Serial null cable modem

Serial null cable modem

Post by noon » Sun, 30 May 2004 16:10:53



I have a Nortel Annex terminal server, of which I want to connect an
E280R and a StorEdge 3310.

The Annex's 8 serial ports are all RJ45.
The E280R has a 25-pin serial port.
The 3310s has a 9-pin serial port.

I have been asking around in the electronics shop here in Sydney (
Tandys,* Smith, Reids behind the QVB ),
but when I say serial null cable, they dont seem to completely
understand what I mean.
( Except for one guy who understood what I meant, but said they are out
of stock ).
They ask, "oh, a crossover" ... but IIRC, a crossover <> null serial cable.
Then there is another cable called a rollover.

So what's the diff between a null cable, a crossover, and a rollover ?

 
 
 

Serial null cable modem

Post by Gary L. Burnor » Sun, 30 May 2004 17:18:19



>I have a Nortel Annex terminal server, of which I want to connect an
>E280R and a StorEdge 3310.

>The Annex's 8 serial ports are all RJ45.
>The E280R has a 25-pin serial port.
>The 3310s has a 9-pin serial port.

>I have been asking around in the electronics shop here in Sydney (
>Tandys,* Smith, Reids behind the QVB ),
>but when I say serial null cable, they dont seem to completely
>understand what I mean.
>( Except for one guy who understood what I meant, but said they are out
>of stock ).
>They ask, "oh, a crossover" ... but IIRC, a crossover <> null serial cable.
>Then there is another cable called a rollover.

>So what's the diff between a null cable, a crossover, and a rollover ?

You were trying to say "null modem" cable which is the same thing as a
crossover. Rollover sounds quite bogus but may be the same thing.

Were both of your computers 25 pin for example, the minimum cable
would be

2-3
3-2
7-7
20-20

With a db9 to 25, it'd be 2,3,(can't remember, darn it) 9 and 3,2,7,20

They're not that hard to make.  Buy the ends and crimp your own if you
can't find one.

 
 
 

Serial null cable modem

Post by Dave Rilet » Mon, 31 May 2004 03:55:55


I bought my null modem cable at blackbox.com
drilett


Quote:> I have a Nortel Annex terminal server, of which I want to connect an
> E280R and a StorEdge 3310.

> The Annex's 8 serial ports are all RJ45.
> The E280R has a 25-pin serial port.
> The 3310s has a 9-pin serial port.

> I have been asking around in the electronics shop here in Sydney (
> Tandys,* Smith, Reids behind the QVB ),
> but when I say serial null cable, they dont seem to completely
> understand what I mean.
> ( Except for one guy who understood what I meant, but said they are out
> of stock ).
> They ask, "oh, a crossover" ... but IIRC, a crossover <> null serial
cable.
> Then there is another cable called a rollover.

> So what's the diff between a null cable, a crossover, and a rollover ?

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Serial null cable modem

Post by Dave Uhrin » Mon, 31 May 2004 04:27:00



> I have been asking around in the electronics shop here in Sydney (
> Tandys,* Smith, Reids behind the QVB ),

If Tandys is the same as USA's Tandys/Radio Shack ask for P/N 26-264,
which is a 9 pin PC null modem.
 
 
 

Serial null cable modem

Post by ML Starke » Mon, 31 May 2004 11:18:46



>I have a Nortel Annex terminal server, of which I want to connect an
>E280R and a StorEdge 3310.

>The Annex's 8 serial ports are all RJ45.
>The E280R has a 25-pin serial port.
>The 3310s has a 9-pin serial port.

If you check the system handbook for the E280R,  you will find there
is a reference in the Server's owners guide for the pins and signals
on the serial ports along with a diagram.  If you aren't fluent in
serial signals and have no interest in learning, then a competent
supplier should be able to help you construct the cabling you need.
Usually, all it takes is a diagram of both sides.

If you want to learn more, the Sun System handbook has the basic info.
Celeste Stokely's site (www.stokely.com) is the definitive Solaris
serial port site. It contains just about all the Sparc serial port
diagrams but examples of serial null modem and serial null terminal
cables (the difference there being the presence or absence of the DSR
DTR DCD signals).

