I am using the Tx pin as a general purpose input.The pin is connected to the out put pin of a CD 40106.We have made about 200 systems like this.We have started getting units back with 87c51 /89c51 failed with tx pin permanently high.

is the failure due to the fact that the power on reset writes a 1 into the pin and if the 40106 is ouputting 0 then a very heavy current flows into the port pin.

There are other pins used similarly but they do not fail.

Note that this a desgin upgrade from 80c31 and we had no such problem in 80c31.

regards