Issue on connect 2 modems with a single phone line

Issue on connect 2 modems with a single phone line

[Park Lee]

Hi,
  I want to try serial console in order to see the
complete Linux kernel oops. 
  I have 2 computers, one is a PC, and the other is a
Laptop. Unfortunately,my Laptop doesn't have a serial
port on it. But then, the each machine has a internal
serial modem respectively.
  Then, can I use a telephone line to directly connect
the two machines via their internal modems (i.e. One
end of the telephone line is plugged into The PC's
modem, and the other end is plugged into The Laptop's
modem directly), and let them do the same function as
two serial ports and a null modem can do? If it is,
How to achieve that?

  Thank you.



=====
Best Regards,
Park Lee


		
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Re: Issue on connect 2 modems with a single phone line

[Lee Revell]

On Wed, 2004-12-15 at 10:42 -0800, Park Lee wrote:
> Hi,
>   I want to try serial console in order to see the
> complete Linux kernel oops. 
>   I have 2 computers, one is a PC, and the other is a
> Laptop. Unfortunately,my Laptop doesn't have a serial
> port on it.

No idea but it would be way easier to use netconsole.  That is, unless
the oops happens before the network is up.

Lee

Re: Issue on connect 2 modems with a single phone line

[Park Lee]

On Wed, 15 Dec 2004 at 13:48, Lee Revell wrote:
> On Wed, 2004-12-15 at 10:42 -0800, Park Lee wrote:
> > Hi,
> > I want to try serial console in order to see the
> > complete Linux kernel oops. 
> > I have 2 computers, one is a PC, and the other is 
> > a Laptop. Unfortunately,my Laptop doesn't have a 
> > serial port on it.
>
> No idea but it would be way easier to use 
> netconsole.  That is, unless the oops happens 
> before the network is up.

Yes, I've tried to use netconsole. But it seemed that
it wasn't fit for my case, because the kernel crash
occurs in the net subsection. When kernel doesn't
crash, the netconsole works well. BUT, when the kernel
crashs, the netconsole cann't send out the oops
message!


=====
Best Regards,
Park Lee


		
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Re: Issue on connect 2 modems with a single phone line

[Pavel Machek]

Hi!

>   I want to try serial console in order to see the
> complete Linux kernel oops. 
>   I have 2 computers, one is a PC, and the other is a
> Laptop. Unfortunately,my Laptop doesn't have a serial
> port on it. But then, the each machine has a internal
> serial modem respectively.
>   Then, can I use a telephone line to directly connect
> the two machines via their internal modems (i.e. One
> end of the telephone line is plugged into The PC's
> modem, and the other end is plugged into The Laptop's
> modem directly), and let them do the same function as
> two serial ports and a null modem can do? If it is,
> How to achieve that?

You'd need phone exchange to do this. Most modems will not talk using
simple cable. With 12V power supply and resistor phone exchange is
quite easy to emulate, but...
								Pavel
-- 
People were complaining that M$ turns users into beta-testers...
...jr ghea gurz vagb qrirybcref, naq gurl frrz gb yvxr vg gung jnl!

Re: Issue on connect 2 modems with a single phone line

[David Lawyer]

On Thu, Dec 16, 2004 at 02:01:38AM +0100, Pavel Machek wrote:
> Hi!
> 
> >   I want to try serial console in order to see the
> > complete Linux kernel oops. 
> >   I have 2 computers, one is a PC, and the other is a
> > Laptop. Unfortunately,my Laptop doesn't have a serial
> > port on it. But then, the each machine has a internal
> > serial modem respectively.
> >   Then, can I use a telephone line to directly connect
> > the two machines via their internal modems (i.e. One
> > end of the telephone line is plugged into The PC's
> > modem, and the other end is plugged into The Laptop's
> > modem directly), and let them do the same function as
> > two serial ports and a null modem can do? If it is,
> > How to achieve that?
> 
> You'd need phone exchange to do this. Most modems will not talk using
> simple cable. With 12V power supply and resistor phone exchange is
> quite easy to emulate, but...

Here's what I once wrote in Modem-HOWTO:

  Most modems are designed to be connected only to telephone lines and
  will not work over just a pair of wires.  This is because the
  telephone company supplies the telephone line with a 40-50 volt DC
  voltage which powers part of the modem.  Recall that ordinary
  conventional telephones are entirely powered by the voltage from the
  telephone company.  Without such a DC voltage, the modem lacks power
  and can't send out data.  Furthermore, the telephone company has
  special signals indicating a ring, line busy, etc.  Conventional
  modems expect and respond to these signals.

