If you are using a Unitronics USB to Serial Adapter (MJ10-22-CS35) please be certain to use the most up to date driver. Drivers are in the attachments below. PL2303_Prolific_DriverInstaller_v1.7.0_32_64bit Win8.zip PL2303_Prolific_DriverInstaller_v110.zip PL2303_Prolific_v1_8_0_WIN8 32bit_64bit.zip