What do you mean by "RESET"? Is it to reset the PLC or to reset the count to zero?
You can use the high speed counter of the T100MD888+ which is occupying input #3 and #4 for channel 1. Connect your encoder outputs to these two inputs, then run a HSCDEF 1,10,100 to define such that when the pulse counts to +100, it will call custom function #10. Inside custom function #10 you can do what you want whether to reset the PLC or simply run HSCPV[1] = 0 or any other "RESET" that you want to do. You can also send data via RS232 port to other devices when this happens.
You can run the same HSCDEF again inside custom function #10 such that this function will be called again when the encoder counts to 100 again. In this case function #10 will be called repeated each time the encoder count reaches 100.