Connect with us

Why do we have cross-over cables.

Discussion in 'Hobby Electronics' started by Sylvia Else, Mar 11, 2009.

Scroll to continue with content
  1. Sylvia Else

    Sylvia Else Guest

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

    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?

  2. Sylvia Else

    Sylvia Else Guest

    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.

  3. They were.

  4. 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

    And that's a fact, Jack.

  5. 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.

  6. Tee hee hee.

  7. The problem is that when these systems of interface were designed, there
    was nearly no automated assembly.

    Hand assembly means failures, unless error conditions are reduced to a
    minimum. Wiring both ends identically means that less errors were made
    in manufacture of said interface devices and systems. Making the switch
    at the hardware itself was easy, and 100% repeatable.

    Prime pass yield was a huge consideration in labor intensive hand
    operation production procedures, and still is. That is why most
    interface cables are pin-for-pin. Particulalry those that have the same
    or very similar connectors on each end.
  8. Guest

    The reasn this situation persists is that it is written into the
    international standards, and a huge installed base of hardware out in
    the field conforms to those standards.

    I used to know about this when I worked for ITT-Creed in the U.K back
    in 1979-1982, in a group that used to send people to the CCIT
    standards committee meetings.

    The concept dates back to the Telex and Teleprinter networks. The ASR
    33 Teletype printer was orginally a data set (IIRR - 1982 is the last
    time I was seriously involved) produced in huge numbers of the AT&T
    network, and its use as a computer terminal was never more than a
    minor spin-off.

  9. Also a very good reason to refrain from the idea. It can easily be
    incorporated into a gender change DONGLE as well.

    Yes, folks, that term was in use LONG before software security keys used

  10. Western Electric ALSO hand assembled practically everything back then,
    and they knew about error rates in cable assemblies, and they knew how
    best to reduce them by making remembering the wiring procedure an easy
    thing to do.
  11. Sylvia Else

    Sylvia Else Guest

    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.

  12. atec 77

    atec 77 Guest

    has to be ?
    no one could be that fucking stupid
  13. JW

    JW Guest

    Many (most? all?) Intel GB NICs will also crossover automatically.
  14. K Ludger

    K Ludger Guest

    Rod Speed of old?
  15. Jasen Betts

    Jasen Betts Guest

    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...
  16. Sylvia Else

    Sylvia Else Guest

    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.
    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.

  17. krw

    krw Guest

    How is the "Rod Speed of old" [*] any different than the Rod Speed
    of new? He still infests many groups.

    [*] Ron Reaugh of old
  18. qrk

    qrk Guest

    For modern ethernet, you don't need cross-over cables. The devices
    figure out the pair sorting. All the NICs, routers, and switches I've
    come across in the past 3 to 5 years have auto-sorting of the pairs.
  19. Franc Zabkar

    Franc Zabkar Guest

    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


    - Franc Zabkar
  20. krw

    krw Guest

    Don't count on it. We found a bunch of 'em that don't work as
    advertised. :-(
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day