I'm attempting to reverse engineer a data communication protocol between 
a 16 key keypad and a radio.  A previous version of this product used 
DTMF tones for this communication making it relatively simple to figure 
out.  A newer version uses TTL level signals with communication in both 
directions occurring on a a single wire.  It is not the Dallas 1 wire 
protocol.  When the keypad is not connected to the radio, the radio 
holds the line high except for pulsing the line low every 500 ms.  When 
the keypad is connected the line is held mostly low except for pulsing 
the line high every 50 ms.  It turns out that all of these pulses are 
actually comprised of a series of more rapid pulses that aren't very 
square (sort of looks like a capacitor is charging and discharging).  I 
suspect that these shorter pulses are related to a pin switching from 
output to input and back.

When a key is pressed on the keypad, the line is first held low so that 
two of the 50 ms pulses are missing and then it's held high for about 15 
ms.  The data is actually encoded on the falling edge of this 15 ms 
pulse.  The falling edge consists of a series of pulses where the number 
of pulses on the falling edge depends on which key has been pressed.  I 
have been attempting to replicate the keypad operation with a PIC, but 
no joy as yet.  Oscilloscope screenshots of this protocol and more 
detailed information can be found at:  www.coastalchip.com/datacomm.htm

I'm wondering if anyone on the list has seen anything like this before.  
It would sure be nice if this turned out to be a known protocol that is 
documented somewhere.  Thanks for any help you might be able to provide.

John Hansen


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