Subject:
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RE: Custom Firmware, IR Problems, and Dead RCXs (long)
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Newsgroups:
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lugnet.robotics
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Date:
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Sun, 27 May 2001 18:55:31 GMT
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Original-From:
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Jim Lee <jlee@proaxis.com&AntiSpam&>
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Viewed:
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529 times
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Hi all,
Just a couple a quick comments on this thread - I'm a firmware design
engineer who's done lots of work with IrDA. I don't, however, have any
experience with the internals of the RCX or the IR Tower, other than owning
them and reading this list. I think that putting a 50 ohm resistor in
series with the IR LEDs will limit you to something less than "near-mode"
permanently. The IR circuitry is *designed* to pump as much current as
possible/available through the IR LEDs, far exceeding their "rated"
capacity. They are kept from being destroyed by limiting the duty cycle.
Say you have an LED rated at 20mA forward current, and you're pumping 1000mA
through it. If you keep the duty cycle at 1/50, everything works fine, and
you gain about a 7x increase in range (inverse square law - sqrt(50)).
You're trading bandwidth for range. Higher current = lower duty cycle =
longer range = lower bandwidth. Lower current = higher duty cycle = shorter
range = higher bandwidth. Now, what everyone in the Lego world calls
"firmware", isn't. It's the operating system. The real "firmware" is what
is permanently stored in the Hitachi processor's ROM (or Flash). What you
are calling firmware gets loaded into volatile RAM, so technically it is not
firmware, but software. So, given that the duty cycle of the IR LEDs is
controlled by software that can be changed easily by the user, then yes, it
is very possible to destroy the RCX by running bad code. A safer design
would have been to put the low level IR code into callable subroutines in
the RCX's "real" firmware, the ROM. I don't know about the issue with the
IR tower, but I suspect that it has more to do with using an unregulated
wall wart with too high a voltage. Just my two cents,
-Jim Lee
-----Original Message-----
From: news-gateway@lugnet.com [mailto:news-gateway@lugnet.com]On Behalf
Of Ralph Hempel
Sent: Sunday, May 27, 2001 11:08 AM
To: lego-robotics@crynwr.com
Subject: RE: Custom Firmware, IR Problems, and Dead RCXs (long)
Andy Gombos wrote:
> Oh, geez, this could be bad. Could changing the firmware to reduce comm
> activity fix this? It seems to be a problem of usage, not specific
> firmware properties (i.e. constant data uplaoding.)
Well, yes and no. The biggest risk is when you use the tower or
the RCX in high-power (far) mode for extended transmissions.
I was thinking about why we have not received more posts about
dead RCXs. My old pbForths (and I think legOS too) set the default
range to low. In fact, the ability to set a high-power range was on
the wish list for the next release of pbForth.
In the process of testing my new serial routines, I managed to burn
out one tower and the RCX twice. Now, the nice folks at LEGO replaced
the tower for me, but that was before I figured out the RCX was dead
too! I'm too embarassed to ask for a replacement unit....besides, it's
easy enough for me to fix.
I'm putting a 50 Ohm resisitor in series between the IREDs on the
RCX main board to avoid future problems like this.
Any other opinions on this issue?
Cheers, Ralph Hempel
--------------------------------------------------------------------
Check out pbFORTH for LEGO Mindstorms at:
<http://www.hempeldesigngroup.com/lego/pbForth>
Buy "Extreme Mindstorms: an Advanced Guide to Lego Mindstorms"
<http://www.amazon.com/exec/obidos/ASIN/1893115844/hempeldesigngrou>
--------------------------------------------------------------------
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Message has 1 Reply:
 | | RE: Custom Firmware, IR Problems, and Dead RCXs (long)
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| (...) You are right, but the RCX and the tower are right at the hairy edge. The additional 50R impedance will leave the near mode essentially unchanged and I'll sacrifice on the high range. Start with the tower. If you use a 9V battery, the IR (...) (23 years ago, 28-May-01, to lugnet.robotics)
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Message is in Reply To:
| | RE: Custom Firmware, IR Problems, and Dead RCXs (long)
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| (...) Well, yes and no. The biggest risk is when you use the tower or the RCX in high-power (far) mode for extended transmissions. I was thinking about why we have not received more posts about dead RCXs. My old pbForths (and I think legOS too) set (...) (23 years ago, 27-May-01, to lugnet.robotics)
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