Telephone wire TWO WIRE null modem cable?

Telephone wire TWO WIRE null modem cable?

Post by Marc Ant » Mon, 29 Apr 1996 04:00:00



I want to make a serial connection between two computers in
the house using regular telephone lines.  I already have a TWO
WIRE telephone line between the two computers, but the
following diagrams asks for a ground (GND) connection, does
anyone know if this is really required, or if there's a way to
get around that third ground wire.  

Thanks.

Marc

Diagram in my communications terminal program:

      Simplest Possible 3-wire Null Modem Cable

      MALE                                    FEMALE
      =============                           ========
      25D RS232 pin     signal       signal   25D   9D (as in AT)
      =============     ======       ======   ===   ==
                  7     GND <------> GND        7    5
                  3     RD  <------- TD         2    3
                  2     TD  -------> RD         3    2

 
 
 

Telephone wire TWO WIRE null modem cable?

Post by James McDuffi » Mon, 29 Apr 1996 04:00:00



> I want to make a serial connection between two computers in
> the house using regular telephone lines.  I already have a TWO
> WIRE telephone line between the two computers, but the
> following diagrams asks for a ground (GND) connection, does
> anyone know if this is really required, or if there's a way to
> get around that third ground wire.

> Thanks.

> Marc

> Diagram in my communications terminal program:

>       Simplest Possible 3-wire Null Modem Cable

>       MALE                                    FEMALE
>       =============                           ========
>       25D RS232 pin     signal       signal   25D   9D (as in AT)
>       =============     ======       ======   ===   ==
>                   7     GND <------> GND        7    5
>                   3     RD  <------- TD         2    3
>                   2     TD  -------> RD         3    2

You should be able to just connect the ground to your metal case... that is the
ideal of a ground isn't it?

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Telephone wire TWO WIRE null modem cable?

Post by -Lurc » Tue, 30 Apr 1996 04:00:00



burst forth, saying:

Quote:>I want to make a serial connection between two computers in
>the house using regular telephone lines.  I already have a TWO
>WIRE telephone line between the two computers, but the
>following diagrams asks for a ground (GND) connection, does
>anyone know if this is really required, or if there's a way to
>get around that third ground wire.  

>Thanks.

>Marc

>Diagram in my communications terminal program:

>      Simplest Possible 3-wire Null Modem Cable

>      MALE                                    FEMALE
>      =============                           ========
>      25D RS232 pin     signal       signal   25D   9D (as in AT)
>      =============     ======       ======   ===   ==
>                  7     GND <------> GND        7    5
>                  3     RD  <------- TD         2    3
>                  2     TD  -------> RD         3    2

Yes, the sigal ground is required. Otherwise you stand a real good
chance of corrupted data. If you look at your house wiring, you may
see that you have extra pairs in the wall. If they lead between the
machines, then you can make use of them. I would suggest doing it
right though and run a cable through the walls/ceiling/attic/cellar to
do the job. Be sure to compy with the building codes though. Else just
run it along the baseboards externally.  Keep in mind that rs-232 is
speced to only fifty (50) feet of cablle length. Speeds, cable
quality, environment and a host of other stuff can effect this
though...
===================================
Sig? We don' need no s*kin' sig!
===================================
 
 
 

Telephone wire TWO WIRE null modem cable?

Post by Bob Hau » Tue, 30 Apr 1996 04:00:00




Quote:>I want to make a serial connection between two computers in
>the house using regular telephone lines.  I already have a TWO
>WIRE telephone line between the two computers, but the
>following diagrams asks for a ground (GND) connection, does
>anyone know if this is really required, or if there's a way to
>get around that third ground wire.  

No, it's required for RS-232.  TXD and RXD voltages are measured
relative to signal ground (pin 7).

OTOH, many modems will talk to each other if you just hook them
together with a twisted pair.  And several companies make so-called
"short haul" modems for directly driving a twisted pair (see the
Black Box or Inmac catalogs for instance).

---

 Wasatch Communications Group               http://www.wasatch.com

 
 
 

Telephone wire TWO WIRE null modem cable?

Post by Mattias Brunsche » Tue, 30 Apr 1996 04:00:00



> >I want to make a serial connection between two computers in
> >the house using regular telephone lines.  I already have a TWO
> >WIRE telephone line between the two computers, but the
> >following diagrams asks for a ground (GND) connection, does
> >anyone know if this is really required, or if there's a way to
> >get around that third ground wire.
> No, it's required for RS-232.  TXD and RXD voltages are measured
> relative to signal ground (pin 7).
> OTOH, many modems will talk to each other if you just hook them
> together with a twisted pair.  And several companies make so-called
> "short haul" modems for directly driving a twisted pair (see the
> Black Box or Inmac catalogs for instance).

