disablement and enablement of a computer serial port.

Peter Easthope

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Jul 26, 2015, 11:42:59 AM7/26/15

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Can anyone explain how a computer serial port is disabled and enabled? Many Web pages cover the software topics but I've found nothing explaining how a BIOS disables a port. Is is done by clearing any interrupt assignment to the port? Is the power to the UART shut off? Other ways? Probably more than one technique has been used but not a large number.

Can anyone cite a Web page which answers this?

Thanks, ... Peter E.

M Philbrook

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Jul 26, 2015, 1:32:05 PM7/26/15

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In article <eca56518-7629-4947...@googlegroups.com>,
petere...@gmail.com says...

>
> Can anyone explain how a computer serial port is disabled and enabled? Many Web pages cover the software topics but I've found nothing explaining how a BIOS disables a port. Is is done by clearing any interrupt assignment to the port? Is the power to the UART shut off? Other ways? Probably more than one technique has been used but not a large number.
>
> Can anyone cite a Web page which answers this?
>
> Thanks, ... Peter E.

if Bios has control of it, it simply flips a bit
in a device manager chip on the board..

From that point, the OS does not see that serial port.

Which also means, there is no IRQ assigned to it or
port addresses etc.. in otherwords, no resources assigned
to it.

USB serial ports are a different story..

Jamie

Peter Easthope

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Jul 26, 2015, 2:09:50 PM7/26/15

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On Sunday, July 26, 2015 at 10:32:05 AM UTC-7, M Philbrook wrote:
> if Bios has control of it, it simply flips a bit
> in a device manager chip on the board.

OK, that would be in evidence when the Bios interface
allows disabling.

> From that point, the OS does not see that serial port.

There must be a customary way for the OS to read the state
of the bit. Otherwise communication software would have
to allow for obscure hardware variations. Can software
flip the bit? Ie. can a port can be enabled and disabled
when the system is running?

> USB serial ports are a different story.

Fortunately I'm only interested in RS-232 style serial ports,
for now at least.

Thanks! ... Peter E.

Johann Klammer

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Jul 27, 2015, 9:04:23 AM7/27/15

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On 07/26/2015 08:09 PM, Peter Easthope wrote:
> On Sunday, July 26, 2015 at 10:32:05 AM UTC-7, M Philbrook wrote:
>> if Bios has control of it, it simply flips a bit
>> in a device manager chip on the board.
>
> OK, that would be in evidence when the Bios interface
> allows disabling.
>
>> From that point, the OS does not see that serial port.
>
> There must be a customary way for the OS to read the state
> of the bit. Otherwise communication software would have
> to allow for obscure hardware variations. Can software
> flip the bit? Ie. can a port can be enabled and disabled
> when the system is running?
>

Depends what you mean by software.
The ancient vt1211 superi/o chip does its configuration via port writes.
You may need to be in kernel context to be allowed to do that(or ioperm()).
You may have to browse datasheets, to find out if others are similar
or if there's a de-facto standard.
Having a look at intels LPC bus specs may be necessary, too.
(they're available online)

Analogman2

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Aug 20, 2015, 2:48:03 PM8/20/15

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What exactly are you tring to do? If you are using Windows, you should be able to disable it using the device manager.