[E3-hacking] E2 PBL response

David Given e3-hacking@earth.li
Tue, 15 Mar 2005 10:34:00 +0000

On Tue, 2005-03-15 at 08:55 +0000, Peter Washington wrote:
> I'm a bit of a newcomer, (complete virgin actually), to newsgroups and mailing list, so please bear with me.

That's okay; I'll help you with the mailing lists if you'll help me with
the hardware...

> 1.	Trust the Maxim datasheets over any other source.
> 	(I think the Hardware Recyling circuit has used 0.1mF to 0.1 micro Farads, lack of the Greek mu)
> 2.	Putting Vcc into the Gnd Pin of any chip will definitely damage it IF any other pin is connected to a voltage around about Vcc.
> The "Magic Blue Smoke" gets boiled off and it stops working ;-(

There's actually good, if rather embarassing, news. The reason why I was
getting a voltage drop of zero across Gnd and Vcc was not, as I thought,
because the MAX232 was short-circuiting the power supply --- it was
because it wasn't connected to the power supply! I hadn't noticed that
the breadboard I was using had a break in the power rails halfway
across, and of course I'd plugged the power supply in on the left and
the circuit in on the right... I think the only useful thing I can say
at this point is "D'oh!".

So after plugging everything in the right place, I powered it all up
again and it seems to be much better behaved. All the voltages seem to
be correct, so the charge pump is pumping and it generally seems to be
doing its thing.

Still doesn't work, though; but seeing as when I connect TX and RX
together on the jack, I get garbage instead of seeing my input echoed
back, I suspect there's a wiring problem somewhere. I'll try to scrounge
up a scope and have a look this evening.

[Incidentally, I've discovered that the HAM radio set use pretty exactly
this kind of interface --- you can buy readymade cables for under twenty
quid. Pricy, but unlike the stuff I build it should work... it might be
worth looking at TI calculator cables, too.)

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+- www.cowlark.com --+ Pat Robertson