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I've been looking into the interface of the RCX sensor port, as well as studying
available information.
This page from Stef Mientki suggests that the port uses a current source:
http://oase.uci.kun.nl/~mientki/Lego_Knex/Lego_electronica/RCX_inside.htm
I believe this to be correct. According to the circuit topology and values I've
deduced, there is a current source of 15.38mA, derived from a +8.4V supply rail
(one diode drop below battery) by two PNP transistors (3Kp = BC858) and a 39 ohm
resistor. Assuming 0.6V across B-E of a transistor, then 0.6V/39R = 15.38mA.
Of course accuracy of this depends on component accuracy, and performance will
vary with battery voltage.
This indicates that sensors should try to draw no more than 10mA and certainly
not more than 15mA in order to get the best performance.
Mark
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I believe this to be correct. According to the circuit topology and values
I’ve deduced, there is a current source of 15.38mA, derived from a +8.4V
supply rail
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This indicates that sensors should try to draw no more than 10mA and certainly
not more than 15mA in order to get the best performance.
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Experiments completely second that, see this V/I curve I measured while
preparing my GP2D12 interface.
Philo
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