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connecting a pc to a router's console port?

Discussion in 'Network+' started by iain14, Jul 9, 2007.

  1. iain14

    iain14 Bit Poster

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    one of the questions in my course work was, to connect a pc with a router's console port, you need to use a rollover cable? does anyone know if this is true as a v.35 seems adaquate or have i just gone blond?
     
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  2. hbroomhall

    hbroomhall Petabyte Poster Gold Member

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    This is very much a Cisco'ism. On Cisco routers the console port is RJ45, but actualy contains RS-232/V.24 signals. A rollover cable is in fact a 'null-modem' cable, and you use a Cisco RJ45 to D25 (or D9) adaptor to connect to a PC.

    V.35 involves balanced signals, so needs a special card in the PC, or a balun to V.24.

    Harry.
     
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  3. iain14

    iain14 Bit Poster

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    thanks for clearing that up for me, thinking about the 9 pin null-modem cable answers that.
     
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  4. MacAllan

    MacAllan Byte Poster

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    Sorry to stick my nose in, but the previous answer was not entirely correct for cisco routers. To connect a PC to most modern cisco router consoles you need a rollover cable, with RJ-45 connectors, which is NOT the same thing as a null modem cable. Rollover cables have pins in exact reverse order:
    1 2 3 4 5 6 7 8 at one end,
    8 7 6 5 4 3 2 1 at the other.

    Of course, life is never so simple, as other varieties of connectors are also used with different models. See http://www.cisco.com/warp/public/701/14.html
    and get nicely confused.

    For the CCNA exam, the answer would be 'rollover cable.'
    N+ exam - you're not likely to get it.

    HTH.

    Edit: the RJ-45 connector at the PC end fits into a RJ-45/DB-9 converter to attach to the PC serial port. Some consumer router models come with a cable with RJ-45 and DB-9 ends on a single cable.
     
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  5. hbroomhall

    hbroomhall Petabyte Poster Gold Member

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    Sorry - but even Cisco says that it is the same *type* of cable.
    See here.
    Which is what makes it a null-modem cable.

    This is a *serial* cable, not ethernet, and the various RS-232 signals have been very carefully mapped to the pins so that the rollover acts as a null-modem.

    Harry.
     
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  6. MacAllan

    MacAllan Byte Poster

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    You're right - I was thinking of the sort of null-modem cable I used to use to directly connect two of my PCs together - same function, just looking different to my flat cisco one so I assumed (like an ass) that they were different. The wiki page on null-modem cables seems to suggest a different pinout, but I guess that is just my misreading of the page. http://en.wikipedia.org/wiki/Null_modem

    Interesting: the answer to the question, then, is that you need a null-modem cable - but I bet on a cisco exam, the answer regarding the console port would be, use a rollover cable...
     
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  7. Theprof

    Theprof Petabyte Poster Forum Leader

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    Well a rollover cable is a type of null modem cable if I am not mistaking.
     
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  8. hbroomhall

    hbroomhall Petabyte Poster Gold Member

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    The RJ45 plug only has 8 connections, compared to the PC's typical 9 or the 'standard' 25. I suspect that Cisco mapped the signals from the 25 pin standard onto the RJ45 deliberately so that a simple cable, like the rollover, would work as expected.

    I would agree. They seem to restrict the use of 'null modem' to situations where the console or AUX ports have D type sockets instead of RJ45.

    Harry.

    Edit: A fairly exhaustive discussion on these cables is here.
     
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  9. r.h.lee

    r.h.lee Gigabyte Poster

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    I agree

    I disagree. The console port on most Cisco routers is an "8 position modular jack." RJ-45 refers to the connector and wire mapping. RJ-48 is another cable type that uses the same connectors but different wire mapping.

    To say that "a rollover cable is in fact a 'null-modem' cable" is like saying that a PC with an Ethernet NIC connected to a hub with a UTP straight through cable, which is connected to a switch with a UTP straight through cable, which is connected to a router with a UTP straight through cable, which is connected to a T1 WAN connection using a DTE serial cable to the CSU/DSU, which is connected to a WAN switch, which is connected to the WAN switch on the other side, which is connected to the CSU/DSU on the other side, which is connected to the serial port of the remote router with a DTE serial cable, which is connected to a switch with a UTP straight through cable, which is connected to a hub with a UTP straight through cable, which is connected to the destination server "is in fact a 'null-modem' cable."

    The fact is that a "rollover cable" is a "rollover cable" and is NOT a "null-modem cable." It is the rollover cable plus the serial port transceiver that makes the overall link _act like_ a "null-modem cable." Just like the above rebuttal case, the entire networking from end to end makes the overall channel _act like_ a crossover cable.

    So in review, a cable is a section of media with connectors at the ends. A transceiver converts wire mapping from one physical connector to another physical connector. A cable and transceiver is NOT a cable.
     
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  10. r.h.lee

    r.h.lee Gigabyte Poster

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    That link describes how to interconnect two Aux ports and not the Console port.
     
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  11. r.h.lee

    r.h.lee Gigabyte Poster

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    MacAllan,

    You were correct before. A null-modem serial cable, which is the full term, has the transmit and receive wires crossed so that the first side's transmit pins are connected to the second side's receive pins and the first side's receive wire is connected to the second side's transmit pins. That makes modems unnecessary which is where the term "null-modem" comes from.

