Internet PLC Forum

I have a fair amount (years) of experience with elecronic hardware at the component level and instrument control experience using Vbasic.  However, I am new to PLCs.  My application involves a simple form of motion control.  I want to take input from a position sensor which outputs data in the a/b quadrature phase form and dump this data into the plc.  I want the plc to interupt the motor that moves the position sensor in certain increments.  For example, when the operator pushes a button the motor will start to move and after things have moved X millimeters I want an output from the PLC to interupt the motor.  I know this is possible using the counters in the PLC but I need it to go both ways.  For example if I want the motor to move the sensor in 100 millimeter increments I want to be able to push a button and have  the work move 100 millimeters to the left.  The next time I may want to make the work move 100 millimeters to the right.  Is it possible to use counters this way?  Can the counter send an interupt on more than one count condition?  If not, is it possible to wire several counter inputs together and have one count up and the other count down?  Once the PLC interupts the motor the counters can be reset.  The PLC does not need to know absolute position of the sensor.