4

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.

Shankensteinium
  • 111
  • 8
  • 2
    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 04 '17 at 07:29
  • 1
    Might help to link to the actual hardware you're talking about and the cable markings. – Journeyman Geek Dec 04 '17 at 07:42
  • 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: http://www.apc.com/shop/us/en/products/P-AP9827?gclid=CjwKCAiA3o7RBRBfEiwAZMtSCa5cbVZfU3ApKS9UePV1zQ4NdZYNc0WRBd07P4ZMPVS3-dYH3Wr8choCM4cQAvD_BwE – Shankensteinium Dec 04 '17 at 07:43
  • 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 04 '17 at 07:45
  • 2
    From the opposite direction: https://superuser.com/questions/1274489/what-is-this-port-labelled-usb-on-my-apc-ups-which-looks-almost-like-a-rj45-ne – u1686_grawity Dec 05 '17 at 22:45

1 Answers1

5

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".

u1686_grawity
  • 426,297
  • 64
  • 894
  • 966
  • 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 04 '17 at 07:55
  • 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 04 '17 at 08:01
  • (And no, you cannot connect a RJ45 serial cable to the Ethernet port.) – u1686_grawity Dec 04 '17 at 08:03
  • TP-Link 1750AC. These are rather new enough that I haven't been able to find much delineation of the device. – Shankensteinium Dec 04 '17 at 08:19
  • 2
    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 04 '17 at 08:22
  • Pretty much. I'm just not adept at exploiting logical backdoors, so I wanted a physical one. – Shankensteinium Dec 04 '17 at 08:25
  • 2
    See https://wiki.openwrt.org/toh/tp-link/tl-wdr7500 for the on-board TTL console pins. – u1686_grawity Dec 04 '17 at 08:29
  • 1
    @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 05 '17 at 23:33