    Since a cable is defined by connector then media then another connector, you were correct that the flat cable used to connect your PC to the Cisco router's console port is indeed a rollover cable. The rollover cable is connected to the transceiver on the PC's serial port. Transceivers convert pinouts from one connector to another, in this case (pun intention your democratic free choice) the serial port pinouts are converted to an 8 position modular jack. A transceiver is also used to connect UTP patch cabling to the AUI port of a Cisco 2501 router by converting the pinouts from 10Base5 to 10Base T. Therefore, the cable itself remains as a rollover cable as it was born from it's mother's womb, err, manufacturer.
     
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  12. hbroomhall

    hbroomhall Petabyte Poster Gold Member

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    While you are correct that using the term 'RJ-45' is not correct here, most people would understand the term to mean the plug (rather loosely) rather than the exact meaning. I feel it is being a bit pedantic to deny the use of RJ-45. And I see Cisco is quite happy to use this term in their docs!


    No - nothing like. For one thing the cable is just that - a cable. Your alternative crosses so many different physical media that I feel the term 'null-modem' not to have any meaning at all!

    I find it helps people to describe the rollover cable as the equivalent of a null-modem cable. And evidently Cisco do as well.

    Not sure where transceivers come into this.

    Harry.
     
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  13. MacAllan

    MacAllan Byte Poster

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    I guess I feel a bit in the middle here, since I see (now) where both of you are coming from. Does this sort of help? :

    A cisco rollover cable, because it reverses (well, keeps in a straight line, really, but it comes to the same thing) the pin-out, can be described as a type of null-modem cable. It meets the criteria that Tx and Rx wires/pins would directly connect. This is shown by the possibility of using it to directly connect two cisco routers via their AUX ports (and presumably their console ones too?)

    But a PC doesn't have a jack-socket (call it what you like) to which the rollover cable can connect. Therefore it needs a transceiver to adapt this to the PC's serial port. But this is a function of making it work for a PC, it doesn't alter its nature of being a null-modem-type of cable. The reference
    http://www.cisco.com/warp/public/701/14.html
    shows the ways in which the adapter alters the pin-out to enable use with a PC serial port, and how a different adapter is used to enable its use with a modem.

    (An adapter that turns it into a modem cable - oh fekk! Now I'm really confused..... :D )

    i.e. A rollover cable is a type of null-modem cable, but for it to work for a PC, an adapter is needed. Without the adapter, it is not functional for connecting the two.

    At least that's how I now see it, - but hey, my record here is hardly startling, now is it? I am NOT an elecrtical nor electronic engineer. Just doing my best to understand.
     
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  14. r.h.lee

    r.h.lee Gigabyte Poster

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    I have a Cisco 2621 router with an ISDN BRI WIC. The ISDN BRI WIC has an "8 position modular jack." It is wired internally to RJ-48 standards and not RJ-45 standards. Therefore, I believe it is misleading to setup the association between "8 position modular jack" with RJ-45 without mentioning RJ-48 as well. I believe even a T1 WIC with an internal CSU/DSU may also have an "8 position modular jack" port. I'm just concerned about establishing the precedent that "8 position modular jack" port hole = RJ-45. So does that mean I can connect an Ethernet switch to an ISDN BRI WIC port? No. Does that mean I can connect an Ethernet switch to a T1 WIC port? No.

    Technically, a "null-modem serial cable" is more closer to a UTP crossover cable than a rollover cable. That's because just like the null-modem serial cable, the crossover cable has the transmit and receive pins crossed, thus the "cross" in "crossover." A rollover cable is a separate third category (pun intention your democratic free choice) after the "straight through" and "crossover" cables.

    For the case of the console cable, you need to convert the "rollover pinout" of the console cable connector to the "serial pinout" for the DB9/DB25 port on the PC. Just like you need to convert the "568B.2 pinout" to the "AUI pinout." The device that does the pintout conversions is a transceiver. That's how transceivers come into this.
     
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  15. hbroomhall

    hbroomhall Petabyte Poster Gold Member

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    I don't blame you! :biggrin
    No - it doesn't need any transceivers. I don't know why R.H.Lee introduced that term here.

    The adaptor is required to tuen a RJ-45 (like) plug into the D25 or D9 required for a PC. It is just wireing - nothing else.

    Harry.
     
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  16. hbroomhall

    hbroomhall Petabyte Poster Gold Member

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    No. Most people understand that there can be alternative signals on the 'RJ45'. If they are going to be 'correct' then they need to use alternative descriptions of the plug. However, many (including Cisco engineers at $employer) know what is meant here.
    Er - there is *no* transceiver involved with a rollover cable to a PC. There is an adaptor from RJ45 (calm down) to the D25 or D9, but there is no electronics involved, just wires, so no transceiver.

    Harry.
     
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  17. MacAllan

    MacAllan Byte Poster

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    Yes, I agree that transceiver was the wrong word: the adapter just redirects the wiring so that the null-modem property of the rollover cable is effective when attaching it to the serial port of a PC (and stops twits like me from thinking the first time they saw one that it attached to the NIC :D :D :D )
     
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