KLM trigger scheme

The KLM trigger system consists of 2 CAMAC ULM2366 modules fabricated by LeCroy and custom-made 12 NIM modules for signal OR, called ``OR-module.'' OR-modules pick up readout board output signals of the trigger layers with high impedance input. Then, OR-module makes an ORed signal for each 4 input channels that correspond to one superlayer. The output signal width of OR-module is made 1.8 $\mu$s in order to mask the trailing output signals of the readout board which hold position information. The output of OR module is sent to CAMAC ULM2366 which produces a trigger signal. In order to reduce accidental trigger rates, the signal length at the input of ULM is shortened to 25 - 50 ns from 1.8 $\mu$s. The length depends on the timing between the input signals and the ULM system clock.

Fig.  shows the signal flow of the KLM trigger system. In the case of barrel KLM, 4 superlayers among 15 superlayers were selected as trigger layers, i.e. layers 02, 03, 10, and 11. The layer numbers start from 00. In order to reduce the inefficiency caused by the gap between forward and backward barrel KLMs, forward and backward superlayers, which are located at the same layer number and the same sector number, are logically connected in ULM, and treated as one superlayer. When 2 out of 4 superlayers in a sector have coincidental hits, a trigger signal is made. Trigger signals from all the sectors are ORed, and then sent to GDL. Since a ULM module has a limited number of input channels, signals from barrel KLM are divided into 2 ULMs. Thus, the barrel trigger output from one ULM is connected in series to another ULM. In the 2nd ULM, all barrel trigger signals are ORed.

In the case of end-cap KLM, 2 layers among 14 superlayers were selected as trigger layers, i.e. layers 04 and 05. In order to avoid fake trigger signals caused by beam backgrounds, the outer superlayers are not used as trigger layers. When both trigger layers have coincidendal hits, a trigger signal is made. Trigger signals from 4 sectors of each part are ORed separately and then sent to GDL.

Thus, the KLM trigger system makes three types of trigger signals; barrel, end-cap forward, and end-cap backward.

Figure: Schematic diagram of the KLM readout board output signals and the trigger system.