After many hours of head scratching I have found why I was able to link a TI84+ to an arduino, but the 83+ wouldn't work reliably. Since the protocol is the same, it had to be a hardware problem. I talked to TI and they said the specs were identical for the tlink port, but I was suspicious. I wondered whether the 83+ couldn't handle the current when the arduino set a line low. It must have been borderline. I had some 1K resistors on hand and put one in series with the ring line and the tip line to restrict the current to 3-5ma. IT WORKED!! now I can have the arduino communicate with the 84 or the 83 with no problems!

Next job: does anyone know what the protocol for an 86 is? It doesn't seem to be the same as the 83/84.

When do I lose the newbie status??
docbill wrote:
After many hours of head scratching I have found why I was able to link a TI84+ to an arduino, but the 83+ wouldn't work reliably. Since the protocol is the same, it had to be a hardware problem. I talked to TI and they said the specs were identical for the tlink port, but I was suspicious. I wondered whether the 83+ couldn't handle the current when the arduino set a line low. It must have been borderline. I had some 1K resistors on hand and put one in series with the ring line and the tip line to restrict the current to 3-5ma. IT WORKED!! now I can have the arduino communicate with the 84 or the 83 with no problems!
Congratulations! You might want to also enable the Arduino's internal pull-up resistors if you haven't already, as I think that should do the trick equally well.

Quote:
Next job: does anyone know what the protocol for an 86 is? It doesn't seem to be the same as the 83/84.

When do I lose the newbie status??
You lose it once you get to 40 posts (see this list of ranks). The physical-layer protocol for the TI-86 is identical to the one for the TI-83/+/TI-84+ family.
The pull_ups are enabled, but they only set the logic level high. When the arduino sets the lines low, the pull_ups obviously can't be in play. The 1k's are in series with the lines to reduce current. The 83+ either can't source or sink (I haven't decided which yet# the current from the arduino. Hence the random nature of the response. It is probably at the edge of what the calculator can handle. The series resistor reduces the current. You may be able to uses less than 1K. It was what I had on hand, but if the output #High) is 3-5 volts, that keeps the current at 3-5ma which doesn't seem to stress the calculator.
  
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