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'Need help with a little hardware RS232 inteface.'
1997\03\10@153248 by Randy Walsh

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Need help with a little hardware RS232 inteface.

Description: I've created a PC stepper motor interface using a 16c84 and a
2003.  I'm currently doing Serial I/O straight from the 16c84 to the PC

Question: If I use 1 of then 2003 drivers to boost the TX line (from pic to
PC) to +12 volts, would this damage the PC?  (I thing that the RS232 spec
is only +10 volts), what must I do to safely drop the voltage to 10 volts?

Thanks in advance (TIA)

Sincerely,
Randy Walsh

1997\03\10@161749 by Mike

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At 12:19 PM 10/03/97 -0800, you wrote:
>Need help with a little hardware RS232 inteface.
>
>Description: I've created a PC stepper motor interface using a 16c84 and a
>2003.  I'm currently doing Serial I/O straight from the 16c84 to the PC
>
>Question: If I use 1 of then 2003 drivers to boost the TX line (from pic to
>PC) to +12 volts, would this damage the PC?  (I thing that the RS232 spec
>is only +10 volts), what must I do to safely drop the voltage to 10 volts?

1.      Pair of diodes,
2.      Resistors
3.      2v7 zeners

And remember you will have some (minor) saturation of the output transistors
in the 2003 anyway.

Most PCs can quite comfortably handle 12v - but of course you should not rely
on continued use at the maximum.

Though shouldn't you have a -ve voltage on the TX line when off - say -3v, this
will help in noise rejection in case  your line floats above 0v when not being
driven !

Rgds

Mike

Politicians, like babies nappies should be changed often - and for the same
reason.

1997\03\10@165343 by Robert Lunn

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>Question: If I use 1 of then 2003 drivers to boost the TX line (from pic to
>PC) to +12 volts, would this damage the PC?  (I thing that the RS232 spec
>is only +10 volts), what must I do to safely drop the voltage to 10 volts?

       The RS-232 spec is +/- 18 volts.  +10 volts is fine.

___Bob

1997\03\10@170510 by myke predko

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Randy Walsh Wrote:

>Need help with a little hardware RS232 inteface.
>
>Description: I've created a PC stepper motor interface using a 16c84 and a
>2003.  I'm currently doing Serial I/O straight from the 16c84 to the PC
>
>Question: If I use 1 of then 2003 drivers to boost the TX line (from pic to
>PC) to +12 volts, would this damage the PC?  (I thing that the RS232 spec
>is only +10 volts), what must I do to safely drop the voltage to 10 volts?

No.  RS-232 is up to 15 Volts.

Where you may have your problems is with the negative voltage generation.
Your PC (like mine) may "read" 0 Volts as a "Mark" and actually require a
negative voltage.

There's a simple circuit for using the RX Negative Voltage.  Please correct
me if I'm wrong:



            |
            |     10K Resistor
 PIC   RXIn |-----^^^^^^-------------------+-------------------- From PC
            |                              |
            |                              >
            |                              < "Pull Down" Resistor
            |                              >
            |                              |
            |                              +-------------------- To PC
            |                              |
            |           Resistor         |^
       TXOut|-----------^^^^^------------|
            |                            |\
            |                              |
            |                            To +5 - +12 Volts
            |

This will send a Negative Voltage nominally and then send a Positive voltage
when the TXOut Line is High.

Have I got the circuit correct?  I typically use a DS275 which does all this
for me.

myke

"Some people say that foreign cars handle best, while others say domestic.
For my money, nothing handles as well as a rental car." - P.J. O'Rourke

1997\03\10@182331 by Randy Walsh

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>
> Randy Walsh Wrote:
>
> >Need help with a little hardware RS232 inteface.
> >


> Myke Predko responded:
> Where you may have your problems is with the negative voltage generation.
> Your PC (like mine) may "read" 0 Volts as a "Mark" and actually require a
> negative voltage.
>
> There's a simple circuit for using the RX Negative Voltage.  Please
correct
{Quote hidden}

voltage
> when the TXOut Line is High.
>

I like this idea of stealing -12V off the RX line.

Forgive my ignorance first, I'm more of a software buff than hardware, but
what if...the RX line is receiving while I'm trying to TX?  Will it's +/-
swing mess up my ability to get -12 volts from it?

Assuming this is true, and assuming I don't TX and RX at the same, then
when I leave the TXOut line low (not in use), the "To PC" line is sitting
at -12V  (Did I get this right?).

What happens when I receive a character from the PC?  It seams to me that I
will then get the same character sent back to the PC over the TX, only
through the pulldown resistor?  Is this correct?

As before, please explain in detail, as I said, I'm a little dense when it
comes to making hardware do its thing.


Thanks again.
Randy

1997\03\10@193309 by Lee Jones

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> Need help with a little hardware RS232 inteface.
>
> Description: I've created a PC stepper motor interface using a 16c84 and a
> 2003.  I'm currently doing Serial I/O straight from the 16c84 to the PC
>
> Question: If I use 1 of then 2003 drivers to boost the TX line (from pic to
> PC) to +12 volts, would this damage the PC?  (I thing that the RS232 spec
> is only +10 volts), what must I do to safely drop the voltage to 10 volts?

RS232 specifies that each line must be in 1 of 2 states.  One
state is a voltage between +6 and +25 at the transmitter output
pin (after losses, +3 to +25 volts at the receiver input pin).
The other state is a voltage between -6 and -25 at the transmitter
(-3 to -25 volts at the receiver).

The range from -3 to +3 is not a valid signal level.  It's there
to provide hysteresis and a noise margin.  Lots of people use 0
volts as the low level.  But don't kid yourself that it's compliant
RS232 or that it will work with all devices.
                                               Lee Jones

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