Here is a small example how to count up and down a position counter.
How to initialise the position counter you must do by yourself.

A_INPUT VAR PORTA.0
B_INPUT VAR PORTA.1

HULP1 VAR BIT
HULP2 VAR BIT

COUNTER VAR WORD

Clear
COUNTER = 32768

START:


HULP2 = A_INPUT & ~ HULP1 'EVERY POSITIVE GOWING EDGE OF A_INPUT
HULP1 = A_INPUT 'GIVES A PULSE OF ONE PROGRAMM CYCLE

IF HULP2 = 1 AND B_INPUT = 1 THEN 'MOTOR TURNS RIGHT
COUNTER = COUNTER + 1
ENDIF

IF HULP2 = 1 AND B_INPUT = 0 THEN 'MOTOR TURNS LEFT
COUNTER = COUNTER - 1
ENDIF