Not all USB-to-serial adapters will work with Motion Planner.
The only USB adapters we have tested that work with all communication modes of Motion Planner is the Cables Unlimited part number USB-2920 (available on www.amazon.com).
This adapter gives a 9pin com port output; a null modem crossover cable is still necessary to connect from the adapter to the 6K, Gemini or Gem6K.
Other USB adapters, such as those made by Keyspan and Belkin, may require custom COM6SRVR.EXE and COM6SRVR.TLB files and were proven to not work with the Gemini GT/GT6 and GV/GV6 Binary mode of communication.
Be sure to contact your USB adapter manufacturer to obtain the latest driver available for your product. Many incompatibility issues can be resolved by using the latest driver.
Previously recommended was BAFO BF-800 (25-pin RS232 version) and the BF-810 (9-pin RS232 version) which does work on Win XP and prior but the drivers do not support Windows 7 (Win7).
Generally, confirm the device uses the FTDI chipset and driver (the Prolific based devices have been shown to not work under Win7 and later).