals from the FPGA are input to DS26c31M differential line drivers (RS422) . The 3in1 cards are connected to the bus in a party line mode and receive the control signals on DS26c32 receivers. The bus is rated for 36 nodes, the maximum used is 24. Repeaters are used half way down the drawer (48 possible 3in1 card positions in the full drawer). Depending on the card being addressed, one of 48 CARD_SEL lines should go high. Twelve card_sel lines are controlled from each drawer card section. A local EPLD (ALTERA EPM7064) on cards 2, 3 and 4 decode a 6 bit tube address and set a local card_sel line if appropridate. The minimal set of control lines that must be operational for control of the drawer is noted above in the 3in1 card discussion.
Each of the 4 drawer card sections has a digitally controlled delay in the TP line. While control of this delay is not strictly required for operation of the drawer, adjusting the relative times of charge injection for speeds charge injection calibration.
The drawer card system provides a precision reference voltage to the 3in1 cards. This reference voltage is used by a local DAC on each 3in1 card to set voltages for charge injection calibration and slow integrator calibration. Two voltage regulators are used in the drawer to provide redundancy in case of failure of a voltage regulator.
Radiation testing will be carried out when prototype boards incorporation TTC and CANbus control are completed. Critical components are located away from the digher radiation levels at the end of the drawer.
We expect to easily meet and exceed radiation testing requirements.