RS232 Parasitic power

Started by Scott Lingerfelt April 11, 2005

Is it possible to power my PIC from an RS232 serial cable connected to my
PC? If so were can I find some schematics or can someone tell me what I
need.

Thanks in advance, Scott


Hello.

Look at slaa096b.pdf on the TI web site. It's a document about a bootstrap
loader for the MSP430, and the power is supplied by the RS232 cable. The
circuitry to get the supply voltage from the RS232 is described there and
you can use it for a PIC too..

Regards

Vito

----- Original Message -----
From: "Scott Lingerfelt" <slingerfelt@slin...>
To: "Piclist" <piclist@picl...>
Sent: Tuesday, April 12, 2005 1:56 AM
Subject: [piclist] RS232 Parasitic power >
>
> Is it possible to power my PIC from an RS232 serial cable connected to my
> PC? If so were can I find some schematics or can someone tell me what I
> need.
>
> Thanks in advance, Scott >
> to unsubscribe, go to http://www.yahoogroups.com and follow the
instructions
> Yahoo! Groups Links



Vito,

I have metered the power of 2 of my PC's and get around 10V
+/- on one and 7V +/- on the other. I feel the voltage is
random and will be to low to run the PIC at 5V. They are
running the MSP430 at 3V. Is it possible to run my PIC at
3V and monitor a 0-5V input to the PIC's A/D by scaling the
input in software?

Here is the direct link for anyone looking for the same.

http://focus.ti.com/lit/an/slaa096b/slaa096b.pdf

Thanks for your help,

Scott

On Tue, 12 Apr 2005 09:35:11 +0200
"Vito Russo" <vrusso@vrus...> wrote:
>
> Hello.
>
> Look at slaa096b.pdf on the TI web site. It's a document
> about a bootstrap
> loader for the MSP430, and the power is supplied by the
> RS232 cable. The
> circuitry to get the supply voltage from the RS232 is
> described there and
> you can use it for a PIC too..
>
> Regards
>
> Vito
>
> ----- Original Message -----
> From: "Scott Lingerfelt" <slingerfelt@slin...>
> To: "Piclist" <piclist@picl...>
> Sent: Tuesday, April 12, 2005 1:56 AM
> Subject: [piclist] RS232 Parasitic power > >
> >
> > Is it possible to power my PIC from an RS232 serial
> cable connected to my
> > PC? If so were can I find some schematics or can
> someone tell me what I
> > need.
> >
> > Thanks in advance, Scott
> >
> >
> >
> > to unsubscribe, go to http://www.yahoogroups.com and
> follow the
> instructions
> > Yahoo! Groups Links
> >
> >
> >
> >
> >
> to unsubscribe, go to http://www.yahoogroups.com and
> follow the instructions
> Yahoo! Groups Links >
>




Well, the voltage on RS232 lines should be between 5 - 15 V +/- (Altough it
should be better to check this on the WEB. By the way, the cable adds
degradation to the voltage level, so the voltage you get on your device
could be a bit lower). If the system stays between theseboundaries it is
RS232 compliant.

The ICs used in embedded application, like the MAX232, usually are on the
low side of the specification (usually you see 5/5.5 V on the output), but
the ICs used on PCs usually have higher voltages.

However, if you want that your device will work with every PC, you should
accept 5V as an input voltage.

You should search for a voltage regulator able to provide 5V output with a
wide range of input, even when the input is very close to 5V. I've never
used one, but maybe you can try a charge pump IC.

Or, you can run the system with a lower voltage, at 3 V for example.
You can use a resistor divider to scale the signal from 5V to 3V (for
example 2k/3k). However this affects the performance, it adds an additional
delay, so if the signal is changing very quickly it could be a problem.

Hope this helps.. (and don't take my hints as the holy bible..)

