var downStrokeField;
function autotab_keydown(field) {
    field.beforeLength = field.value.length;
    downStrokeField = field;
}

function autotab_keyup(field, maxLen, nextField) {
    if (field == downStrokeField && field.value.length > field.beforeLength && field.value.length >= maxLen) {
        nextField.focus();
        nextField.select();
    }
    downStrokeField = null;
}