Why do we have cross-over cables.

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

  1. Sylvia Else

    Sylvia Else Guest

    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.
     
    Sylvia Else, Mar 11, 2009
    #1
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  2. Sylvia Else

    Sylvia Else Guest

    VWWall wrote:
    > Sylvia Else wrote:
    >> 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?
    >>

    > 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.


    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.

    >
    > 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.


    Ditto.

    Sylvia.
     
    Sylvia Else, Mar 11, 2009
    #2
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  3. On Mar 10, 10:53 pm, Sylvia Else <> wrote:
    > VWWall wrote:
    > > Sylvia Else wrote:
    > >> 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?

    >
    > > 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.

    >
    > 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.
    >

    They were.
     
    Richard Henry, Mar 11, 2009
    #3
  4. On Wed, 11 Mar 2009 16:53:43 +1100, Sylvia Else
    <> wrote:

    >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.



    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.
     
    StickThatInYourPipeAndSmokeIt, Mar 11, 2009
    #4
  5. On Wed, 11 Mar 2009 16:53:43 +1100, Sylvia Else
    <> wrote:

    >> 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.

    >
    >Ditto.



    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.
     
    StickThatInYourPipeAndSmokeIt, Mar 11, 2009
    #5
  6. On Tue, 10 Mar 2009 22:58:44 -0700 (PDT), Richard Henry
    <> wrote:

    >>

    >They were.



    Tee hee hee.
     
    StickThatInYourPipeAndSmokeIt, Mar 11, 2009
    #6
  7. On Tue, 10 Mar 2009 23:45:51 -0700, (Dave Platt)
    wrote:

    >I agree, it does make a great deal of sense to set up the interfaces
    >in a symmetrical fashion whenever possible. I've grown fond of the
    >Yost method of doing serial-port hookup... there's no differentiation
    >between DTE (e.g. terminal or PC) and DCE (e.g. modem), and you can
    >hook either to the other.
    >
    >10BaseT could also have been designed with this sort of symmetry in
    >mind, I suppose, if we'd started out using the sort of
    >highly-intelligent auto-adaptive interfaces that we use today on e.g.
    >100BaseT.



    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.
     
    StickThatInYourPipeAndSmokeIt, Mar 11, 2009
    #7
  8. Sylvia Else

    Guest

    On Mar 11, 6:24 am, Sylvia Else <> wrote:
    > 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?


    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.

    http://www.iso.org/iso/livelinkgetfile?llNodeId=21523&llVolId=-2000

    --
    Bill Sloman, Nijmegen
     
    , Mar 11, 2009
    #8
  9. On Tue, 10 Mar 2009 23:45:51 -0700, (Dave Platt)
    wrote:

    >Also (minor issue) it's a trifle easier to extend a straight-through
    >cable with another... you just use a second straight-through cable and
    >a one-to-one butt-heads splicer. You can't do this with two crossover
    >cables, as you'll end up crossing everything over twice and creating
    >the equivalent of a straight-through cable... your splicer needs to
    >include a *third* crossover!



    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
    it.
     
    StickThatInYourPipeAndSmokeIt, Mar 11, 2009
    #9
  10. On Wed, 11 Mar 2009 00:31:27 -0700 (PDT), wrote:

    >On Mar 11, 6:24 am, Sylvia Else <> wrote:
    >> 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?

    >
    >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.
    >
    >http://www.iso.org/iso/livelinkgetfile?llNodeId=21523&llVolId=-2000



    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.
     
    StickThatInYourPipeAndSmokeIt, Mar 11, 2009
    #10
  11. Sylvia Else

    Sylvia Else Guest

    StickThatInYourPipeAndSmokeIt wrote:
    > On Wed, 11 Mar 2009 16:53:43 +1100, Sylvia Else
    > <> wrote:
    >
    >>> 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.

    >> Ditto.

    >
    >
    > 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.


    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.
     
    Sylvia Else, Mar 11, 2009
    #11
  12. Sylvia Else

    atec 77 Guest

    Sylvia Else wrote:
    > 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.

    TROLLTROLLTROLL
    has to be ?
    no one could be that fucking stupid
     
    atec 77, Mar 11, 2009
    #12
  13. Sylvia Else

    JW Guest

    On Tue, 10 Mar 2009 22:40:42 -0700 VWWall <> wrote in
    Message id: <>:

    >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.


    Many (most? all?) Intel GB NICs will also crossover automatically.
     
    JW, Mar 11, 2009
    #13
  14. Sylvia Else

    K Ludger Guest

    "atec 77" <"atec 7 7 "@hotmail.com> wrote in message
    news:gp7tt9$itv$...
    > Sylvia Else wrote:
    >> 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.

    > TROLLTROLLTROLL
    > has to be ?
    > no one could be that fucking stupid



    Rod Speed of old?
     
    K Ludger, Mar 11, 2009
    #14
  15. Sylvia Else

    Jasen Betts Guest

    On 2009-03-11, Sylvia Else <> wrote:

    >> 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.

    >
    > 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.


    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...
     
    Jasen Betts, Mar 11, 2009
    #15
  16. Sylvia Else

    Sylvia Else Guest

    Jasen Betts wrote:
    > On 2009-03-11, Sylvia Else <> wrote:
    >
    >>> 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.

    >> 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.

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


    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.

    >
    > 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...
    >


    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.
     
    Sylvia Else, Mar 11, 2009
    #16
  17. Sylvia Else

    krw Guest

    In article <49b78df5$0$4195$5a62ac22@per-qv1-newsreader-
    01.iinet.net.au>, says...>
    > "atec 77" <"atec 7 7 "@hotmail.com> wrote in message
    > news:gp7tt9$itv$...
    > > Sylvia Else wrote:
    > >> 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.

    > > TROLLTROLLTROLL
    > > has to be ?
    > > no one could be that fucking stupid

    >
    >
    > Rod Speed of old?


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

    [*] Ron Reaugh of old
     
    krw, Mar 11, 2009
    #17
  18. Sylvia Else

    qrk Guest

    On Wed, 11 Mar 2009 16:24:45 +1100, Sylvia Else
    <> wrote:

    >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.


    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.
     
    qrk, Mar 11, 2009
    #18
  19. Sylvia Else

    Franc Zabkar Guest

    On Wed, 11 Mar 2009 16:24:45 +1100, Sylvia Else
    <> put finger to keyboard and composed:

    >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.


    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.
     
    Franc Zabkar, Mar 11, 2009
    #19
  20. Sylvia Else

    krw Guest

    In article <>,
    says...>
    > On Wed, 11 Mar 2009 16:24:45 +1100, Sylvia Else
    > <> wrote:
    >
    > >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.

    >
    > 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.


    Don't count on it. We found a bunch of 'em that don't work as
    advertised. :-(
     
    krw, Mar 11, 2009
    #20
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