Subject:
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Re: Faulty Output
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Newsgroups:
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lugnet.robotics
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Date:
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Wed, 14 Aug 2002 19:35:41 GMT
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Viewed:
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574 times
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I'm sorry to have to say this, but it looks like you've blown
one of the MX10400 motor controllers within the RCX itself. These
chips were made by a company called Melexis. I can't remember the web
address (probably something stupidly simple like www.melexis.com, but
I don't think it was). The only thing you would be able to do is
replace the chip (have fun!) but getting a replacement is a problem. I
couldn't find any locally (Toronto) and our friends at Melexis don't
respond at all to e-mails.
The controller contains an "H" bridge - a circuit which
connects the motor leads to either +ve or -ve supply lines depending
on which direction you want the motor to turn.
The problem could be in the logic, within the chip, that turns
on two diagonally opposite transistors to select forward (or two
others for reverse) in which case you will just have to live with an
output that can only drive a light or turn a motor on and off (and
maybe use a polarity switch to mechanically reverse the motor's
power). If this is the case, you can go on using the RCX indefinately
with no further ill effects, even if you try to run it in the
direction it with the fault.
Worse yet - and your statement that you can draw power from
the port when the RCX is turned off suggests this - you could have a
short in one of the transistors. This would allow the motor to operate
when the diagonally opposite transistor is turned on, but would create
a short when used in the opposite direction. This situation would
ultimately do more harm in the long (or maybe even short) run if the
output is set in the direction with the fault. Note that you wouldn't
have to have anything plugged into the port for this to be
destructive. All it would take is the programming oversight of setting
the motor to the wrong direction.
So that the rest of us can avoid the same problems, can you
describe what you did (if anything) that caused the fault? It's
strange, really. The MX10400 has built in current limiting and termal
protection circuits to prevent most problems.
Matthias Jetleb
P.S.: As I finish this, one other *correctable* possibility comes
to mind: A faulty (aka "cold") solder joint. This could happen on it's
own or be triggered by a dripping the unit. Open it up and check the
back of the circuit board (the side opposite the LCD) and there you
will find three MX10400 chips. The one closest to the terminals for
output C is most likely the chip in question. Look *VERY* closely at
the board (use a microscope if necessary) check to see if one of the
pins of the IC is raised slightly off the board or if there appears to
be a tiny hairline crack in the solder. If you can't see it, try this:
take the lid off the RCX and find a way to connect something (a
light?) to the output. Turn the output on in the direction of the
fault. With the output on try twisting the circuit board and/or
applying pressure at various points with an insulator (eraser end of a
pencil works well for me). If the output comes to life, it's
definately a crack in the solder or, more rarely, one of the copper
traces. They can be tough to find (assuming that the break isn't under
one of the components whereupon it will be impossible) but if you do
find it, either re-flow the solder - if it's a "cold" solder joint at
one of the pins - or, if it's a crack in a copper trace, scrape a
little of the green film off the trace on either side of the crack and
bridge it with solder. Use a soldering iron intended for surface mount
work (what do you mean you don't have one???!!! I thought everyone
does!) or try this: Take a piece of thick(er) solid wire (telephone
wire is OK) and wind some around the end of a fairly high-powered
iron. Do it with the power off (grin) and leave a short (maybe 1cm)
section projecting from the tip. Cut that wire at a sharp angle to
form a point. It won't be pretty (nor easy to work with) but in a
pinch, it does work.
P.P.S.: If Melexis had ever sent me the data sheets for their chip,
I'd have been able to give you more specific test information. It
occurs to me after writing all this that there is yet another
possiblity. I've allways assumed that the MX10400 generates it
direction control signals internally. I know it generates the PWM
(motor power control signal) internally, but that doesn't necessarily
mean that direction selection can't come from an external source like
the H8 CPU. If that's the case, there could be a cold solder joint at
the CPU end on whatever pin selects the direction if in fact that's
how it works. I'd kill for a schematic of the RCX's.
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Message has 1 Reply:
 | | Re: Faulty Output
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| Thankyou very much for your help. I have no idea what caused my rcx to malfunction. I received my rcx in '98 and have used it very infrequently. Previously I had only used output A and B. Recently I got a new motor and started using Ouput C for the (...) (22 years ago, 15-Aug-02, to lugnet.robotics)
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Message is in Reply To:
| | Faulty Output
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| I am writing to see if anybody has any idea on how to fix a badly functioning output on my RCX 1.0 . The problem that I have encountered is that output C does not respond to any forward commands. However, it does respond to backwards commands. I (...) (22 years ago, 13-Aug-02, to lugnet.robotics)
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