problem of counting via incremental encoder

Hi all. I just want to know the speed of a roller. So i used incremental encoder of 200pulses and interface it with s5-95U PLC. In PLC i used an up counter to calculate the no of pulses. The problem i am facing is when encoder moves slowly it is giving exactly 200 pulses but when encoders moves fast plc counter mises a lot of pulses. Can anyone help me in this matter.

The pulses are coming too quickly, you will have to use a High Speed Counter input instead of a standard input. I have no experience with the S5 so cannot give you any part numbers.