  One way around this is to make a simple power supply to emulate a
  telephone line.  See Connecting two computers using their modems,
  without a telephone line <http://www.jagshouse.com/modem.html>.  In
  most cases there are better way to connect computers together such as
  using network cards or just cables (null-modem) between the serial
  ports.  Using modems has the advantage of increasing the distance as
  compared to a null-modem cable, since it's using a twisted pair.  But
  it isn't nearly as fast as network cards.

			David Lawyer

Re: Issue on connect 2 modems with a single phone line

[Ondrej Zary]

Pavel Machek wrote:
 > Hi!
 >
 >
 >>  I want to try serial console in order to see the
 >>complete Linux kernel oops.
 >>  I have 2 computers, one is a PC, and the other is a
 >>Laptop. Unfortunately,my Laptop doesn't have a serial
 >>port on it. But then, the each machine has a internal
 >>serial modem respectively.
 >>  Then, can I use a telephone line to directly connect
 >>the two machines via their internal modems (i.e. One
 >>end of the telephone line is plugged into The PC's
 >>modem, and the other end is plugged into The Laptop's
 >>modem directly), and let them do the same function as
 >>two serial ports and a null modem can do? If it is,
 >>How to achieve that?
 >
 >
 > You'd need phone exchange to do this. Most modems will not talk using
 > simple cable. With 12V power supply and resistor phone exchange is
 > quite easy to emulate, but...
 > 								Pavel

I've tried it with 2 older Microcom modems and a piece of cable and it 
worked. Opened terminal on both sides, wrote ATDT123 (or any number you 
like) and ATA on the other. But if the laptop modem is a software modem, 
this will not work.

-- 
Ondrej Zary

Re: Issue on connect 2 modems with a single phone line

[Steve Bromwich]

On Wed, 15 Dec 2004, Park Lee wrote:

> Hi,
>   I want to try serial console in order to see the
> complete Linux kernel oops.
>   I have 2 computers, one is a PC, and the other is a
> Laptop. Unfortunately,my Laptop doesn't have a serial
> port on it. But then, the each machine has a internal
> serial modem respectively.
>   Then, can I use a telephone line to directly connect
> the two machines via their internal modems (i.e. One
> end of the telephone line is plugged into The PC's
> modem, and the other end is plugged into The Laptop's
> modem directly), and let them do the same function as
> two serial ports and a null modem can do? If it is,
> How to achieve that?

Hi,

This used to come up every now and then on the UK USR HST Fidonet echo.
You can do this back to back with a crossover phone cable with some
modems. Other modems require a circuit with a battery inline and resistors
(see http://www.repairfaq.org/ELE/F_ASCII_Schem_Tel.html#ASCIISCHEMTEL_010
for an example of the sort of thing you'd need, but I'd imagine you'll
need to change it for your local standards). You will need to set up your
modem on your laptop to auto-answer and save to NVRAM (usually ATS0=1&W),
and also disable DTR detection and again save to NVRAM (usually AT&D0&W).
On the other end you'll need to disable dial tone detection (usually ATX3)
and then dial a dummy number (ATDT0).

As was already noted, if the modem in the laptop is a softmodem you won't
be able to do this.

Cheers, Steve

Re: Issue on connect 2 modems with a single phone line

[Brad Campbell]

David Lawyer wrote:
> On Thu, Dec 16, 2004 at 02:01:38AM +0100, Pavel Machek wrote:
> 
>>Hi!
>>
>>
>>>  I want to try serial console in order to see the
>>>complete Linux kernel oops. 
>>>  I have 2 computers, one is a PC, and the other is a
>>>Laptop. Unfortunately,my Laptop doesn't have a serial
>>>port on it. But then, the each machine has a internal
>>>serial modem respectively.
>>>  Then, can I use a telephone line to directly connect
>>>the two machines via their internal modems (i.e. One
>>>end of the telephone line is plugged into The PC's
>>>modem, and the other end is plugged into The Laptop's
>>>modem directly), and let them do the same function as
>>>two serial ports and a null modem can do? If it is,
>>>How to achieve that?
>>
>>You'd need phone exchange to do this. Most modems will not talk using
>>simple cable. With 12V power supply and resistor phone exchange is
>>quite easy to emulate, but...
> 
> 
> Here's what I once wrote in Modem-HOWTO:
> 
>   Most modems are designed to be connected only to telephone lines and
>   will not work over just a pair of wires.  This is because the
>   telephone company supplies the telephone line with a 40-50 volt DC
>   voltage which powers part of the modem.  Recall that ordinary
>   conventional telephones are entirely powered by the voltage from the
>   telephone company.  Without such a DC voltage, the modem lacks power
>   and can't send out data.  Furthermore, the telephone company has
>   special signals indicating a ring, line busy, etc.  Conventional
>   modems expect and respond to these signals.

