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PICList Thread
'using pic with out RS232 chip'
1998\11\21@204452 by engelec

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Hi to all engineers.

I was working with 16c62X comparator based pics and I found
that using comparator input with 2.0v (TTL) reference I can
connect pic directly to serial port thru 10-20k resistor.
serial port has +12 -12 volts right if reference voltage
assigned to 2.0V above that wouldn't damage the pic
so when input voltage = 5v we have logic 1 if input voltage = +12
still we have logic 1 if input voltage = -12 or 0 that is 0
correct me if I am wrong.

Andre Abelian

1998\11\22@103917 by Dave Johnson

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>I can
>connect pic directly to serial port thru 10-20k resistor.
There's a great app note from National Semiconductor about the
interoperation of RS232, 422, 485, and TTL in all the various
combinations. Very useful. You can download it at
http://www.national.com/an/AN/AN-972.pdf.

Dave Johnson

1998\11\22@121817 by Peter L. Peres

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On Thu, 22 Oct 1998, Andre Abelian wrote:

> Hi to all engineers.
>
> I was working with 16c62X comparator based pics and I found
> that using comparator input with 2.0v (TTL) reference I can
> connect pic directly to serial port thru 10-20k resistor.
> serial port has +12 -12 volts right if reference voltage
> assigned to 2.0V above that wouldn't damage the pic
> so when input voltage = 5v we have logic 1 if input voltage = +12
> still we have logic 1 if input voltage = -12 or 0 that is 0
> correct me if I am wrong.

You are right ;) You can also use a spare analog op-amp or comparator to
boost RS232 in the other direction.

Peter

1998\11\22@123021 by Dave VanHorn

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> You are right ;) You can also use a spare analog op-amp or comparator to
> boost RS232 in the other direction.
>
> Peter

Depending on the situation, you can also send it just using the PIC
itself, and invert the output logic level. Not something I'd reccomend
for industrial environments, but it works. The common 232 receivers
don't need negative voltage, just <1V, and they need at least 3V on the
high side.

On the RX side, a resistor in series with the incoming signal, and a
pair of diodes at the pin to +5V and Gnd will clip off the negative side
and limit the current, then the pic can decode it. Again, the polarity
is inverted, but that's a fast hack.

1998\11\22@142945 by engelec

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Thanks Peter.

I started to  work on pic16c621 UART for parallel lcd's to communicate
serially. it will have back light on/off , jumpers to change baud rate ,
PWM to change brightness. I am not sure if PWM is necessary or not.
I need your recommendations what baud rate range should I make.

Andre Abelian



>
>
> You are right ;) You can also use a spare analog op-amp or comparator to
> boost RS232 in the other direction.
>
> Peter

1998\11\23@051504 by Peter L. Peres

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> baud rate

The faster, the better ;)

> PWM

four steps are usually enough.

Peter


'using pic with out RS232 chip'
1998\12\28@114301 by John Payson
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|Depending on the situation, you can also send it just using the PIC
|itself, and invert the output logic level. Not something I'd reccomend
|for industrial environments, but it works. The common 232 receivers
|don't need negative voltage, just <1V, and they need at least 3V on the
|high side.

The big thing to note when trying to send serial "directly"
between an RS-232 port (like the PC) and the PIC is that the
effective rise times and effective fall times won't match;
this isn't generally a problem at slower baud rates, and it
may to a fair extent be corrected in software, but don't ex-
pect to go 115,200 baud without having to tweak things a little
bit...

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