I ran into a problem loading new SW into an ATMEL 89C51 CC01 with my brandnew Dell Notebook. It's TXD is approx +6V in non initialized state, then after opening the com port it goes to approx -6V and sends its data with +6V again.. The +6V causes the ATMEL uP not to correspond to the Connect command.

A low power Mosfet (BS170), a 1M 4k7 1k5 a diode and a 0.1uF Condenser did the trick to prevent a long +6V level to come through to the RXD pin of the ATMEL uP..

If anybody needs the detailed schematics, I can provide it.