But be carefull: Some modems require a 60 V voltage level on the
2-wire-cable. There are only some modems which accept a "dumb"
twisted-pair between them. But it works: Because it's much cheeper
to hire a "dumb" twisted-pair cable instead of hiring a 9.6 kBd-line
we use this method over a distance of ablut 7 kilometers without
any problems.

Regards,
                     Mattias

 
 
 

Telephone wire TWO WIRE null modem cable?

Post by Rob Janss » Tue, 30 Apr 1996 04:00:00



Quote:>I want to make a serial connection between two computers in
>the house using regular telephone lines.  I already have a TWO
>WIRE telephone line between the two computers, but the
>following diagrams asks for a ground (GND) connection, does
>anyone know if this is really required, or if there's a way to
>get around that third ground wire.  

It is really required.  Unless you have some other ground connection
between your systems.
Use two pairs of wires for a computer connection, and you will have
no problems.  (this is also required when you are using ethernet TP)

Rob
--
+------------------------------------+--------------------------------------+


+------------------------------------+--------------------------------------+

 
 
 

Telephone wire TWO WIRE null modem cable?

Post by Marc Ant » Tue, 30 Apr 1996 04:00:00


Quote:>You should be able to just connect the ground to your metal case...
>that is the ideal of a ground isn't it?

I thought I heard something like this before but I wasn't sure.
It's easy to find the metal case to my desktop PC but I'm not
sure exactly where that would be on my plastic enclosed notebook
..I guess I'll open it up and have a look.

Thanks.

-Marc

Quote:>> I want to make a serial connection between two computers in
>> the house using regular telephone lines.  I already have a TWO
>> WIRE telephone line between the two computers, but the
>> following diagrams asks for a ground (GND) connection, does
>> anyone know if this is really required, or if there's a way to
>> get around that third ground wire.

>> Thanks.

>> Marc

>> Diagram in my communications terminal program:

>>       Simplest Possible 3-wire Null Modem Cable

>>       MALE                                    FEMALE
>>       =============                           ========
>>       25D RS232 pin     signal       signal   25D   9D (as in AT)
>>       =============     ======       ======   ===   ==
>>                   7     GND <------> GND        7    5
>>                   3     RD  <------- TD         2    3
>>                   2     TD  -------> RD         3    2

 
 
 

Telephone wire TWO WIRE null modem cable?

Post by bill davids » Tue, 30 Apr 1996 04:00:00



| I want to make a serial connection between two computers in
| the house using regular telephone lines.  I already have a TWO
| WIRE telephone line between the two computers, but the
| following diagrams asks for a ground (GND) connection, does
| anyone know if this is really required, or if there's a way to
| get around that third ground wire.  

The interesting thing is going to be getting around the other lines
required for hardware flow control. While people do get by with
software flow, don't be astonished if you have problems, depending
on what you are going to try to do with this setup.

The RS-232 spec calls for +/- 12v from ground, so find a ground.
Before connecting between your computer and the ground in a box, do
check the voltage! People who build computers do ill-advised things
with power supplies many times.
--

  What do you mean I shouldn't do thing like that at my age?
  At my age if you don't do things like that you might die of natural causes!

 
 
 

Telephone wire TWO WIRE null modem cable?

Post by Marc Ant » Wed, 01 May 1996 04:00:00


Okay, a telephone line, used to connect two computers via
modem together, uses only two wires...what am I missing here?  
I guess the difference is the modem, and or the telephone
carrier signals....I don't plan to use a modem so I suppose
I'll have to put in the extra effort to get the extra wires in.

Thanks.

-Marc

 
 
 

Telephone wire TWO WIRE null modem cable?

Post by Wayde Ni » Wed, 01 May 1996 04:00:00



Quote:> Okay, a telephone line, used to connect two computers via
> modem together, uses only two wires...what am I missing here?  

As I understand it...

A modem does without the extra ground since it generates audio tones to be
transmitted over the line in refence to the carrier tone. When you're
talking serial (RS232 anyway...) directly over the wires, you're supposed
to have a common ground line between the two sides to act as the signal
reference (voltages are not absolute values, they represent a 'potential
difference' between two charges). You can often get away without this
common ground, just hope that each end generates signals in the same
range....

Hope this helps,

> ------------------------------------------------------------------------- <
> Wayde Nie                              Computing and Information Services <
> John Hodgins Eng. Bldg., Rm 122        McMaster University, CANADA        <

> ------------------------------------------------------------------------- <

 
 
 

Telephone wire TWO WIRE null modem cable?

Post by Derek Snid » Wed, 01 May 1996 04:00:00


: I want to make a serial connection between two computers in
: the house using regular telephone lines.  I already have a TWO
: WIRE telephone line between the two computers, but the
: following diagrams asks for a ground (GND) connection, does
: anyone know if this is really required, or if there's a way to
: get around that third ground wire.  

