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PICList Thread
'[PIC]: Why full on RS232'
2001\02\14@150750 by Peter May

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I asked this a week or so ago but didn't get much a response so I will ask
again.

I have written a small windows application that communicates with a PIC
project. All it does is poll the project for the values of the analog
inputs. I have 3 wires connected between the 2. Ground and a send and
recieve data with a resistor in line with each. It seems to work fine.

Why should I use a MAX232 and all the associated pins etc out of the 9 pin
plug etc?

I do however wonder if there is a way of connecting a RTS pin from the
commputer to an interupt pin on the PIC so it can go and do things an answer
the comp only when it polls it...

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2001\02\14@153059 by Drew Vassallo

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>inputs. I have 3 wires connected between the 2. Ground and a send and
>recieve data with a resistor in line with each. It seems to work fine.

This works fine for short cable lengths, but still has the potential to
glitch now and then.  It's definitely NOT production-ready, but it will get
you through the debugging phase.  The logic levels don't span the full range
on the '1' bit (logic low), as they only go to 0V with this setup, while the
computer is looking for anything 0 to -5V.

--Andrew
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2001\02\14@161238 by M. Adam Davis

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Peter May wrote:
>
> I asked this a week or so ago but didn't get much a response so I will ask
> again.
>
> I have written a small windows application that communicates with a PIC
> project. All it does is poll the project for the values of the analog
> inputs. I have 3 wires connected between the 2. Ground and a send and
> recieve data with a resistor in line with each. It seems to work fine.
>
> Why should I use a MAX232 and all the associated pins etc out of the 9 pin
> plug etc?

You should use the MAX232 when you want to make more than one device and
you don't want the headaches of constant user support.  For one offs the
resister direct connect is perfectly fine and reliable as long as you
don't use the internal PIC UART (which cannot invert it's output)

> I do however wonder if there is a way of connecting a RTS pin from the
> commputer to an interupt pin on the PIC so it can go and do things an answer
> the comp only when it polls it...

Sure.  Both the DTR and RTS pins are controlled by the computer, and are
standard rs-232 signals (+12 and -12v).  You can hook them up to a pin the
same way you hook up the computer's transmit line to the pic - through a
resistor.

-Adam

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2001\02\14@174536 by David Duffy

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At 07:04 AM 15/02/01 +1100, you wrote:
>I asked this a week or so ago but didn't get much a response so I will ask
>again.
>
>I have written a small windows application that communicates with a PIC
>project. All it does is poll the project for the values of the analog
>inputs. I have 3 wires connected between the 2. Ground and a send and
>recieve data with a resistor in line with each. It seems to work fine.
>
>Why should I use a MAX232 and all the associated pins etc out of the 9 pin
>plug etc?

The MAX232 is there as a voltage level converter. You don't have to use any
more
pins on the DB9 just because of it. Note that the MAX232 inverts the levels
as well.
I assume you're bit-banging the comms - not using a PIC hardware UART?

>I do however wonder if there is a way of connecting a RTS pin from the
>commputer to an interupt pin on the PIC so it can go and do things an answer
>the comp only when it polls it...

You should be able to use the other receiver in the MAX232 chip for this.
Regards...

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