I have used analogue modems back to back for years and have *never* come across a modem that sourced 
anything other than it's ringing signal (via an opto) from the phone line. Every single modem I have 
here will talk to the others over a straight telephone cable.

Analogue modems use a line transformer to couple to the phone network usually with a decoupling 
capacitor on the phone end of the network to prevent large current flows through the transformer. 
They use a standard AC analogue signal. Nothing more than an audio transformer linkage.

Now, sometimes a modem needs coaxing to ignore the lack of dial/call progress tones, but they should 
always talk to each other regardless of line voltage.

ATA on one end and ATD on the other will normally get them talking.
As a test I just looped my internal AMR winmodem to my Xircom Realport V90 modem and got a solid 
28.8k link. No problem.

If the fluid is salty enough you could probably get analogue modems to talk over wet string (I have 
certainly passed RS485 over wet string before).

-- 
Brad
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Re: Issue on connect 2 modems with a single phone line

[Manu Abraham]

On Sat December 18 2004 11:01 am, Brad Campbell wrote:
> David Lawyer wrote:
> > On Thu, Dec 16, 2004 at 02:01:38AM +0100, Pavel Machek wrote:
> >>Hi!
> >>
> >>>  I want to try serial console in order to see the
> >>>complete Linux kernel oops.
> >>>  I have 2 computers, one is a PC, and the other is a
> >>>Laptop. Unfortunately,my Laptop doesn't have a serial
> >>>port on it. But then, the each machine has a internal
> >>>serial modem respectively.
> >>>  Then, can I use a telephone line to directly connect
> >>>the two machines via their internal modems (i.e. One
> >>>end of the telephone line is plugged into The PC's
> >>>modem, and the other end is plugged into The Laptop's
> >>>modem directly), and let them do the same function as
> >>>two serial ports and a null modem can do? If it is,
> >>>How to achieve that?
> >>
> >>You'd need phone exchange to do this. Most modems will not talk using
> >>simple cable. With 12V power supply and resistor phone exchange is
> >>quite easy to emulate, but...
> >
> > Here's what I once wrote in Modem-HOWTO:
> >
> >   Most modems are designed to be connected only to telephone lines and
> >   will not work over just a pair of wires.  This is because the
> >   telephone company supplies the telephone line with a 40-50 volt DC
> >   voltage which powers part of the modem.  Recall that ordinary
> >   conventional telephones are entirely powered by the voltage from the
> >   telephone company.  Without such a DC voltage, the modem lacks power
> >   and can't send out data.  Furthermore, the telephone company has
> >   special signals indicating a ring, line busy, etc.  Conventional
> >   modems expect and respond to these signals.
>
> I have used analogue modems back to back for years and have *never* come
> across a modem that sourced anything other than it's ringing signal (via an
> opto) from the phone line. Every single modem I have here will talk to the
> others over a straight telephone cable.
What about power ? The opto-coupler will not work without power.

>
> Analogue modems use a line transformer to couple to the phone network
> usually with a decoupling capacitor on the phone end of the network to
> prevent large current flows through the transformer. They use a standard AC
The capacitor is used to prevent DC saturation of the transformer core rather 
than doing current limiting, A capacitor cannot do current limiting. When the 
lag changes by changing the capacitance value,  general concept is that a 
capacitor can limit current which is very much wrong.

Manu

> analogue signal. Nothing more than an audio transformer linkage.
>
> Now, sometimes a modem needs coaxing to ignore the lack of dial/call
> progress tones, but they should always talk to each other regardless of
> line voltage.
>
> ATA on one end and ATD on the other will normally get them talking.
> As a test I just looped my internal AMR winmodem to my Xircom Realport V90
> modem and got a solid 28.8k link. No problem.
>
> If the fluid is salty enough you could probably get analogue modems to talk
> over wet string (I have certainly passed RS485 over wet string before).

