stepper motor optional encoder reset

I have a SLC500 talking to a small AB Ultra 100 drive via a 1746-hstp1 stepper module. This setup has an optional encoder wired to the stepper input module. So, we have 2 encoders on this machine. For the current position, we are using the one wired to the "optional" second encoder input. When the drive hits a home prox, we are moving a 512 into the drive config word(setting bit 9 of the config word to 1). This resets the drive position but the optional encoder count is not changed. Does anyone have any idea how I can reset this? I did try setting bit 14 of the config word, which is labeled preset encoder position, but it made no change. Any help is appreciated. Thanks

Russ