USB-A to USB-C Mysteries

In My Humble Opinion
1

I have an external accessory for my PC tablet. It connects to the PC via USB for both power & data. It happens to be an RFID reader/writer, which means very low power draw. But the device type in and of itself probably isn’t significant.

The design is a few years old. Its body has a female USB-C connector and it came packaged with a male USB-C to male USB-A cord. So plug the C end into the device, then when you plug the A end into the typical female A connector on a PC, the PC recognizes the device, the LED lights up, etc. Typical plug & play behavior for a USB external device. And it works great.

Until … [cue ominous music]

I replace my old tablet with a new one. Which also involved going from Win10 to Win11. Which new tablet also has no USB-A connectors, only USB-C connectors. So I find a USB-C to USB-C cable in my stash, plug it in, and … nothing. The PC does not see the device. Or maybe it does, but Win11 works differently and plug & play needs to be enabled, or maybe the device failed in the ~6 months since it was last used, or maybe the cable is bad or … So many possible failure modes - how to diagnose them?

Try several different C-C cords. Same results. Try plugging my phone into the various cords as a known-good control. I find some cords are power only; the PC can’t see the phone. So makes sense it can’t see the RFID thing either. But of the several cords that work fine to provide both power & data between phone & PC, none do anything with the RFID device; it’s dead.

My old tablet with the A connectors is long since recycled, so can’t test that as a control. I have no other computers. Lots of fruitless Googling about win11 vs USB and some Chinglish from the device manufacturer.

Then in desperation I remember; I think I have an adapter from female USB-A to male USB-C. I do! Plug the male C end of the adapter into the PC, get out the RFID device’s C-A cord and … Success! The PC recognizes it immediately and all is well! Try the same thing with other C-A cords in my stash and all work fine. Net of the couple that my phone proves are power only, but that’s as expected.

Holy WTF, Batman!

That is one glitch I never expected to encounter. I have always believed that USB is USB is USB. The connector shapes change but are electrically identical. The maximum speed of ports has changed too, but the protocol is smart enough to handle all the speeds so the latest whizbang ports can still talk successfully to the oldest and slowest and dumbest ports. If the connectors mate, it will work. Yeah, right.


With all that lead-up, it’s now audience participation time …

Anybody ever have similar USB connector shape follies? Anyone here with enough tech knowledge to explain this issue? Why does C-to-A-to-A-to-C work, but C-to-C does not? Anything else interesting or entertaining along these lines?

2

As we’re in IMHO … here’s what Google’s AI has to say about it ..

The combination of a USB-C to USB-A cable with a USB-A to USB-C converter, or a “dongle,” works because the converter’s resistors trick the device into expecting legacy power levels.

More details..

A USB-C to USB-C cable might not work due to being a passive, non-powered cable or a cable that’s missing resistors needed for communication. The combination of a USB-C to USB-A cable with a USB-A to USB-C converter, or a “dongle,” works because the converter’s resistors trick the device into expecting legacy power levels. The USB-C to USB-A cable, acting as a unidirectional device, sends a signal that the converter translates to work with the C-type port.

Why the USB-C to USB-C Cable Fails

Lack of Resistors (E-Marker Chips):
For a USB-C to USB-C connection to work correctly, especially for data and high-power charging, the cable often needs e-marker chips or terminating resistors to negotiate with the connected devices. If these are missing, the devices may not recognize each other or communicate properly, especially for fast charging.

Passive Cable, No Host Recognition:
A simple, non-powered USB-C to USB-C cable can’t establish a host/device relationship without proper power or data negotiation. The devices might only recognize the cable as a power source, or not at all.

Why the Adapter Combination Works
Legacy “Trick” via Adapters:
The adapter contains resistors that simulate a legacy power source, making the device think it’s connected to a traditional USB-A port.

Unidirectional Communication:
The USB-C to USB-A cable is directional, with resistors that ensure it works in one direction—from the USB-C device to the USB-A port, or vice versa, depending on the intended use.
Converter Completes the Connection:
When you use the C-to-A cable with the A-to-C converter, the converter completes the connection by providing the necessary resistors or “trick” to enable communication that the C-to-C cable lacked.

