Hi

Sorry for not very clear.

I use two boards. One board got a 8255 as the in/out port. The other got a 89c2051 to control the extenal device. I want to control the 89c2051 board (send command from 8255 board). However whenever I connect a pin of the Port A(output port of 8255), to a pin of port 3 from 892051, and then set the pin of 8255 to high (5V). The voltage drop to nearly 1V at once.

I don't know why it happens, even I did clear all code from flash rom of 892051, so leave it as a black chip.

Could anyone have any suggestions ?

Quang