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A light sensor connected to port 2 can cause on output from port 1. To see
this (if you are using lejos), download the View object and hook up a light
sensor to port 2. On my RCX, the port 1 raw values fluctuate between about
1010 and 1019. A similar effect can be observed by viewing port 2 and
connecting the light sensor to port 3. These are the only combinations show
this kind of cross talk. If the light sensor is connected to a port that is
activated (so the red light glows), the cross talk effect is not visible.
This caused a problem because I was using port 1 to multiplex several touch
sensors, so S1 was in raw mode. A light sensor was connected to port 2 but
was not activated. The SensorListener monitoring S1 suffered repeated
calls to its stateChanged() method before any touch sensor was bumped.
Which took me some time to figure out. The solution is to discard any raw
values from S1 that are larger than 1000. (or else active S2 earlier)
Has anyone else been bitten by this undocument feature?
Roger
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Message has 2 Replies:
 | | Re: Sensor port cross talk
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| This is certainly a known defect in the input circuitry in the RCX. I know nothing about other kernels apart from the standard Lego one (via NQC) but the results of my examining the problem indicated that the ports are scanned in reverse order, 3 - (...) (22 years ago, 1-Aug-02, to lugnet.robotics.rcx.java)
| | | RE: Sensor port cross talk
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| (...) If you've ever worked with multi-port A/D converters on a micro, and scanned all of the inputs you've been bitten by this one. It's caused by two things, parasitic capacitance on the chip itself and leakage between the lines going to the A/D (...) (22 years ago, 1-Aug-02, to lugnet.robotics.rcx.java, lugnet.robotics.rcx)
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