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To preface this question: I'm not able to get to a parts store any time soon. I'm well aware that I could buy what I need, but I'm trying to make do with the tools and hardware I already have.

I found a cable that is 10pin RJ-45(48?)-to-USB. The jack has leads on 1(red), 4(brown), 7(black), 9(white), and 10(green). Not sure what's Tx, Rx, ground, data, uni/bi-di, etc. Nothing is striped. Upon cutting the jack from the end of the cable and peeling back the insulation, there was no lead for 4(brown) that I could see. Could the metal insulation have been grounded to 4(brown)? Is there a way to salvage this cable with an 8pin RJ-45 to be used as a console cable for my Linux box router?

EDIT: The cable appears to be part of a UPS from one of the several home security systems in the building.

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    Does it have a chip inside, or is it just a passive cable? (Also, does your router really have a RJ-45 console port? Most "Linux boxen" only have a DB-9 port.)
    – u1686_grawity
    Dec 4, 2017 at 7:29
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    Might help to link to the actual hardware you're talking about and the cable markings.
    – Journeyman Geek ♦
    Dec 4, 2017 at 7:42
  • I honestly don't know. I don't have the tools to take apart the barrel near the end to check. It's possible. This seems to be it: apc.com/shop/us/en/products/…
    – Shankensteinium
    Dec 4, 2017 at 7:43
  • You could grab a multimeter in continuity test mode and check which of the RJ48 pins are directly connected to the USB pins. (If you find that all of them are directly connected, nope, that's not going to work.)
    – u1686_grawity
    Dec 4, 2017 at 7:45
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    From the opposite direction: superuser.com/questions/1274489/…
    – u1686_grawity
    Dec 5, 2017 at 22:45

1 Answer 1

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Despite the similar "serial" name, serial ports and USB use completely incompatible signalling. To connect a serial console port to USB, you would need an active cable with a converter chip.

Make sure that's what you have: when the USB end is connected to a computer, it should be immediately recognized as a COM port by your OS (even if the other end is dangling free).

(The "USB-to-RJ45" cables sold for APC UPS will not work. They're passive adapters, and the UPS actually speaks the full USB protocol – not RS232 – over its RJ port.)

You could in theory assemble a console cable yourself. Most "USB-to-serial" cables sold nowadays use chips like Prolific PL2303, perhaps FTDI FT232RL or Microchip MCP2200 (search for "USB UART converter"). I don't know what additional parts may be required.

If your router really has a RJ-45 console port, the pinout is likely called "rollover cable".

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  • I don't have enough hardware for a functioning serial end. Must the router be the device that receives the RJ-45 in the console port? Is there no other way to access the console through LAN or USB?
    – Shankensteinium
    Dec 4, 2017 at 7:55
  • Well usually that's the whole point of "console" – to be used when LAN access is broken... What hardware are you using? If it's a regular server, it'll likely have a BMC in it (with IPMI/iLO/iDRAC) for console-over-IP access. If it's a repurposed PC, it might have Intel AMT for the same. If it's a dedicated router... pretty unlikely.
    – u1686_grawity
    Dec 4, 2017 at 8:01
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    Like most combo routers, it doesn't even have a RJ45 console port, so your cable would have been useless anyway. (It might have TTL serial pins inside, though. If you try to use those, I think you'd need a 2nd converter for RS232 voltages to TTL voltages; search various modding communities like OpenWRT.) But chances are the router will have Telnet or SSH access over the LAN – that's probably what you wanted for "console through LAN".
    – u1686_grawity
    Dec 4, 2017 at 8:22
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    See wiki.openwrt.org/toh/tp-link/tl-wdr7500 for the on-board TTL console pins.
    – u1686_grawity
    Dec 4, 2017 at 8:29
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    @Shankensteinium: Not sure if I understand the situation correctly, but if it's just about accessing a TP-Link 1750AC, installing OpenWRT on it as described in the link provided by grawity is really easy, and (normally) doesn't need exploiting backdoors. Then the TP-Link is completely configurable for your needs.
    – dirkt
    Dec 5, 2017 at 23:33

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