Dual Max232 installation: Capacitors?

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evang...@gmail.com

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Apr 14, 2021, 6:51:05 PM4/14/21
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Hello! assembly on my kit's been going swimmingly - I'm just putting on the final touches after the switch marathon now. I do have one question, though: Does installation of the twin Max232s for external 232-centric terminal usage need any additional caps? I've noticed spots on the board, but either the site doesn't specify or i've missed something. Thanks!

John H. Reinhardt

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Apr 14, 2021, 6:58:16 PM4/14/21
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On 4/14/2021 5:51 PM, evang...@gmail.com wrote:
> Hello! assembly on my kit's been going swimmingly - I'm just putting on the final touches after the switch marathon now. I do have one question, though: Does installation of the twin Max232s for external 232-centric terminal usage need any additional caps? I've noticed spots on the board, but either the site doesn't specify or i've missed something. Thanks!
> --

Yes.  There are 5 small caps needed to make the Max232 work.
https://www.ti.com/lit/gpn/max232 - Section 12.2

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John H. Reinhardt
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Tom Lake

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Apr 14, 2021, 6:58:55 PM4/14/21
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Here's a quote from the manual (emphasis mine):

"More elegant would be to use a MAX232 (or MAX202, or any of the pin-compatible substitutes) level conversion chip. The footprint for that chip and the 5 0.1uF capacitors it needs (check the MAX232 you use, the old ones use bigger caps) are at the top of the PiDP-11 board. Wire the serial port's RX and TX pins to the TTL level connector headers, and your RS-232 terminal to the RS-232 headers. Again, DB-9 or DB-25 connectors can be mounted on press-outs on the back panel."

Anton Lavrentiev

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Apr 14, 2021, 10:52:48 PM4/14/21
to Tom Lake, [PiDP-11]
I used 5 1uF tantalum capacitors, per the MAX232 official specs, and everything works as expected.

A word of advice: don't buy MAX232s from overseas, although they might be cheaper -- that did not work for me -- they were running extremely hot, ouch!
Replaced with the ICs bought directly from TI, and voila! --  no problems with temps, all runs cool.

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Brian Denley

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Apr 14, 2021, 11:07:05 PM4/14/21
to Anton Lavrentiev, Tom Lake, [PiDP-11]
Anton:
Sure they’re not supposed to be 0.1 uF ?  (Or did I buy the wrong caps?)
Brian

Brian Denley
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On Apr 14, 2021, at 10:53 PM, Anton Lavrentiev <anton.la...@gmail.com> wrote:



August Treubig

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Apr 14, 2021, 11:10:45 PM4/14/21
to Brian Denley, Anton Lavrentiev, Tom Lake, [PiDP-11]
There are two (at least) different types of 232’s.  One uses 1 mfd, the other uses .1.

So check carefully which you have.....
Aug

Sent from my iPad

On Apr 14, 2021, at 10:07 PM, Brian Denley <b.de...@comcast.net> wrote:

Anton:

Anton Lavrentiev

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Apr 15, 2021, 12:39:39 AM4/15/21
to Brian Denley, Tom Lake, [PiDP-11]

10.2 typical application shows 1uF caps (small print says MAX202 can operate with smaller .1uF caps); 10.2.2 mentions 1uF caps again.

Alan Abbott

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Apr 15, 2021, 5:06:46 AM4/15/21
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On 15/04/2021 03:52, Anton Lavrentiev wrote:
> I used 5 1uF tantalum capacitors, per the MAX232 official specs, and
> everything works as expected.
>
> A word of advice: don't buy MAX232s from overseas, although they might
> be cheaper -- that did not work for me -- they were running extremely
> hot, ouch!
> Replaced with the ICs bought directly from TI, and voila! --  no
> problems with temps, all runs cool.
>
Interesting advice. I have just got some 'cheap ones'. I was going to
test them on  some bread board first.

I have a PIDP11 and PIDP8 I was going to put them in.

Alan

"Thos who do not learn from Dilbert are doomed to repeat it"

evang...@gmail.com

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Apr 15, 2021, 12:30:31 PM4/15/21
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Thanks for the info, all! I'd noticed the similarities to the reference circuit due to a bungled attempt at building one of Searle's SBCs, but I...didn't make the connection. Funny how that works!

Marc Howard

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Apr 15, 2021, 1:08:07 PM4/15/21
to evang...@gmail.com, [PiDP-11]
There have been many variants of the Maxim 232 over the years.  One even had the capacitors internal to the package.

BTW, tantalums used to be the cat's meow but these days ceramic MLC's are generally used.  Higher density, no polarity issues, better characteristics, etc. and they don't have a nasty tendency to catch fire and disintegrate when used near (or even below) their rated voltage.

Where I work we can't even use tantalums unless there is 1) No other way, and , 2) We derate the voltage rating by 250%.

Marc Howard


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Robert Wilkinson

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Apr 15, 2021, 7:22:03 PM4/15/21
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These ceramic MLC's sound great, so I trotted over to Mouser to see if I could get some.  There don't seem to be any 220uF which I wanted, but might have to settle for 33uF.  Minimum quantity 8, long lead time, total cost $7,439.36

Ouch !

Marc Howard

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Apr 15, 2021, 7:56:18 PM4/15/21
to Robert Wilkinson, [PiDP-11]
There have been spot shortages of MLCs just like semiconductors in general.  It's a very intensive manufacturing process.  However, demand is huge as they are smaller and their electrical characteristics are much better than tantalums and just about anything else.  They're used extensively on processor boards.  Mostly they're rated very low voltage (2.5 or 4 V) as most semi's use 1.2V or so in the core.

But you didn't look hard enough if you can use a 220uF / 6V part.  DigiKey has 186000 in stock for $1.41 each: https://www.digikey.com/en/products/detail/murata-electronics/GRM31CR60J227ME11L/6155740

Marc

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Bill Saltzstein

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Apr 15, 2021, 7:57:53 PM4/15/21
to marc....@ieee.org, Robert Wilkinson, [PiDP-11]
I have to believe that at that pricing that Robert is looking at a full reel price, not individual components...

Robert Wilkinson

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Apr 15, 2021, 8:43:25 PM4/15/21
to Bill Saltzstein, marc....@ieee.org, [PiDP-11]
Many thanks for all that information.  I'll certainly start using them.

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