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I still see quite often that RJ45 DB9 console cable is attached with some devices, when I do not even have a computer with old serial port. So I need to either get the USB - DB9 adapter or USB - RJ45/Console cable separately. Is it just that it takes time to move away from the old serial or I am missing something?

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  • Why the down vote?
    – Maksim Luzik
    Sep 18, 2014 at 8:20
  • 1
    Because you've not read serverfault.com/help/dont-ask .
    – Jenny D
    Sep 18, 2014 at 10:53
  • This question isn't answerable by us but rather by the manufacturer of said equipment/cable.
    – joeqwerty
    Sep 18, 2014 at 14:07
  • Good computers have serial ports. Adequate computers support USB to Serial adapters.
    – Tom O'Connor
    Sep 18, 2014 at 22:27

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Take a counter-example. We have some equipment that has a mini-USB serial port (or rather, a RS-232 signalling protocol carried over a Mini-USB socket/plug; thus not really USB per-se.

This is a big pain in the proverbial, as trying to find a mini-USB adaptor was quite challenging (particularly on the local market). We ended up having to order the particular adaptor from the vendor.

If they had used micro-USB, I could have made an appropriate cable myself (although this would still require acquiring a micro-USB plug, soldering etc.)

At least with a RJ45, crimping equipment is widely available so I don't need to worry quite so much about the physical adaptation. Note that RS-232 etc. don't specify the physical adaption; DB-25 was common around the time of the dial-modem, then the smaller DB-9, and then RJ-45. It's worth noting that RJ-45 also has much greater cost efficiencies, and with a small adaptor cable (easily are yourself), you could turn a regular Ethernet cable into an appropriate rollover serial-cable (much to the relief of tech-support personnel at the time)

So, to answer your question: - cost of RJ-45 socket is very cheap - cost and available of cable components is low and easy - USB miniaturisation is an on-going process and makes it harder to integrate. - also, extra circuitry would be required to drive the line-logic and implement USB-serial profile on the device; rising cost. Such cost is pointless anyway as serial-interfaces are increasingly de-emphasised for day-to-day configuration. (Which makes it all the more important to find a suitable cable, or to be able to make one with materials at-hand when things are going a bit pear-shaped.

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  • Thanks for the answer Cameron. I am not sure if my questions was really understood. I am fine with the RJ-45 socket on the devices, but what I do not understand is why the cables have serial connector on the other side, when serial connector is dying. There seem to be already RJ45 to USB cables, but the RJ45 to Serial cables are still quite popular. Do most of the network administrators still use then RJ45 to serial cables with computers that have serial port available?
    – Maksim Luzik
    Sep 18, 2014 at 10:36
  • Although I'm sure you could find an example of a RJ-45 rollover cable (suitable for Cisco) to USB [with USB to serial microcontroller embedded into it], I think most people just carry a USB to DB-9 adaptor (although I think they get pretty small these days). Compatibility has been known to be an issue with cheaper devices (particularly with regard to RTS timing in some versions of Windows). In my experience (not that I do network admin for a job), this is what I have experienced / witnessed.
    – Cameron Kerr
    Sep 18, 2014 at 12:03
  • Who said "serial connector is dying"? I'm sorry you didn't bother to buy equipment with an RS-232 port, but it's alive, well, and useful.
    – MadHatter
    Sep 18, 2014 at 20:33

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