Subject: [E3-hacking] Line level conversion

From: David Given <[email-masked]>
Date: Mon, 14 Mar 2005 15:13:45 +0000

I'm now the proud owner of a MAX232CPE, and hopefully I should be able
to breadboard up a line level converter tonight. Then we'll discover
whether I've fried my UART by plugging it directly into an RS232 port.

I can't seem to find, however, any consistency in the circuits I'm
supposed to use. The datasheet's standard circuit uses 1µF capacitors;
Maplin's circuit in the catalogue uses 22µF; the circuit here:

http://www.seattlerobotics.org/encoder/aug97/cable.html

...uses 0.1µF. Hardware's not my strong point. Can anyone clarify? How
critical are these sizes?

-- 
+- David Given --McQ-+ 
|  [email-masked]    | Quidquid latine dictum sit, altum viditur.
| ([email-masked]) | 
+- www.cowlark.com --+

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From: Ralph Corderoy <[email-masked]>
Date: Mon, 14 Mar 2005 16:51:46 +0000

Hi David,

> supposed to use. The datasheet's standard circuit uses 1uF capacitors;
> Maplin's circuit in the catalogue uses 22uF; the circuit here:
> 
> ...uses 0.1uF. Hardware's not my strong point. Can anyone clarify?
> How critical are these sizes?

Not my strong-point either, but here's another circuit diagram from the
Hardware Recycling Initiative to throw into the pot.  0.1mF.

    http://hri.sourceforge.net/tools/#uart

Cheers,

Ralph.

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From: David Given <[email-masked]>
Date: Mon, 14 Mar 2005 21:16:26 +0000

On Mon, 2005-03-14 at 16:51 +0000, Ralph Corderoy wrote:
[...]
>     http://hri.sourceforge.net/tools/#uart

I eventually wired it up with 22µF; unfortunately I made a wiring error
the first time I powered it up, and I think I've fried the MAX232. It's
registering pretty much open circuit across GND and Vcc, which I suspect
is wrong... anyone know how to diagnose one?

(The wiring error was to connect GND to Vcc --- I plugged it into the
wrong rail on the breadboard. I'm a little suprised that this would kill
the chip, but there you go. I'll have to get another one and try again.)

BTW, I'm using a USB plug as my power source. Pins 1 and 4 provide a
nice handy 5V supply. I'm not sure yet whether you can use the E2's own
USB socket; it depends whether the socket's powered up by default or
not, so for the time being I'm using my PC, making sure all the various
grounds are tied together.

Sigh.

-- 
+- David Given --McQ-+ "They laughed at Newton. They laughed at Einstein.
|  [email-masked]    | Of course, they also laughed at Bozo the Clown."
| ([email-masked]) | --- Carl Sagan
+- www.cowlark.com --+

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From: Matt Evans <[email-masked]>
Date: Mon, 14 Mar 2005 21:48:37 +0000

Ohh bad news, David :(

(I would say the exact value of the caps really isn't that crucial, IME 
it can be in the same ballpark and you'll just get some lack of 
smoothing on the generated levels.  In all cases I'd trust Maxim's 
datasheets over Maplin's catalogue though!)

Yes reverse polarity might be a killer (though open-circuit across 
Gnd/Vcc isn't really an indication of this).

Re USB, it may well be that the 5V for USB is always present (they're 
regulating 5V somewhere to provide the 5V out of the EXP port, after 
all) - I shall try to check this out when I have it in a suitably 
unbooted state.

Better luck next time :/

-Matt