I have used a DTMF decoder in my project to identify the pressed key of a telephone. I am using AT89C51 microcontroller to read the DTMF decoder output and transfer to PC for further processing. The problem is that when DTMF decoder output is connected to any port of Microcontroller the DTMF decoder stops working. The same problem is faced when I tried with parallel data transfer using PPI card or Printer port. I have used buffer while using PPI and printer port. please provide some suggestions.