Why do we have cross-over cables. - Page 5

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Re: Why do we have cross-over cables.
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Starting at paragraph 40.4.4.

Thanks.

Sylvia.

Re: Why do we have cross-over cables.
On Sun, 15 Mar 2009 12:11:11 +1100, Sylvia Else

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 If thousands of iterations are tried at both ends, a matchup will occur
within milliseconds. IF that were the way it happens.

  Still, it would be meaningless (read negligible)to the observer, and
only needs to be done one time when the cable is attached.

Re: Why do we have cross-over cables.

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situation.
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Add some conditions.  There is an orderly passing sequence.  Each time
you get the microphone you may speak a small permitted say or not.
When done speaking or if you have nothing to say you immediately pass
the microphone.

  .

Re: Why do we have cross-over cables.
On Thu, 12 Mar 2009 11:19:57 +1100, Sylvia Else

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Design, Sylvia, design...

JF  

Re: Why do we have cross-over cables.
On Thu, 12 Mar 2009 11:19:57 +1100, Sylvia Else

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Yes.  It is part of the IEEE 802.3 series.  See:

http://standards.ieee.org/getieee802/802.3.html

  .

Re: Why do we have cross-over cables.
On Wed, 11 Mar 2009 16:24:45 +1100, Sylvia Else

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An even more logical approach is used by USB OTG ("On The Go"). A
device can be either a host (power provider) or a peripheral (power
consumer) depending on the status of a fifth pin. After power-up, both
devices can negotiate to swap functions. The USB data interface is
bidirectional.

See http://en.wikipedia.org/wiki/USB_On-The-Go

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

Re: Why do we have cross-over cables.

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Cabling was even more ambiguous when you consider hardware handshaking.  In
the
earlier years, I have seen handshaking on various pins including 4, 5, 6, 8,
20, 21 and
one printer used pin 19.

The situation was not solely a serial problem either, one TI printer with a
parallel
interface used an inverted data available strobe.  I presume this was to
force the
purchase of their over priced cables.  It was easily possible to build a
transistor
inverter in the centronics head to resolve the issue.



Re: Why do we have cross-over cables.

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That's how the Macintosh serial ports and printers/modems etc. were
wired.  It works.

The old RS-232 days were different: it made sense to the original
builders that a data communication device would attach onto a
data terminal device, SO they were specified with mating connectors.
The DCE (data communication equipment :3D%3D modem) had a female
plug, the DTE (data terminal equipment :3D%3D terminal, computer) had
a male plug.  The connection was through simple extension cables,
also convenient for insulation-displacement (ribbon cable) and
for mass production.

I think,  historically, the decision to use male and female
connectors (which allows docking modules without a cable)
was the breakpoint.

In the case of UTP wiring, the two wiring schemes are 568A and
568B (and everyone uses 568B, the "crossover cable" is 568A on one
end).  Computer-to-hub/router/switch uses straight-through cable,
computer-to-computer uses crossover cable, hub/router/switch to
hub/router/switch uses crossover cable (or a "special" crossover
port).
This is becoming irrelevant, though, with new chipsets: all the
gigabit Ethernet I've seen has logic that corrects the signal for
wire arrangement (the gigabit protocol requires bidirectional use
of all the wire pairs anyhow, so send/receive is just a software
setting).


Re: Why do we have cross-over cables.

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Wow, did you ever get that one wrong.  See:

http://en.wikipedia.org/wiki/TIA-568B

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Re: Why do we have cross-over cables.
On Wed, 11 Mar 2009 16:24:45 +1100, Sylvia Else

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You missed something, and there's a good reason why this condition
persists.

In a minimalist DB25 connection between DTE and DCE we have this:

     DTE                                  DCE
    DB25M             CABLE              DB25F
 +---------+                          +---------+
 |     TXD 3>>3--------------------3>>3 RXD     |
 |     RXD 2>>2--------------------2>>2 TXD DCE |
 | SIG GND 7>>7--------------------7>>7 SIG GND |
 +---------+  \                   /   +---------+
               DB25F         DB25M

Notice that the cable conductors do not cross and are wired 1:1 from end
to end.

