In an error correcting system in which error corrections for each of a plurality of steps of an independent variable in a device which has an error dependent upon the independent variable are stored in a register in adjacent storage positions thereof and wherein a location marker differing from any stored correction value is also stored in the register and moved as said device steps so as to always be adjacent the correction corresponding to the step of the device, the correction values being stored for a range of the device between a first limit and a second limit, the device having a greater overall range than the range between said first and second limit and being able to go to a position below said first limit, loss of synchronization between the device and the location marker when the device is moved below said first limit is prevented by detecting when said location marker reaches said first limit and causing it to move in a direction from said first limit toward said second limit as said device is moved below said first limit and in a direction toward said first limit as said device is moved from below said first limit back toward said first limit and again reversing the direction of movement of said location marker when said first limit is again reached with said independent variable moving in a direction toward said second limit permitting synchronization to be maintained even when going below the range for which correction values are stored.