USB C to Ethernet adapter for PLC work: link light on, software still can't find the PLC

New laptop, no RJ45 port, nothing but USB C on both sides. So you grab a USB C to Ethernet adapter, run a cable to the PLC, and the link light comes on. Then TIA Portal or RSLinx shows nothing. I've been on that call more times than I can count, from both ends of the phone.

The dongle is almost never broken. Windows treats it as a brand new network card with no static IP on it, or your static IP got saved on a different "copy" of the same dongle, or the Wi-Fi sits in the same subnet, or the Siemens software never registered the adapter at all. Below is what to look at when you buy one, how I set it up so it still works next month, and what I check, in order, when it doesn't.

What actually matters on a USB C to Ethernet adapter for a PLC

Most of what the box brags about doesn't matter here. Gigabit, USB 3.2, a 5 Gbps model: a PLC port won't use any of it. An S7-1200 port runs 10/100. A 1756-EN2T is 10/100 too. A CompactLogix 5380 can do 1 Gbps, but you're downloading a few megabytes of project, not streaming video. On a 10/100 port a gigabit dongle just negotiates down, and nobody notices.

What does matter is a short list, and very little of it is printed on the front of the box.

 

The two chip names to look for are ASIX and Realtek, and both work. ASIX's AX88179A, for one, does 10/100/1000 with Auto-MDIX, leaves the factory with a unique MAC, and runs on the in-box driver in Windows 10 and 11. What I'd avoid is the no-name listing that won't say which chip is inside. When that one acts up, you don't even know which driver to go looking for.

Set the USB C to Ethernet adapter up once, then leave it alone

The setup I use takes two minutes and saves the fight with Windows every time after.

Wi-Fi stays on the office or plant network: internet, email, the manual you need to look something up in. The USB C to Ethernet adapter gets a static IP in the PLC's subnet and nothing else. Say the PLC is 192.168.10.20. On the adapter:

·       IP address: 192.168.10.250, or anything free in the same subnet

·       Subnet mask: 255.255.255.0, same as the PLC

·       Default gateway: blank

·       DNS: blank

Leave the gateway empty. Put one in and Windows has two default routes to choose from, and it doesn't always pick the one you want. With it blank, internet traffic stays on Wi-Fi and only 192.168.10.x goes out the dongle.

The other trap is a Wi-Fi network that uses the same range as the machine. If plant Wi-Fi hands you a 192.168.10.x address too, the laptop has two doors into one subnet and you don't control which one a packet takes. Change one side, or turn Wi-Fi off for the session.

The setup that holds up: Wi-Fi keeps the internet, and the USB C to Ethernet adapter sits on the PLC's subnet with a static IP and no gateway.

Some laptops switch Wi-Fi off the moment a wired link comes up. On many Dell Latitude laptops that's a BIOS setting called Wireless Radio Control, under Power Management. Set it to Disabled and both stay up.

Corporate laptop where every IP change is an IT ticket? Ask for the dongle to be handed to a VM where you have admin rights, and change the IP there.

Why Windows forgets the IP you set

This one causes more confusion than everything else here. You set the static IP, it works, you pack up. Next site, you plug the same USB C to Ethernet adapter into the port on the other side of the laptop. Now it's "Ethernet 3", it's back on DHCP, and with no DHCP server on the machine network it lands on a 169.254.x.x address. Your static IP is still there. It's saved on "Ethernet 2", which isn't plugged in.

That's Windows doing what it's built to do. A USB device can report a serial number, but it's optional. If it has one, Windows knows the device on any port. No serial, and every new port means a new device, plus a new adapter entry with default settings.

Two ways to live with it. Use the same USB port every time, or open ncpa.cpl after plugging in and set the IP on whichever entry is actually connected. I also stick a label with the IP right on the dongle body. Give it a year and there are three of them in the bag, on three different subnets, all looking the same.

Without a USB serial number, the same USB C to Ethernet adapter is a new device on every port, and the static IP stays behind on the old entry.

Link light on, PLC invisible: the order I check things

When a PLC won't answer through the USB C to Ethernet adapter, I go in this order and don't skip ahead. Most of the time it's settled by step 3.

1.    Link LED at both ends, on the dongle and on the PLC port. No link usually means cable, port or dongle, and nothing in the programming software will fix that. A plain patch cable straight from dongle to PLC works, no switch in between, as long as one end does Auto-MDIX.

2.    ipconfig. Open a command prompt, run ipconfig, find the adapter that's actually connected. Your static IP should be on it. A 169.254.x.x address means you set the IP on another entry.

3.    Ping the PLC. Replies mean the network part is done, and whatever is left lives in the programming software. A timeout with the right IP on the dongle is usually a subnet mismatch, the wrong PLC address, or Wi-Fi sitting in the same range.

4.    Make sure the software is using the dongle. Siemens and Rockwell fail differently here, both below.

The check order for a PLC that won't show up through a USB C to Ethernet adapter. Most cases end before the software step.

