In SX Microcontrollers, SX/B Compiler and SX-Key Tool, Tracy Allen wrote:

Mike,

The rx input on the MAX3100 expects pulses of the first form, like the first picture you posted, normally high (+5 volts) with pulses going to ground (0 volts).   However,  the signal in that picture is too weak.  It only drops from 5 down to 3 and shows a long decay time constant.  Try bringing the keyboard closer to the receiver, or use a more sensitive receive circuit.  Is that in fact the signal presented to the rx input?

I think you are reading the bit time correctly.   At ~100 microseconds per bit, it is probably 9600 baud.   That is the nearest standard baud rate.   

It does seem strange that there is so much action when you press just one key.  Maybe it sends several bytes per key.   Can you set the 'scope to capture just the action that follows a single key press, for example, one tap on the letter "A"?

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