Serial Port Current
Mike Mudford
I am wondering how much current I can draw from the serial port lines
??
Cheers
~Mike
PS: Please email me at:
mi...@world-net.co.nz
Alexander
The original IBM PC/AT adapter used SN 75150-P chips to drive serial
port. I assume that you are able to draw at least their limit value.
Hope this helps,
Alexander.
Roy Battell
>Mike Mudford wrote:
>
> Hi there,
> I am wondering how much current I can draw from the serial port lines
> ??
>
Less and less as power saving becomes a big issue.
So just because you can draw (say) 10mA at about 8V from an
'old' machine doesn't mean you can get even 1mA at 5V from
a modern lop-top, and increasingly desktop machines as well as
they start to use similar chip sets.
So if you are thinking of a commercial product (rather than a
one-off that only needs to work with what you have) you really
need to do a lot of investigation to make the thing reliable.
>
>> Cheers
>> ~Mike
>>
>> PS: Please email me at:
>> mi...@world-net.co.nz
>
--
Roy Battell.
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Martin P. Rinne
wrote:
>So just because you can draw (say) 10mA at about 8V from an
>'old' machine doesn't mean you can get even 1mA at 5V from
>a modern lop-top, and increasingly desktop machines as well as
>they start to use similar chip sets.
I was under impression that some standard for RS232 specifies that any
RS232 line should be able to source maximum of 7mA... maybe I'm wrong.
-------
Martini
-------
Allan Herriman
wrote:
Let me see...
From ITU-T V.28(*) :
The magnitude of the output voltage shall be between 5V and 15V for
loads between 3000 ohms and 7000 ohms.
This means you can only count on 1.67mA (=5V/3k).
(*) ITU-T V.28 is a copy of the interface characteristics of (I think)
rev C of RS-232. RS-232 is now up to rev F. I'm not sure what the
differences are.
Regards,
Allan.
Roy Battell
<mar...@lbi.ee> writes
>On Thu, 5 Aug 1999 07:52:29 +0100, Roy Battell <ma...@vutrax666.co.uk>
>wrote:
>
>>So just because you can draw (say) 10mA at about 8V from an
>>'old' machine doesn't mean you can get even 1mA at 5V from
>>a modern lop-top, and increasingly desktop machines as well as
>>they start to use similar chip sets.
>
>I was under impression that some standard for RS232 specifies that any
>RS232 line should be able to source maximum of 7mA... maybe I'm wrong.
>
in the interests of saving power. They work fine on connections
to modems and mice, so what does the buyer care.
The reality isn't theoretical - designs that have worked
solidly for the last 10 years are suddenly failing because they
can't produce a stabilised 5V 2mA supply from 'new' 'RS232' ports.
If you do testing of hardware stuff on old 'gash' equipment
for fear of damaging the ports on more integrated designs, you
end up with something that doesn't work on the 'latest' designs.
Just wanted to stop the guy falling into this trap.
>
>-------
>Martini
>-------
--
Roy Battell.
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Steve
>
> On Thu, 5 Aug 1999 07:52:29 +0100, Roy Battell <ma...@vutrax666.co.uk>
> wrote:
>
> >So just because you can draw (say) 10mA at about 8V from an
> >'old' machine doesn't mean you can get even 1mA at 5V from
> >a modern lop-top, and increasingly desktop machines as well as
> >they start to use similar chip sets.
>
> I was under impression that some standard for RS232 specifies that any
> RS232 line should be able to source maximum of 7mA... maybe I'm wrong.
>
> Martini
> -------
------------------
If it did it wouldn't matter. This was done on laptops to conserve power
years ago, and they tested them to be the "sole exception" to whatever
commonly held current/voltage spec for RS232 that there was. Since then
numerous people violated another of the spec's premises, that nobody
would ever try to use serial port current to power anything. The two
violations together are intolerant of each other. The invention of the
mouse was what did it, since there is no good way to power them except a
keyboard inline bullet-tap.
-Steve
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