Serial Port voltage drops with Linux

[Richard B. Johnson]

On Sun, 23 Jan 2000, Alistair Riddell wrote:

> On Mon, 17 Jan 2000, Richard B. Johnson wrote:
> 
> > Hmmm. If his hardware is trying to get power from the modem control
> > lines the hardware is broken by design and should be fixed. There
> > is no RS-232C specification for this mode of operation. 
> 
> And serial mice draw their power from where? ;-)
> 

>From little bits of pizza left laying around.

Seriously, the use of any RS-232C bits for power is completely and totally
broken. In the old XT days a serial board containing two UARTs, plugged
into an 8-bit slot had lots of power available because DSR was just
connected to the 12-volt line. Further, the line-transceivers ran from
+/- 12 volts and were capable of driving a model railroad.

Then we got chips on the motherboard and low-power chips in laptops.
They all comply with the RS-232C standards but they don't drive
mice. Some almost do, depends upon how hungry the mouse is. This
is why mouse-ports (bus mice) were provided. One of the wires in
that cable is power.

So if somebody is designing a new device, as was the case that started
this thread, and they are using "power" from RS-232C, the design is
broken.

Cheers,
Dick Johnson

Penguin : Linux version 2.3.39 on an i686 machine (800.63 BogoMips).


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