I have a circuit that uses an old mylar type pad for input of numbers 
and letters. The problem is the pad is beyond repair.
So I opened the device and found  that it is controlled by a TMP47C443 a 
4bit micro made by toshiba.

The pad has about 16 keys, but a sample of how they connect is below:

1 - pins R80 + R62
2 - pins R81 + R62
3 - pins R82 + R62
4 - pins R80 + R61
5-  pins R81 + R61
6 - pins R82 + R61

R80 - R82 are listed as:
4 bit I/O port with latch.
When used as input port, external interrupt input pin, the latch must be 
set to 1.

R62-R62 are listed as:
4 bit I/O port with latch.
Every bit data is possible to be set, cleared and tested by the bit 
manipulation instruction.

The circuit interfacing the TMP47C443 chip to the keypad is pins R80-R82 
each pass through  a signal diode with the anode side connected to keys.
The R62-R62 pins connect directly to the keypad but have a 10K resistor 
tied to +5v.

What I really want to do is connect a pic to this circuit so that when 
the pic pulls a pin(s) High or Low the corresponding key is entered on 
the device.
The device uses a total of 10 pins to connect to its keypad and I have 
several pics with more than enough pins .I have already tried pulling
  the various pins high and low on the device but unless I connect the 
exact pins like a switch it will not register as a keypress.
  I wanted to do this with just a pic and not use additional parts .  
Can someone point me to how this other micro is interfacing the keypad
  so I can emulate it with a pic.





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