 

Siemens: the dongle isn't in the interface list

With TIA Portal, the first time you go online with a CPU you pick the PG/PC interface type and the actual interface in the Go online dialog. Pick the dongle there, not the built-in card or the Wi-Fi.

Sometimes the USB C to Ethernet adapter just isn't on the list. Windows sees it, ping works, Siemens doesn't. On Windows 10 this is what clears it:

1.    Restart Windows and don't open any Siemens software.

2.    In Device Manager, uninstall the network adapter. Keep the driver if Windows asks.

3.    Right away, without restarting, rename C:\ProgramData\Siemens\Automation\Simatic OAM\data\LogDevices to LogDevices_old. ProgramData is hidden, so turn on hidden files in Explorer first.

4.    Restart and let Windows find the adapter again.

After that the TCP/IP interface for the dongle should show up in the PG/PC list again. One warning: if a TCP/IP interface parameter set is already listed, don't delete it. Other software may be using it.

Rockwell: RSLinx is looking through the wrong card

The EtherNet/IP driver in RSLinx Classic browses through one adapter, picked by its IP. With Wi-Fi, a dock and two dongles on one laptop, it's easy to end up browsing the wrong card, or an address that's gone since you swapped dongles. Communications, Configure Drivers, open the EtherNet/IP driver, pick the dongle's address. If an adapter carries more than one IP, only the one it's using right now is offered. Browsing across a router needs directed broadcast on that router, so just put the laptop in the PLC's subnet.

MAC addresses, licenses, and the USB C to Ethernet adapter

Every dongle carries a MAC address, set by the chip maker or by whoever built the dongle. ASIX ships the AX88179A with a unique MAC, but it also gives manufacturers tools to rewrite both the MAC and the USB serial number. A careless factory can flash a whole batch the same. One such dongle works fine. Two on the same plant network fight over the address, and the symptoms look like a flaky switch. Buying a six-pack for the crew? Run getmac /v on each one and write the numbers down before they go out.

Dell laptops add a twist. On supported models, with the network driver loaded, MAC Address Pass-through makes a Dell USB C dongle or dock show up with the laptop's own MAC from the BIOS. The MAC printed on the dock label isn't what the network sees. Plug the same dongle into a non-Dell laptop and it's back to its own MAC. On a network that filters by MAC or uses DHCP reservations, that difference is the whole problem.

Licenses can bind to a network adapter too. A FactoryTalk activation locked to an adapter that isn't there right now (a dock, a dongle in the bag, a laptop adapter that powers down when idle) shows as unavailable. Bind activations to something that's always present, like the hard disk serial or the built-in Ethernet port, not to a USB C to Ethernet adapter you'll leave in a cabinet someday.

When I don't want a dongle

For programming and monitoring, a USB C to Ethernet adapter is all I carry. I'd rather fry a cheap dongle than the onboard port of a laptop. There are a few cases where I want something else, though.

Wireshark captures of fast I/O traffic. Frames can cross USB in batches, so the timestamps tell you more about the USB bus than about the wire. Fine for seeing who's talking. Not for timing.

Then there's the dongle that lives in a panel. Consumer dongles aren't built for a cabinet, and the plug works loose. Industrial versions exist with a screw-lock USB C, a DIN rail clip, surge and ESD protection, and a 0 to 55 °C rating.

Anything you find in a cabinet that you didn't put there. There are pentest tools built to look like an ordinary USB Ethernet dongle, with a small Linux computer and remote access inside. Buy from a known source and pull anything nobody can account for.

FAQ

Do I need a crossover cable with a USB C to Ethernet adapter?

Usually not. The S7-1200 port crosses over on its own, and so does an AX88179A dongle. Auto-MDIX on one end is enough for a standard patch cable.

Is a gigabit USB C to Ethernet adapter faster for PLC programming?

Not on a 10/100 port like the S7-1200 or 1756-EN2T. A CompactLogix 5380 port can run 1 Gbps, but project downloads are too small for the difference to be worth paying for.

Why does Windows make a new "Ethernet" entry every time I plug the adapter in?

Usually because the dongle doesn't report a USB serial number, so Windows treats it as new hardware on each port. Use the same port every time, or set the IP on whichever entry is connected.

Can I use a USB C to Ethernet adapter inside a virtual machine?

Yes. Pass it to the VM as a USB device and the guest gets a real network card with its own IP. VMware and VirtualBox do this; Hyper-V doesn't pass USB devices through.

When the dongle checks out and the PLC port doesn't

If a known-good USB C to Ethernet adapter links on every other PLC and one port still stays dark, the port is the suspect, not the laptop. IQElectro stocks the parts that sit on the other end of that cable: the 5069-L306ER CompactLogix 5380 controller, the 1756-EN2T EtherNet/IP module for ControlLogix racks, and the 1783-US08T Stratix 2000 unmanaged switch, if you want a small switch between laptop and PLC for firmware updates. Everything ships with a free 2-year warranty.