I am using a OEM 750 on my 4th axis right now. Does yours have the indexing card installed? If so it needs to be removed and jumpers put in place of the card. The card is by the db25 connector and mounts to the board by a header and just pulls off. You may have a hard time finding jumpers as the pins are odd spaced. I ended up finding a matching socket and soldering the pins shorted to make a header.