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'[EE]: MAX3232 operation'
2000\11\21@043956 by D Lloyd

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Hi,

Quick question for MAX3232 afficionados (!)

Do you know if the charge pump is supposed to be clocking all the time or
whether it runs in a sort of standby scheme where there is an occasional
transition to "gnd" ? I just dropped the system voltage to 3.3V and
replaced my MAX232 with the 3V part and RS232 RX to the PIC still works but
the attached PC sees nothing from the PIC, via the MAX, so I am wondering
if it is something to do with the output RS232 levels being borderline.
However, the MAX chip is throwing out data on pin 14 (TX out) at +/- 5.5V
which might appear to be correct.

The MAX device is not short of current and has 100nF polyester caps
surrounding it, so I see no reason why it should not be working (it is
generating the correct output voltages)......apart from the PC RX being
damaged or there being some threshold problem.

Any comments?

Dan

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2000\11\21@061944 by Simon Nield

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Does the 3v3 part have the same signal polarity as the previous part ?

Regards,
Simon

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2000\11\21@064028 by D Lloyd

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part 1 1653 bytes content-type:text/plain; charset=us-ascii
Hi,

My test code was simply 'get'ting a character from the UART and writing it
to E^2. On pressing ESC on the terminal, the PIC would throw all the E^2
content to the terminal.

I assumed the polarity was correct as the data being written to the E^2
matched the code of the key press. Eg 0x31 for a '1' being pressed. Also,
the PIC returned data correctly for the ESC character (seen on a scope both
sides of the 3232) , although the terminal did not display the data. I
didnt expect just the TX to be inverted by the device change. If both TX
and RX were inverted, nothing should be working properly, yes?

The 3232 is supposedly pin compatible with the '5'232 part. Looking at the
data sheet, they are the same pinout except that the '5'232 part has a 400K
pullup to the +5V rail before the TX buffer. The 3232 data sheet does not
show this resistor.......but it does state that it is pin, package and
functionally compatible with the '5'232. Maybe I ought to try a pull up on
the TX TTL side?

Dan





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Does the 3v3 part have the same signal polarity as the previous part ?

Regards,
Simon

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2000\11\21@074145 by Simon Nield

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Sounds like the signal polarity probably isn't it then...
If you can easily switch back to using the 5V part to check that the rest of the system still works
then I would try that. If that doesn't work either then you know the problem is somewhere else.

Good luck,
Simon

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2000\11\21@080301 by Alan B. Pearce

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Check the values of the capacitors you are using in the charge pump. The MAX3232
data sheet shows three different sets of values - one set for working at 3.3V,
one set for working at 5V, and another set if you want to be able to change
voltages. I have not actually used any of these, but this was what I noticed in
the DS.

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2000\11\21@082019 by D Lloyd

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part 1 1350 bytes content-type:text/plain; charset=us-ascii
Hi,

Yeah, I looked at that. The caps are all 0.1uF, as the DS suggests. This is
the same for 5 or 3.3 V operation. I did notice the charge pump not "going"
all the time which seemed odd. I seem to remember a lot of clocking going
on on those capactor pins but there wasnt much of that, even though the
voltages were being created correctly.

I was going to change the caps for some 1uF tantalums, just to make sure,
though........

Odd!

Dan



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Subject:  Re: [EE]: MAX3232 operation

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Check the values of the capacitors you are using in the charge pump. The
MAX3232
data sheet shows three different sets of values - one set for working at
3.3V,
one set for working at 5V, and another set if you want to be able to change
voltages. I have not actually used any of these, but this was what I
noticed in
the DS.

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2000\11\21@083120 by Bob Ammerman

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I've got a system with a MAX3226 which puts the charge pumps to sleep after
a bit. A transition on either an RS232 of TTL input wakes them up. It means
that I had to include a 'noise' character or two before my data block in the
protocol that drives it.

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

{Original Message removed}

2000\11\21@084510 by D Lloyd

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part 1 3121 bytes content-type:text/plain; charset=us-ascii
Hi,

I should bother to read the DS rather than scan it. It seems the charge
pumps are disabled if the voltage levels > |5.5|V, so this is why they
appear discontinuous. So, at least I know that bit is working properly. The
DS implies that the 3232 does not have an auto shutdown mode.

I'll look at it again, tonight, with a serial comms analyser rather than
the PC.

Thanks for everyone's help.

Dan






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Subject:  Re: [EE]: MAX3232 operation

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I've got a system with a MAX3226 which puts the charge pumps to sleep after
a bit. A transition on either an RS232 of TTL input wakes them up. It means
that I had to include a 'noise' character or two before my data block in
the
protocol that drives it.

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

{Original Message removed}
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part 3 136 bytes
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2000\11\21@144725 by Dan Michaels

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Dan Lloyd wrote:
>Hi,
>
>Quick question for MAX3232 afficionados (!)
>
>Do you know if the charge pump is supposed to be clocking all the time or
>whether it runs in a sort of standby scheme where there is an occasional
>transition to "gnd" ? I just dropped the system voltage to 3.3V and
>replaced my MAX232 with the 3V part and RS232 RX to the PIC still works but
........


Does your PIC operate at 3.3v?

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2000\11\22@031717 by D Lloyd

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Hi,

Yes, the PIC is fine. The PC isn't.

MAX232 = gives TX differential of 17V.
MAX3232 = gives TX differential of 12V.

So, the Toshiba laptop I was trying to talk to has a non-standard serial
port, it would seem.
I thought data was supposed to be valid at >+/- 3V?!


Thanks everyone for help and comments,

Dan





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Dan Lloyd wrote:
>Hi,
>
>Quick question for MAX3232 afficionados (!)
>
>Do you know if the charge pump is supposed to be clocking all the time or
>whether it runs in a sort of standby scheme where there is an occasional
>transition to "gnd" ? I just dropped the system voltage to 3.3V and
>replaced my MAX232 with the 3V part and RS232 RX to the PIC still works
but
........


Does your PIC operate at 3.3v?

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