The DRS4 Rev.2.0 with one Multiplexer and a Comparator I think may be is ok for me. Since I wanted to implement counter in FPGA, I have gone through firmware but its very difficult for me to get the flow of data from DRS to USB. Could you help me to get familiar with or to get flow of the data from DRS to the USB?
unfortunately I cannot teach you VHDL, you should take a course somewhere. Actually you don't have to touch the USB part, you just have to implement a new register in the firmware, and connect it to a VHDL counter attached to the trigger input. I will do this in a couple of weeks myself, so you can also just wait. I will however only do this for the V4 board, so you have to port it back to the V2 firmware.