Quote:>I have been asking around in the electronics shop here in Sydney (
>Tandys,* Smith, Reids behind the QVB ),
>but when I say serial null cable, they dont seem to completely
>understand what I mean.
>( Except for one guy who understood what I meant, but said they are out
>of stock ).

Well it doesn't help that your local "electronics" outlets don't seem
to have properly skilled employees, but this is usually the rule not
the exception.  That's probably why there are so many internet
suppliers of cables, including Black Box, which, while pricey, can
either provide or build just about any cables you would need.

Vendor sometimes provide these cables.  Sun only provides the adaptors
for RJ45 to DB25 or DB9 to be used with a shielded cat 5 cable.

Quote:>They ask, "oh, a crossover" ... but IIRC, a crossover <> null serial cable.
>Then there is another cable called a rollover.

>So what's the diff between a null cable, a crossover, and a rollover ?

Here's a diagram of a null modem cable 25pin to 25pin from the Stokely
site:

DB25              DB25
-----             -----
1 GND  ---------- 1 GND
2 TD   ---------- 3 RD
3 RD   ---------- 2 TD
4 RTS  ---------- 5 CTS
5 CTS  ---------- 4 RTS
7 GND  ---------- 7 GND
6 DSR  ---+
          +------ 20 DTR
8 DCD  ---+
             +--- 6 DSR
20 DTR ------+
             +--- 8 DCD

I've heard the term "crossover" used interchangeably with "null modem"
but I don't think it's correct.

Looks like we aren't the only ones confused by these terms, as this
discussion on the Ars Technica forum shows:

http://www.veryComputer.com/

I've seen diagrams of a "rollover" cable and it basically looks like
this:

1 > 8
2 > 7
3 > 6
4 > 5
5 > 4
6 > 3
7 > 2
8 > 1

Here's an example from the Cisco website:

http://www.veryComputer.com/

Good luck and happy cabling!

 
 
 

Serial null cable modem

Post by Darren Dunha » Thu, 03 Jun 2004 01:38:26



> I have a Nortel Annex terminal server, of which I want to connect an
> E280R and a StorEdge 3310.
> The Annex's 8 serial ports are all RJ45.
> The E280R has a 25-pin serial port.
> The 3310s has a 9-pin serial port.

Do you have the adapters for going from an Annex to a DB25 or DB9?
There's no "standard" for RJ45 connections, and the Annex uses different
pinouts than Cisco/Sun do.  

You'll likely have to find the documentation for the Annex which has the
pinout and either crimp your own cables, or get the little RJ-45 to
DB-25 housings and pin your own converter.

I used to make dozens and dozens of them several years ago, but I can no
longer find my old cheat sheet for Annex use.

Quote:> I have been asking around in the electronics shop here in Sydney (
> Tandys,* Smith, Reids behind the QVB ),
> but when I say serial null cable, they dont seem to completely
> understand what I mean.
> ( Except for one guy who understood what I meant, but said they are out
> of stock ).

I doubt anyone at a general electronics shop is going to be able to help
you with the Annex.

Quote:> They ask, "oh, a crossover" ... but IIRC, a crossover <> null serial cable.
> Then there is another cable called a rollover.
> So what's the diff between a null cable, a crossover, and a rollover ?

The terms are fuzzy, so there's no one definition.  

"null modem" was first used years ago as a cable that replaced a
modem-line-modem connection between equipment.  It kept all the data and
control lines present.  It's now used as a general term for a cable that
swaps the send and receive lines sufficiently for two computers (or
other DTE) to be connected together with a cable for communication.

"rollover" and "crossover" I hear used by different people for each of
two different items.

On a UTP cable (usually 8 pin or so), you can wire one end backwards.
On the Cisco/Sun RJ-45 pinout, that would give you most of the
null-modem characteristics.  (It will not on an Annex compatible cable).

The other common crossover/rollover cable is for ethernet networks.  The
"crossover" cable is created having the pair of cables wired to pins 1 &
2 on one end go to pins 3 & 6 on the other end, and vice versa.  I don't
know of any serial adapters that make use of such cables, but I suppose
it would be possible.

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