Regards

Vito ----- Original Message -----
From: <slingerfelt@slin...>
To: <piclist@picl...>
Sent: Tuesday, April 12, 2005 1:41 PM
Subject: Re: [piclist] RS232 Parasitic power >
> Vito,
>
> I have metered the power of 2 of my PC's and get around 10V
> +/- on one and 7V +/- on the other. I feel the voltage is
> random and will be to low to run the PIC at 5V. They are
> running the MSP430 at 3V. Is it possible to run my PIC at
> 3V and monitor a 0-5V input to the PIC's A/D by scaling the
> input in software?
>
> Here is the direct link for anyone looking for the same.
>
> http://focus.ti.com/lit/an/slaa096b/slaa096b.pdf
>
> Thanks for your help,
>
> Scott
>
> On Tue, 12 Apr 2005 09:35:11 +0200
> "Vito Russo" <vrusso@vrus...> wrote:
> >
> > Hello.
> >
> > Look at slaa096b.pdf on the TI web site. It's a document
> > about a bootstrap
> > loader for the MSP430, and the power is supplied by the
> > RS232 cable. The
> > circuitry to get the supply voltage from the RS232 is
> > described there and
> > you can use it for a PIC too..
> >
> > Regards
> >
> > Vito
> >
> > ----- Original Message -----
> > From: "Scott Lingerfelt" <slingerfelt@slin...>
> > To: "Piclist" <piclist@picl...>
> > Sent: Tuesday, April 12, 2005 1:56 AM
> > Subject: [piclist] RS232 Parasitic power
> >
> >
> > >
> > >
> > > Is it possible to power my PIC from an RS232 serial
> > cable connected to my
> > > PC? If so were can I find some schematics or can
> > someone tell me what I
> > > need.
> > >
> > > Thanks in advance, Scott
> > >
> > >
> > >
> > > to unsubscribe, go to http://www.yahoogroups.com and
> > follow the
> > instructions
> > > Yahoo! Groups Links
> > >
> > >
> > >
> > >
> > >
> >
> >
> >
> > to unsubscribe, go to http://www.yahoogroups.com and
> > follow the instructions
> > Yahoo! Groups Links
> >
> >
> >
> >
> >
> >
> >
> to unsubscribe, go to http://www.yahoogroups.com and follow the
instructions
> Yahoo! Groups Links




--- In piclist@picl..., <slingerfelt@c...> wrote:
> Vito,
>
> I have metered the power of 2 of my PC's and get around 10V
> +/- on one and 7V +/- on the other. I feel the voltage is
> random and will be to low to run the PIC at 5V. They are
> running the MSP430 at 3V. Is it possible to run my PIC at
> 3V and monitor a 0-5V input to the PIC's A/D by scaling the
> input in software?

Probably not... On most devices it is not allowed to have a signal
input at a higher voltage than the device itself. Use a couple of
resistors to divide the input voltage.

>
> Here is the direct link for anyone looking for the same.
>
> http://focus.ti.com/lit/an/slaa096b/slaa096b.pdf
>
> Thanks for your help,
>
> Scott
>
> On Tue, 12 Apr 2005 09:35:11 +0200
> "Vito Russo" <vrusso@e...> wrote:
> >
> > Hello.
> >
> > Look at slaa096b.pdf on the TI web site. It's a document
> > about a bootstrap
> > loader for the MSP430, and the power is supplied by the
> > RS232 cable. The
> > circuitry to get the supply voltage from the RS232 is
> > described there and
> > you can use it for a PIC too..
> >
> > Regards
> >
> > Vito
> >
> > ----- Original Message -----
> > From: "Scott Lingerfelt" <slingerfelt@c...>
> > To: "Piclist" <piclist@picl...>
> > Sent: Tuesday, April 12, 2005 1:56 AM
> > Subject: [piclist] RS232 Parasitic power
> >
> >
> > >
> > >
> > > Is it possible to power my PIC from an RS232 serial
> > cable connected to my
> > > PC? If so were can I find some schematics or can
> > someone tell me what I
> > > need.
> > >
> > > Thanks in advance, Scott
> > >
> > >
> > >
> > > to unsubscribe, go to http://www.yahoogroups.com and
> > follow the
> > instructions
> > > Yahoo! Groups Links
> > >
> > >
> > >
> > >
> > >
> >
> >
> >
> > to unsubscribe, go to http://www.yahoogroups.com and
> > follow the instructions
> > Yahoo! Groups Links
> >
> >
> >
> >
> >
> >
> >



Well, the voltage on RS232 lines should be between 5 - 15 V +/- (Altough it
should be better to check this on the WEB. By the way, the cable adds
degradation to the voltage level, so the voltage you get on your device
could be a bit lower). If the system stays between theseboundaries it is
RS232 compliant. 
 
 
RS232 is 25v, being -12.5 to +12.5.  No doubt someone uses 15v for long cables...  A valid signal is between 3 - 12.5v +/-, so -3 to +3 is undefined, ie not a 0 or 1 bit.
 
I don't think they ever expected people to make laptops where the signal level is 5v.
 
Still, mice work well on serial power, and they had PICs in them - 16C54 from memory.
 
Tony


Have you tried to use the max232 inverted? Sometime ago, somebody from
Maxim told something about that, using max232 as a DC-DC converter
(what it really is) but in reverse direccion. Feeding it with RTS (for
example) and obtaining 5V at 16pin. I'll try to check on this.

I hope to find it, it was a looooong time ago.

Bye!
____________________
Enzo Gomez, EE