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I bought this RJ45 to USB cable: USB to RJ45 Console Cable RS232 Serial Adapter for Laptop Computer Cisco Router USB RJ 45 8P8C Converter Console Cable 1.8m 3m.

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I connected one side of the cable to the USB port of my laptop, and the other side to one of the four LAN ports of the router. I expected my laptop to be connected to the Internet, but it did not work - my computer was not connected. I tried to switch to a different LAN port, but it still did not work. I also did not see any lights near the LAN port.

Did I use this cable correctly? Is it really meant to connect my computer to the LAN?

Giacomo1968
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Erel Segal-Halevi
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3 Answers3

43

That RJ45 to USB cable is not an Ethernet adapter.

It might superficially look like one, but it’s not.

That cable you purchased is a known as console cable (aka: Cisco cable or rollover cable). It’s meant/designed to connect your PC to a server (or other enterprise level device) via RS232; aka serial connection or link. That is not an Ethernet adapter, and cannot be used as one at all.

Generally, what you are looking for is a USB to Ethernet adapter that looks something like this:

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You would plug the USB side into your PC and then connect the Ethernet end to your LAN via an Ethernet cable. These are fairly common to come by and should be pretty affordable. Just make sure to check if the USB A to RJ45 adapter you purchase explicitly states it’s an Ethernet adapter.

Giacomo1968
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12

The 8P8C connector (commonly known as RJ45, even though this is incorrect) is used for a lot of different things.

While nowadays its most common use is for (some kinds of) Ethernet, it can also be used:

  • For RS-232 or other similar serial ports (instead of the more common DB-25 or DE-9 connectors), especially for console access on Cisco routers and other devices;
  • For analogue telephony (aka POTS);
  • For digital telephony (aka ISDN);
  • For proprietary telephony systems;
  • Sometimes even USB;
  • And many other things.

The adapter you bought is for the first type: to connect to an RS-232 serial port presented as 8P8C (RJ45). It is completely unrelated to Ethernet.

What you need is a USB Ethernet adapter. They come in many different shapes and sizes, and have varying characteristics and performances (100 Mbit/s, Gigabit, 2.5GbE, USB 2, USB 3, USB-A, USB-C...). You need to make sure the description of the product includes "Ethernet", along with the specs you want.

Some docks (especially USB-C docks) also include Ethernet ports, so it may be an alternative if you need other interfaces (more USB, HDMI, etc.).

If your computer has a USB-C port, you're probably better off using an adapter using that kind of port. Given the low prices of such adapters, picking a 2.5GbE adapter is probably a good idea even if you won't use it at its maximum potential if your switch/router doesn't support such speeds.

While there are a few adapters that take the form of a cable (with a male "RJ45" at the other end like the one you bought), most will have a female connector, and you will then need an Ethernet cable to connect that to the corresponding connector on your switch or router. The advantage is that you can easily change the cable if you need shorter or longer distances (or if the cable or plug gets damaged).

jcaron
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3

Over the last few decades, it has become increasingly common for companies designing consumer standards to design "single-purpose" connectors to support their standard. USB, Firewire, HDMI, DVI, Displayport and PCIe all did this.

Combine this with the high level of homogeneity in the computer industry today and it's easy for someone who only deals with consumer-level equipment to get the idea that a connector is synonomous with the interface that it carries.

But that is a dangerous mindset to get into. Many connectors were intended to be general-purpose from the start. Even when a connector was originally designed with a specific purpose in mind it was often re-used for other purposes. Connecting incompatible equipment can result in stuff not working at best, and in some cases can cause damage.

The "office connectivity" world kind of went in the opposite direction to the "consumer equipment" world, using "RJ45" connectors for everything. Ethernet yes, but also token ring, analog voice, ISDN, serial and so-on. The idea was/is that the expensive part of the job was running cable and it was better to install a bunch of general purpose cable upfront than to install specific cables for specific tasks.

Professional networking equipment tends to use "RJ45" connectors for it's serial console ports, both because they are smaller than traditional 9 way D subs, and because of the aforementioned structured cabling. You occasionally see this on servers too, but it's less common. The cable you have purchased is NOT an ethernet adapter, it is a USB to serial adapter supplied with a connector suitable for use with professional networking equipment.

If you want to buy a USB to Ethernet adapter you should look for product that specifically mention Ethernet and does NOT mention "serial console". That said, there are visual clues that can help distinguish the two products when going through a junk draw.

  • USB to ethernet adapters most often come in the form of a "dongle" with a "RJ45" socket on the body of the dongle and a short cable with a USB plug. Serial console adapters are more often built into the USB plug, with a caprive cable.
  • "RJ45" sockets on Ethernet controllers usually have a metal shell and indicator LEDs.
  • Cables intended for serial console use are usually flat, ethernet cables are usually round.
  • Serial console cables are usually a light blue colour.
plugwash
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