In Summary
The adapter and cable combination works because the adapter is essentially a mediator that allows a non-functional C-to-C cable to interact with a device that needs a specific type of communication signal. Your standard C-to-C cable likely lacks the necessary components for a successful host/device negotiation, which the adapter provides.

3

I encountered this issue a couple years ago and bought adapters which seems to have fixed the problem. Really flummoxed me at the time. So now my travel pouch includes adapters and cables to meet the needs.

Here is a Reddit thread on the issue from a few years ago: How come some devices only work with USB-A to USB-C?

4

I don’t know what’s going on, but here’s my guess.

With USB-A to anything, both devices know that the side that gets the A port is controlling the connection, and the other side is taking orders.

USB-C is the same on both sides, and the devices have to negotiate the connection. When both devices understand this, it’s an upgrade. It means the phone can tell the charger, “this is how much power i want now”, for instance. There are a lot of things that really work better with USB-C to USB-C.

But i suspect your accessory doesn’t know how to negotiate the connection, and as a result, it’s confused, and just isn’t talking to the computer at all.

When i read the first paragraphs, i was going to suggest buying a cheap USB-C to USB-A adapter, and i see you already tried it and it works. I’d either stick with that, or find out if there are newer drivers/firmware you can install on the tablet and/or device. Honestly, having found a solution, I’d stick with it.

It’s a little weird that the device came with a USB-C port and without the software to understand it, but if it was sold just as USB-C was rolling out, it’s not crazy

5

I suspect that was the case.

IMO they chose USB-C on the device just for physical size. I’d bet a previous iteration of the device had been equipped with Micro-USB. And they were counting on the USB-C to USB-A cord to do the “negotiating” by presenting the simple “I’m a dumb satellite device” interface to the computer.

The device is still being sold by Amazon today the same way; USB-C device connector with a C-A cord. With (of course) no mention in the paperwork, the ad copy, or anywhere, that it’ll only interface successfully to a PC via USB-A connection.

6

Sounds right - As stated, if early in adoption, the C connector in a peripheral would just be replacing a prior-generation Micro-USB or similar with only the minimal essential reconfiguration, and the peripheral still “expecting” for data use to have an A plug with the appropriate configuration on the dock end.

(…And yes, the implicit expectation that by the time C-to-C became the norm, you would be buying last month’s model of both things at both ends. ‘Cause that’s how tech rolls.)

7

The relevant standard here is USB Power Delivery, which — despite its incredibly dumb name — is a Type-C-specific thing that requires an active negotiation first. Both devices have to first establish a data channel via one of the “safe” pins, discuss what voltages each can handle, and settle on an agreement before any real power is sent.

But, as you saw, many cheaper USB-C devices do not support USB-PD, even if they have a Type-C port, and so no power gets sent at all… and yes, using a C to A to C setup will “fool” the devices into supplying legacy USB power, as the AI said and you already confirmed.

It’s an incredibly annoying problem because many manufacturers either do not really understand this, or worse, know this and purposely market their devices as USB-C compatible anyway, leaving out the part where it only works with a C-to-A adapter. I’ve bought and returned many such devices, including a bike light, electric toothbrush, walkie talkie, etc.

To make matters worse, even when a device does support USB-PD, there is no clear guarantee or labeling of the wattage it can actually supply. A USB-PD phone or laptop charger can provide anywhere from like 5W to 200W and you’ll never know without reading the fine print.

USB-C is a great idea hampered by terrible execution. What looks like a single standard is actually like 20 different ones: Overview | Understanding USB Type C: Cable Types, Pitfalls and More | Adafruit Learning System

If you buy “expensive” USB-C things like high-end laptops and phones, you can usually expect them to work as you think they ought to. But that is definitely not true on the budget end of things, or with commodity electronics. It’s a total shitshow that makes me miss the simplicity of the old RS-232 serial ports :frowning:

Contrast this with the Apple/Intel variant of USB-C, Thunderbolt. It still looks like USB-C, but it has a set of more strictly defined standards and better trademark enforcement, so two Thunderbolt-branded devices (and cables!) are much, much more likely to interoperate together well. But even then it’s not perfect, and there are plenty of counterfeits.