Re: Issue on connect 2 modems with a single phone line

[Brad Campbell]

Manu Abraham wrote:
> On Sat December 18 2004 11:01 am, Brad Campbell wrote:

>>I have used analogue modems back to back for years and have *never* come
>>across a modem that sourced anything other than it's ringing signal (via an
>>opto) from the phone line. Every single modem I have here will talk to the
>>others over a straight telephone cable.
> 
> What about power ? The opto-coupler will not work without power.
> 

If you read what I wrote you will note the opto-coupler is used for ring detection only and then 
only triggers when it sees an AC ring pulse. You don't need ring detection to run modems back to 
back and therefore it's not a problem.

As for the Capacitor and Transformer, you are correct. I merely mis-communicated my message.
Point is the same, you can connect 90% of Analogue modems back to back with only a bit of wire and 
they will talk. (I say 90% as I'm sure there is one out there somewhere that uses the phone line 
voltage for something but I have *never* seen one). Remember that off-hook phone voltages can get 
down to around 3v while on hook you may have 50v.

Don't theorize about it, try it.

-- 
Brad
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Re: Issue on connect 2 modems with a single phone line

[Bill Davidsen]

Manu Abraham wrote:
> On Sat December 18 2004 11:01 am, Brad Campbell wrote:
> 
>>David Lawyer wrote:
>>
>>>On Thu, Dec 16, 2004 at 02:01:38AM +0100, Pavel Machek wrote:
>>>
>>>>Hi!
>>>>
>>>>
>>>>> I want to try serial console in order to see the
>>>>>complete Linux kernel oops.
>>>>> I have 2 computers, one is a PC, and the other is a
>>>>>Laptop. Unfortunately,my Laptop doesn't have a serial
>>>>>port on it. But then, the each machine has a internal
>>>>>serial modem respectively.
>>>>> Then, can I use a telephone line to directly connect
>>>>>the two machines via their internal modems (i.e. One
>>>>>end of the telephone line is plugged into The PC's
>>>>>modem, and the other end is plugged into The Laptop's
>>>>>modem directly), and let them do the same function as
>>>>>two serial ports and a null modem can do? If it is,
>>>>>How to achieve that?
>>>>
>>>>You'd need phone exchange to do this. Most modems will not talk using
>>>>simple cable. With 12V power supply and resistor phone exchange is
>>>>quite easy to emulate, but...
>>>
>>>Here's what I once wrote in Modem-HOWTO:
>>>
>>>  Most modems are designed to be connected only to telephone lines and
>>>  will not work over just a pair of wires.  This is because the
>>>  telephone company supplies the telephone line with a 40-50 volt DC
>>>  voltage which powers part of the modem.  Recall that ordinary
>>>  conventional telephones are entirely powered by the voltage from the
>>>  telephone company.  Without such a DC voltage, the modem lacks power
>>>  and can't send out data.  Furthermore, the telephone company has
>>>  special signals indicating a ring, line busy, etc.  Conventional
>>>  modems expect and respond to these signals.
>>
>>I have used analogue modems back to back for years and have *never* come
>>across a modem that sourced anything other than it's ringing signal (via an
>>opto) from the phone line. Every single modem I have here will talk to the
>>others over a straight telephone cable.
> 
> What about power ? The opto-coupler will not work without power.
> 
> 
>>Analogue modems use a line transformer to couple to the phone network
>>usually with a decoupling capacitor on the phone end of the network to
>>prevent large current flows through the transformer. They use a standard AC
> 
> The capacitor is used to prevent DC saturation of the transformer core rather 
> than doing current limiting, A capacitor cannot do current limiting. When the 
> lag changes by changing the capacitance value,  general concept is that a 
> capacitor can limit current which is very much wrong.

I think you're quibbling over terminology here, blocking the DC 
component with a decoupling capacitor does result in less current even 
though it's not "current limiting" in some strict sense of the term. 
Less voltage results in less current, even if the load on the other side 
were purely resistive. None of this is germane to the original 
discussion of using back to back modems, of course.

-- 
bill davidsen <davidsen@tmr.com>
   CTO TMR Associates, Inc
   Doing interesting things with small computers since 1979