Stepper motor is open loop control, so you just have to specify the number of pulses you want to send and the PLC will handle the rest. No encoders are normally used with stepper motor.
However, if you want to connect an encoder you can use the HSC (High Speed Counter) to connect to a quadrature encoder for up/down counting.
PMON is used for measuring pulsewidth and frequency which is useful if your purpose is to measure the speed. But I wonder why you need to measure the speed since the speed is set by the STEPSPEED command already.
PIDDEF lets you define the proportional, integral and derivative multiplier. You do need to have knowledge of PID type digital control to use the function properly. Usually you start with proportional term only, and if the steady state error is not acceptable then add in some integral factor. D term is not necessary if P+I can do the job, in that case just leave the D term as zero.
Since the M-series PLC do not handle floating point, you usually represent fraction using fixed point arithmatic. E.g. Use a single digit to represent 0.01. i.e 123 represents 1.23 so on. That allow you to specify fractional P or I terms without using floating point.
