What is this Oscilloscope picture telling me?

Chuck Rice

I am getting some results that I do not understand. I have my propeller connected to a parallax position controller. Using FullDuplexSerial, I transmit one byte to the position controller, and it responds with 2 bytes. As you can see from the picture, it is doing that, and if I turn the wheel, it increments and decrements. But I am expecting the signal from the position controller to be between 0 and 5 volts, or at least 0 to 3.3 volts. but from the scope, it looks like the high returned is 3.5v or so and the low is about 2.8 volts. I have also attached a picture of the interface circuit I built.

So what is the scope telling me that I am doing wrong?

T Chap

I don't know the rest of the circuit, but you obviously have some voltage division going on.

Mike Green

If you're measuring the signal on the Propeller side of the 220 Ohm resistor, this is roughly what would be expected for a high voltage. The controller (and the pullup resistor) will be supplying +5V and the Propeller's substrate diode will conduct since the voltage is above 3.3V + one diode drop or roughly 3.9V. The 220 Ohm resistor will limit the current supplied by dropping the voltage to 3.9V or below.

I don't know why you're seeing 2.8V for a logic low. It should be much lower, more like 0.3V. I suspect your actual circuit is not what you've drawn.

I think the position controller has its own pullup and series resistor. You might check the documentation for that.

Phil Pilgrim (PhiPi)

I suspect you've forgotten to reverse the direction of DIRA to make it an input when reading from the position controller.

-Phil

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Chuck Rice

I suspect the problem was that FullDuplexSerial was fighting itself over the pin. I switched back to SimpleSerial and now the scope shows a much better picture. The sharing of the pin for tx and rx confuses me. When they are one and the same, I thought that I could set mode bit 2 on to float the pin, but that did not seem to work and I am not sure why. Mode bit 2 on is %0100, isn't it? I am always confused about MSBit, LSBit, BIT0, MSByte, LSByte, Byte0, etc. Every language uses them differently!

Thanks for your responses. If you do understand where am misunderstanding things, I would enjoy additional enlightenment. Simple serial does work now, so I am good for the moment. [noparse]:)[/noparse]

Post Edited (Chuck Rice) : 12/8/2008 12:08:03 AM GMT

T Chap

I suspect your low coming back was meeting with 5V through the 12k, and meeting with 3v3 through the 560 (held high with the Prop), arriving at the level you were seeing. If the Prop pin were an input, you would get a response that made sense.

If the Prop was an output, and was low, you would read a low if the device was outputting low. If the Prop was high, and the device was low, you would get some division between the several resistors, and get somewhere between 5 and GND depending on the actual equation in your case.