Trigger Hardware and Software

The Hall A trigger was designed by the University of New Hampshire. Here we give a brief overview of the hardware arrangement, the logic of the trigger, and the usage of the software control. Diagrams of the hardware layout are shown in three accompanying figures for the E-arm, H-arm, and coincidence circuit.

Scintillators make the main, so-called S-Ray, trigger in each spectrometer arm, and a coincidence is formed between the spectrometer arms. The S-Ray trigger is formed by requiring that scintillator paddles in planes S1 and S2 both fired (and both phototubes in each paddle), and that the paddle combinations in S1 and S2 belong to an allowed set. A memory lookup (MLU) decides if the combination is valid. The allowed combinations are for ``S-Ray'' tracks that are at an approximately 45 degree angle with respect to the hall floor, with a tolerance of $\pm$1 paddle on either S1 or S2. The timing of this trigger is determined by a strobe on the MLU which in most events comes from the right-side PMTs of the S2 plane. The coincidence is formed in an overlap AND circuit with a 110 nsec window. The electron arm singles triggers are called T1 (event type 1) in CODA, the hadron arm singles are T3, and the coincidences are T5. If T1 doesn't exist, a looser trigger called T2 is considered (similarly for T3/T4 on H-arm). This looser trigger requires any hit in 2 out of the following 3 detectors: S1,S2, and Cerenkov (in the case that a Cerenkov detector is used), or 1 out of 2 from S1 and S2 if there is no Cerenkov detector in the spectrometer. These looser triggers are prescaled, and a sample of about 5 Hz from them allows for an accurate measurement of inefficiencies. The trigger design is quite flexible and it is relatively easy to add detectors to define new trigger types or to modify existing ones, so long as the detector is fast enough. The trigger supervisor also allows for the possibility of 2nd level triggers which could be used for a later decision.

Here we describe the software control of the CAMAC modules involved in the trigger. There are four types of modules that are controlled: ) Discriminators; ) Delay Units; ) Memory Lookup Unit ) AND/OR Modules A graphical user interface called XTrigMang was written by T. Smith (UNH). XTrigMang is used to download the trigger and read back values. This GUI reads in a default setup file and with one button, called ``Download All'', one may load the default setup. To start XTrigMang, login to adaqh2 as the ``adaq'' account, then type ``gotrigger'' to go to the correct directory (which contains the default setup in trigsetup.settings), and type ``XTrigMang'' there. For experiments that never change the trigger, one only needs to ``Download All'' to ensure the trigger is set up after power is turned on for the crates. For coincidence experiments it is anticipated that the only change one needs to make is the delay on the hadron arm to accommodate momentum changes. Proton momenta above 360 MeV/c are at present accommodated by the hardware (it would be a fairly easy hardware change to go lower). For coincidence experiments, instead of using XTrigMang directly, one must download the trigger with a script called trigsetup. Login to adaqh2 as ``adaq'' account and type ``trigsetup''. It asks for the momentum of the H-arm (under the assumption that a proton is in the H-arm and that the E-arm has an electron with respect to which the H-arm must be delayed). Then trigsetup loads the correct default setup file and starts XTrigMang, which pops up. At this point, you simply press the ``Download All'' button in XTrigMang to setup the trigger.

If individual modules need to be modified for test purposes etc. (e.g. to change thresholds), one may press on the buttons in the XTrigMang GUI and pop up the components in an obvious way. Each component has four buttons which it is essential to understand: ) The ``Set'' button. One must enter the choice and then press ``Set''. This loads your choice into memory on the workstation (not on the CAMAC crate). ) The ``Down'' button. This sends whatever choice is in memory to the CAMAC crate. ) The ``Show'' button. This reads back from CAMAC what is actually in the module. ) The ``Read'' button. This shows what is in memory on your workstation. Note that ``read'' is quite different from ``show''. A typical operation to modify a trigger module would be to Set the value, Download it, and then Show to check that the value is in the CAMAC module.

Figure 7.1: Electron Arm Trigger Circuit.

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Figure 7.2: Hadron Arm Trigger Circuit.

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Figure 7.3: Coincidence Trigger Circuit.

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