Peter --
To a very limited extent, at least "some" power can be 'stolen' in this
manner, but the amount is quite limited.
The amount available depends entirely on the drivers that are driving your
cable. You should consult the documentation for the actual drivers that are
in use for the V/I curves for your load impedance.
Of course, as soon as you start to lean on device dependent characteristics,
your application becomes, by definition, "not portable".
Two common methods are --
1. Rectify one of the adtive interface lines to charge a capacitor. This
requires that the interface line remain active to maintain the charge and
the capacitor must have sufficient energy storage to handle at least one
message frame.
2. Dedicate one of the static interface lines (such as DTR) to providing
power. Again, see the driver's spec sheets for available V/I.
I consider it much better, to "hard drive" the DTR line by connecting it to
a power source. Alternately, consider using an 'unused' RS232 signal as a
supply line.
I have also observed that you can purchase suitable cable with a pair of
'power conductors, typically 18AWG, plus the necessary signal conductors in
smaller gauge. This cable is intended for use in multi-drop links which
normally use RS-485 type protocol. The power leads supply power to the drop
points. See Belden, Alpha, et al.
You should also be aware that a number of IC manufactures, MAXIM for
example, make RS-232 interface devices with "auto wake-up features" which
may be useful where minimum power consumption is important.
Good luck, Bob Smith
--- Avoid computer viruses, Practice safe hex ---
-- Specializing in small, cost effective
embedded control systems --
http://www.smithmachineworks.com/embedprod.html
Robert L. (Bob) Smith
Smith Machine Works, Inc.
9900 Lumlay Road
Richmond, VA 23236 804/745-1065
[EMAIL PROTECTED]
----- Original Message -----
To: "Multiple recipients of list CHIPDIR-L" <[EMAIL PROTECTED]>
Sent: Friday, November 28, 2003 1:29 AM
> Does anyone have any experience in using an RS232 port to power a
> periphereal device? The device includes a small PIC micro that will
> need to send data back to the computer it's plugged into, so I need to
> power the RS232 driver as well.
>
> Any practical advice such as how much current and voltage I can rely on
> will be appreciated.
>
> Peter Vogel
> ZapTV Pty Ltd
> 30 Adeline St, Faulconbridge 2776
> Australia
> Tel: 02 4751 8735
> Fax: 02 4751 2601
> email: [EMAIL PROTECTED]
>
> --
> Author: Peter Vogel
> INET: [EMAIL PROTECTED]
>
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--
Author: Robert Smith
INET: [EMAIL PROTECTED]
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