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The following information is my own opinion and not
endorsed or verified in any way by LEGO.
That being said, I think that now is the time to relate
some experiences I've had over the past couple of months
with my RCX and IR tower - and it boils down to this...
*********************************************************
If you use custom firmware like legOS, pbForth, or leJOS
(not the standard LEGO firmware or any software that uses
it, like NQC or RoboLab) you may be risking damage to
your RCX and IR tower!
If you use an unregulated wall-wart (adapter) you may
risk damaging your RCX and IR tower.
*********************************************************
I've spent the past couple of spare weekends making some
upgrades to pbForth, but recently my RCX and tower both died.
I have a couple of theories about this and would like to offer
them up for discussion. Please use the robotics.rcx group
for followups so we can all follow the thread....
1. I do a lot of upload/download sequences. In fact, I've been
developing an interrupt-driven comms library for the RCX.
In testing the library I end up doing a ton of transmissions
in both near and far range.
2. To conserve batteries, I use a RadioShack adapter through a
Y cable to the tower and RCX (Obviously, I have a 1.0 RCX)
The adapter is 273-1656A, which is rated for 800mA at 9V.
The actual output (unloaded) is about 12 volts - near the posted
limit for the RCX.
3. During the past few weeks, comms were becoming more unreliable.
This caused me lots of grief as I was thinking my software was
to blame. Finally, my tower died. I confirmed this using a
video-camera. It can see IR and I verified the tower was
not sending any chars. So I opened it up to see what I could see.
5. In off-line discussions with others on this list, it was pointed
out that the far-range current limit resistor in the tower is
only 5.6 Ohms. This would allow about 1A of peak current through
the IREDs. Even at a 50% duty cycle for the carrier, the 500mA
exceeds the normal operating range of the IREDs by about a factor
of 5 if all 1 bits are being sent.
6. I discovered that the NPN drive transistor for the IREDs in the
tower had failed - probably due to the very high current. I
replaced it and also replaced the 5.6 Ohm resistor with a 75
Ohm (actually 2x150 Ohms in parallel) to reduce the peak far
range current. This repaired the tower, but now my RCX did not
work either.
7. I opened up the RCX and found that one of the IREDs had failed
and was an open circuit. I replaced the IRED with RadioShack
276-143C which is described as a high-output IRED.
8. The drive transistor for the IREDs on the RCX *does* get hot when
doing steady transmissions from the RCX. Using the normal
firmware should not be a problem, but extended serial comms may
fatigue the part beyond its limits.
9. Windows 2000 on some computers may turn the IR tower to transmit
all of the time. I think this may have something to do with the
reports of tower failures under Windows2000. Leaving the tower
in near-range may avoid this problem.
Soooo, the upshot is that I think those of us that use custom firmware
should be aware that we may be running the risk of damaging the RCX
because some of the hardware may be stressed beyond the design limits.
The use of standard LEGO firmware only uses the IR for relatively
short bursts, and should not be a problem.
Use of unregulated wall-warts or adapters may cause damage to the RCX
as well.
Please post any replies or comments to the lugnet.robotics group
Best Regards,
Ralph Hempel - P.Eng
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