Why do we have cross-over cables.

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Or maybe the question should be why do we have cables that are not
crossover.

My first encounter with this concept came when connecting serial ports
together. Turned out there were two kinds - data set, and data terminal.

Not content with that, when UTP cables came out, we had a similar situation.

Maybe I've missed something, but it's always seemed to me that a logical
approach would be to define some pins as input and some as output, and
for cables to connect input pins to output pins, thus obviating the need
for two different ways of wiring up connecting cables.

Did I miss something? Is there a reason this situation persists?

Sylvia.

Re: Why do we have cross-over cables.
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Consider the case when you want to connect two of the same type
together. For example, a normal PC is considered a data terminal.  If
you want to connect two together, the transmit pins on one must connect
to the receive pins on the other.  Hence the crossover cable, often
called a "null modem" for the serial cable case.

The same reasoning applies to CAT5 cables, but in this case there are a
total of four pairs, two transmit and two receive.  Both sets are not
always used, but the ones in use must be "crossed over" to connect two
of the same type equipment.  Many routers can sense the type of cable
required and will automatically do the crossover internally if required.

Hope all this is not too confusing. :-)

--
Virg Wall, PE

Re: Why do we have cross-over cables.
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If the transmit and receive pins on a data set (typically a modem) were
transposed in the original design, then the same cables could have been
used in all cases.

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Ditto.

Sylvia.

Re: Why do we have cross-over cables.
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They were.

Re: Why do we have cross-over cables.
On Tue, 10 Mar 2009 22:58:44 -0700 (PDT), Richard Henry

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 Tee hee hee.

Re: Why do we have cross-over cables.
On Wed, 11 Mar 2009 16:53:43 +1100, Sylvia Else

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 Serial cables are pin for pin. This makes creating one an easy process
with low error rate during assembly, which was ALL by hand at that time.

  Switching the gear was far more reliable than switching cable
conductors around, and counting on low paid assemblers to do so
consistently.

  And that's a fact, Jack.

Re: Why do we have cross-over cables.
On Wed, 11 Mar 2009 16:53:43 +1100, Sylvia Else

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 Same reason.  MAKING an RJ-11 cable is easier if the crimped on
connector is wired the same way ALL the time.  Especially during field
service scenarios.  The error rate in cables is much higher if specific
cross-overs have to be made from end to end. Also, one would need to
observe the one completed end to reference what would be needed on the
other end.  Doing them all the same practically guarantees success.
Requiring cross-over practically guarantees a much higher prime pass
yield in manufacturing circles, and a higher failure rate in field
installations as well.

 And that's a fact, Jack.

Re: Why do we have cross-over cables.
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OK, I can accept the field installation error issue, though the
existence of two different standard ways of wiring the plugs seems at
least partly to defeat the goal of reducing errors.

I'd have thought that in a mass-production environment it would be as
simple as having one person do one end and another person do the other end.

Sylvia.

Re: Why do we have cross-over cables.
On Wed, 11 Mar 2009 19:04:07 +1100, Sylvia Else

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 You still do not understand.  When these standards were made, and when
such production began "mass production" was individual hand assembly, and
regardless of who does which end (passing it off to another wastes yet
more time), other than pin for pin wiring can and does increase prime
pass yield failure rates.

  If the wires change, the error rate goes up.  If ALL assemblers wire
ALL cables the same on ALL ends, the likelihood for error decreases
exponentially, because they all learn, and inspect the same pin-out every
time. Errors get made less often, and inspection error get made less
often.  With the advent of cable test fixtures, the only place left for
the errors to occur is at the individual cable builder's bench.  That
builder's learned mindset, if hard wired to one spec, becomes less prone
to error.

Re: Why do we have cross-over cables.
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Wire coloured x goes to pin y. You just need two groups of people,
who've learned different schemes.

Sylvia.

Re: Why do we have cross-over cables.
On Thu, 12 Mar 2009 10:47:12 +1100, Sylvia Else

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  You have no concept of hand assembly processes.

 One person wire both ends.  That ONE person only needs to examine the
wiring of that one cable, one cable at a time.

  Gathering up a bunch of cables to pass them off to yet another
assembler has an attached cost that reduces profit, and cable fab shops
pinch every penny. That's why they hire unskilled workers.

 Then there is the error rate thing, which invariably also goes up.

 When a single lost cable occurs, it costs the company more than twice
the value of the lost labor.  Usually about 5 times when the error is
caught in-house.  It can be 16 times the cost if caught in the field.

  Rework is a very costly aspect of lean electronics manufacture.  If
error rates are high, it quickly becomes the most costly expenditure.

 Yes, cables are simple.  But that just means that the assemblers *think*
they can be more casual.  Otherwise there would be ZERO failures for a
simple 9 pin interconnect.  That is only 18 connections, after all, and
that is if all pins were used.  Yet still, we have much error.

 The cables that have historically shown to be the least error prone
assemblies are, in fact, pin-for-pin matings.

Re: Why do we have cross-over cables.
On Wed, 11 Mar 2009 19:04:07 +1100, Sylvia Else

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Part of the issue was to make cable testing easier for low end techs,
they are pin for pin.
  

Re: Why do we have cross-over cables.

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this would presumably require the DTE to have outputs for carrier and
ring although thise concepts apply only to DCE.


the standard was developed before the invention of the smartmodem(tm)
which could indicate those two conditions as serial data.


Early modems ware loosely speaking analogue filters coupled to serial
line drivers, if you were lucky you could pulse dial by toggling the
DTR line with the correct cadence...



Re: Why do we have cross-over cables.
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Just means those pins would be tied to ground. This would have required
more than the 9 pins on modern serial ports, but would have been easily
done with the original 25 pin standard.

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Did something similar with a telephone in my teens, by pulsing the
handset rest.

Shame no TV suspense movie ever used it - devious villain leaves a phone
with its dial detached so that he can call his imprisoned victims - but
one captive knows better and calls the police despite the absence of a dial.

Sylvia.

Re: Why do we have cross-over cables.
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IIRC it was used in "War Games" (1983)



Re: Why do we have cross-over cables.

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  Want to play a game?

Re: Why do we have cross-over cables.
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Hannibal Lecter used that trick to make phone calls from prison in the
movie _Manhunter_. I used to tap the handset cradle to make free calls
as a teenager.

--
    W
  . | ,. w ,   "Some people are alive only because
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Re: Why do we have cross-over cables.
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I can't see why it would make the calls free.

Sylvia.

Re: Why do we have cross-over cables.

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Old type phone boxes in the UK had the dial linked to the coin mechanism.
You had to pay to use the dial. Handset rest pulsing cut that out.



Re: Why do we have cross-over cables.
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Must have been very old. I remember the push button A/push button B
type, and then the later ones where you put the coins in when the other
party answered (I hated them!). I don't remember any where there was a
link between the dial and the coins.

Sylvia.

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