8

Boy, a serial port would certainly add a lot of thickness and weight to my phone.

Doesn’t Thunderbolt add a totally new dimension of complexity? It uses the same physical interface, but has still different rules.

9

Maybe there’d actually be enough space then for a replaceable battery and headphone jack! Sigh, I feel old…

Whoops, I simul-edited my post as you were posting this. Yes it does… sometimes Thunderbolt makes the situation better (when you buy Thunderbolt-branded devices from major US brands). It doesn’t really help with the aftermarket and counterfeit situation as much, unfortunately.

Edit: I think Thunderbolt is intended as a superset of regular USB-C, i.e., your uber-fancy Thunderbolt 5 port should still support any off the shelf standards-conforming USB-C device. But “standards-conforming” is doing a lot of heavy lifting there… it’s like a 50/50 chance whether something that looks like USB-C is actually USB-C with proper Power Delivery and the other standards. You never really know until you try it, unless you get one of those voltage & data speed tester dongle things.

10

Baofeng? :smiley:

Including, excitingly, that the connector is physically symmetric but the pinout is not. I have an adapter that splits the USB-C port of a Raspberry Pi so that it can receive power (from a barrel plug) over USB-C while also allowing data to be transferred, so the Raspberry Pi can be plugged in to a computer and enumerate as a USB peripheral without drawing too much current from the computer. The adapter’s USB-C ports are directional, but there is no way to know whether you’ve plugged the USB-C cable the “right way up” until you try and learn that it doesn’t connect to your PC.

(It is also possible for a rechargeable USB-C lamp to suddenly decide it’s fine taking all 65W the universal charger will give it instead of the 5W it is designed for. It becomes very bright, and then it becomes on fire. Ask me how I know)

11

Heh, indeed. To Baofeng’s credit, though, it was like 10-20% the price of the US-designed Rocky Talkie, which I believe supports USB-PD… but even there I’m not sure. It, too, comes with a USB-A-to-C adapter… sigh.

The whole GMRS handheld market is a mess. I eventually just gave up and made sure to remember to carry the extra USB-A-to-C chargers. I have to psychologically trick myself into remembering “nevermind how it looks… these are not real USB-C…”.

Heh, USB-C is something I hope I never have to tinker with at a DIY level. It’s bad enough at the consumer level already!

12

@DETXL This does make me wonder though… is there some sort of dongle that can map the standard pinouts of USB-C into a USB-A, keeping the same physical connectors but ignoring or remapping the newer pins? Basically a C to A + A to C setup like the OP described, but in a single, smaller dongle? For the express purpose of “I want to charge this fake USB C device from my real USB C charger”?

I don’t think those USB-C data blocker “condoms” would work in this scenario, right?

13

I just want to say thanks for the simplest, most easily understood summary of how this works.

14

It’s clever engineering to find a way for the device to continue to produce light without any current draw. :exploding_head:

15

The ultimate compatibility hack!

16

Thanks! And it’s not just for charging, USB-C devices also negotiate data at both ends. (Of course, that doesn’t work if you have a cheap, charge-only cable, either.)

17

Remarkable, I have that same device and was going to make the same recommendation a few hours ago. I got distracted and you did it already! But here it is again, super useful dealie and I can confirm it works to 30 volts(!).

18

Any chance the port is a USB 4, Not USB C? They use the same connector but have different protocol. My new mini computer has 4 USB A ports and a USB 4 port but it is clearly marked.

19

If that was directed at me/OP, the tablet (Surface Pro 11) has two identical USB-C shaped ports described as

2 X USB-C®/ USB 4 ports with support for:
Charging
Data transfer
DisplayPort 1.4a
Surface Thunderbolt™ 4 Dock and other accessories

20

Another reason is that the EU has mandated that consumer electronics use USB-C for charging.

Since the EU is one of the largest markets, most manufacturers have converted all their products to use USB-C power, except for possibly some smaller manufacturers who only market their electronics outside the EU.