: Diagram in my communications terminal program:
:  
:       Simplest Possible 3-wire Null Modem Cable
:  
:       MALE                                    FEMALE
:       =============                           ========
:       25D RS232 pin     signal       signal   25D   9D (as in AT)
:       =============     ======       ======   ===   ==
:                   7     GND <------> GND        7    5
:                   3     RD  <------- TD         2    3
:                   2     TD  -------> RD         3    2

Most every phone wire cable has four wires inside.
It wouldn't be a bad idea to make use of them.

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Telephone wire TWO WIRE null modem cable?

Post by David Colquho » Fri, 03 May 1996 04:00:00




>> >I want to make a serial connection between two computers in
>> >the house using regular telephone lines.  I already have a TWO
>> >WIRE telephone line between the two computers, but the
>> >following diagrams asks for a ground (GND) connection, does
>> >anyone know if this is really required, or if there's a way to
>> >get around that third ground wire.
>> No, it's required for RS-232.  TXD and RXD voltages are measured
>> relative to signal ground (pin 7).
>> OTOH, many modems will talk to each other if you just hook them
>> together with a twisted pair.  And several companies make so-called
>> "short haul" modems for directly driving a twisted pair (see the
>> Black Box or Inmac catalogs for instance).
>But be carefull: Some modems require a 60 V voltage level on the
>2-wire-cable. There are only some modems which accept a "dumb"
>twisted-pair between them. But it works: Because it's much cheeper
>to hire a "dumb" twisted-pair cable instead of hiring a 9.6 kBd-line
>we use this method over a distance of ablut 7 kilometers without
>any problems.
>Regards,
>                     Mattias

What the obove posters say is valid , just anote about tieing RS232
signal ground to the metal case of your computer.
DON'T. especially if you live in the U.S.
It can be dangerous , dammage your PC and your body.
It is called a 'ground' but is realy just a reference wire for the Tx and Rx
lines.
You will need 3 wires to use RS232 without modems.

BTW. make sure that none of the wires you may use is connected to any
telephone exchanges or telephones.

If at all possable , run some co-axial cable between the two machines and
use network cards.
The benefits are great , you can telnet (terminal) file-share and use a
common modem for internet access...

Good luck.
and Happy hacking.

--
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              *** ALL replies to NewsGroup Please ***
------------------------------------------------------------------------

 
 
 

Telephone wire TWO WIRE null modem cable?

Post by George Bonse » Fri, 03 May 1996 04:00:00


One OTHER thing to watch out for is manufacturers that tie signal ground
to chassis ground of the computer. If the two computers are on different
power circuits and there is a difference in ground potential between the
two, it could really light up your life AND mess up your computer.

In cases where the two systems are on different power circuits, get a
RS-232 to RS-422 adapter (they are very small, fit inside the connector)
and run RS-422 between the conputer. It is a four-wire differential
interface that does not use a ground reference.


> What the obove posters say is valid , just anote about tieing RS232
> signal ground to the metal case of your computer.
> DON'T. especially if you live in the U.S.
> It can be dangerous , dammage your PC and your body.
> It is called a 'ground' but is realy just a reference wire for the Tx and Rx
> lines.
> You will need 3 wires to use RS232 without modems.

> BTW. make sure that none of the wires you may use is connected to any
> telephone exchanges or telephones.

> If at all possable , run some co-axial cable between the two machines and
> use network cards.
> The benefits are great , you can telnet (terminal) file-share and use a
> common modem for internet access...

> Good luck.
> and Happy hacking.

> --
> ------------------------------------------------------------------------
>                      Sorry, Mail is broken.
>               *** ALL replies to NewsGroup Please ***
> ------------------------------------------------------------------------

 
 
 

1. Null modem wiring configuration?

: I am setting up a second terminal into my Linux box and would like to
: know if anyone knows the exact configuration to use when you have DB9
: ports at both ends.  I need to know specifically which pins to hook to
: which.  It is not a 'straight through' hook-up.  I would have just
: bought a cable but they do not sell 50' null-modem cable so I went and
: bought the components and the spool-type cable.  I read the
: Serial-Howto and it is precise about the DB25 but tells you only the
: minimum about the DB9s.  Also, what do I do with the shielding itself
: (i.e. which pin is 'frame ground')? Any help appreciated. BTW, the
: 'terminal' is an old 286 and the 'server' is a 486-66.

The pins on a DB9 connector are:
1 DCD
2 RD
3 TD
4 DTR
5 SG
6 DSR
7 RTS
8 CTS
9 RI

Dumb terminals only need like rd,td,sg

Matt
--
----------------------------------------------------------------------
 Matt Falenski  Westmoreland County Community College  Data Center WS

      Do you need a silencer if you are going to shoot a mime?
----------------------------------------------------------------------

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