High Speed Counter Problem

Hi Forum

High speed counter on Siemens CPU226 is playing up. CV reading from it does not match to PV and keeps increasing and increasing. Eg PV set at 1800 but CV just keep increasing past this point. I cant clearly see if it jumps above the 1800 point so does not  register as having reached the value.

Is this a problem with PLC or the Encoder?

thanks