But now assume someone wants two computers to talk to each other through
their serial posts, and we wind up with this:

     DTE                                  DTE
    DB25M                                DB25M
 +---------+                          +---------+
 |     TXD 3>                        <3 TXD     |
 |     RXD 2>                        <2 RXD     |
 | SIG GND 7>                        <7 SIG GND |
 +---------+                          +---------+

How would you suggest wiring up the ports to accomplish that?

JF  

Re: Why do we have cross-over cables.

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The way the Yost wiring system works is one way around this.

It uses an RJ-45 jack.  Every device (DCE or DTE) is wired up to put
the "data I am sending to you" on pin 6 (yellow wire), and to look for
"data from you" on pin 3.  In the case of DCE, "data I am sending" is
RD and "data I am getting from you" is TD;  in the case of DTE, the
opposite is true.

The 8-pin cable used for Yost is wired up with a "flip over", so it
connects pin 3 at each end to pin 6 at the opposite end.

There's a similar pairing-and-cable-flipping between "terminal ready"
signals (DTR from the DTE and Carrier Detect from the DCE) and "flow
control" signals (RTS from DTE, CTS from DCE).

So, using the same cable, you can connect together a DCE and DTE, or
two DCEs, or two DTEs.  Every device has the same sort of connection
port, and any two ports can be connected successfully using an RF-45
8-wire flipover cable (which is *not* the same as an Ethernet
crossover cable).

The magic here depends on two things:  you're always using a flipover
cable, and the devices at either end (DCE or DTE) are responsible for
connecting the RJ-45 goezintas and goesoutas to their correct internal
signal points.  [In practice, the latter is usually done by wiring up
DTE- and DCE-specific "DB-9/25-to-RJ-45 adapters").

I've used the Yost methodology at home for several years and quite
like it.

--
Friends of Jade Warrior home page:  http://www.radagast.org/jade-warrior
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Re: Why do we have cross-over cables.
On Wed, 11 Mar 2009 16:58:08 -0500, John Fields

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 The port would have to have an expensive multi pin switch incorporated
into the panel next to it.  Very bad.

Re: Why do we have cross-over cables.
On Wed, 11 Mar 2009 16:53:56 -0700, FatBytestard

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---
Much simpler is a cable with pins 2 and 3 crossed, an that's what's
done.

JF  

Re: Why do we have cross-over cables.
On Thu, 12 Mar 2009 04:11:35 -0500, John Fields

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 Exactly.  Far cheaper as well.

 Computer talks to modems. When computer needs to talk to another
computer via the same ties directly, it is obvious that a conflict will
be present.  The 'null modem cable' was part of the spec, even before one
was ever used.

Re: Why do we have cross-over cables.
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Yes, clearly with the endpoints wired as they are we need different
cables depending on the kind of endpoints we're connecting. But that
wasn't my quesion.

My question related to why this situation was created in the first
place, since it is not a logical necessity.

Sylvia.

Re: Why do we have cross-over cables.
On Thu, 12 Mar 2009 10:54:59 +1100, Sylvia Else

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  He already showed how it WAS and still IS a necessity.

Re: Why do we have cross-over cables.
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Because the modem is "Data Communications Equipment", or DCE, and the
computer/teletype is "Data Terminal Equipment", or DTE. When  you use
a computer to simulate a modem (or a server on the other end of the
line) you need to turn the sense of the connector around. This is
why reversing cables used to be called "null modem" cables.

Hope This Helps!
Rich


Re: Why do we have cross-over cables.
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No it doesn't. It entirely misses the point, which is why there is a
distinction betwen DCE and DTE.

Sylvia.

Re: Why do we have cross-over cables.
On Thu, 12 Mar 2009 11:32:00 +1100, Sylvia Else

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 Actually, it is YOU that is missing the entire point.

Re: Why do we have cross-over cables.
On Thu, 12 Mar 2009 11:32:00 +1100, Sylvia Else

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Are you a lesbian?

JF  

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