The MX-4 is a memory expansion board designed to allow owners to run the last versions of ACC Software. The MX-4 was designed to run V3.6 and the last version for the 850, V3.8. The MX-4 plugs into the "BUSS" connector on the main board. It comes complete with the V3.8 eprom set. To install the MX-4, simply remove the MX-2 board if required, plug the MX-4 into the BUSS connector near the 8085 40 Pin CPU IC. Remove Eproms U6 through U8 and install the new U8. That's it! You now have V3.8 without the Computer Interface Board. The MX-4 is available for $175.00 plus $10.00 shipping.