USB-C Cables

[Steerpike]

With my new LG Gram 17, I'm thrilled to have a laptop that finally uses 'USB-C PD' for charging, opening a world of possibilities in terms of chargers and accessories.  The fact that USB-C supports Video, fast charging, docking stations (with ethernet), etc is a really great development.  But today, when I looked on Amazon for a new USB-C cable to use with the new 65W USB-C PD power supply (charger) I just bought ... oh my, what a confusing situation!   

 

This is the power supply I bought - Amazon.com: Anker USB C Charger, 715 Charger (Nano II 65W), GaN II PPS Fast Compact Foldable Charger ...  

It's GaN, and supports PD and PPS. Great!  So now it's time to buy a cable for it ... 

 

It seems like you can buy all manner of USB-C cables, that will support different combinations of features.  This cable, for example - Amazon.com: Anker New Nylon USB C to USB C Cable 100W 10ft, USB 2.0 Type C Charging Cable Fast Charge ... sounds great, and will charge up to 100W, supports 'fast charge', and mentions '480 mbps', but has this note tucked away at the bottom of the page - "Note: This is a data transfer and charging cable, and does not support video output". 

 

So ... how does one navigate this minefield of cable selection? Other than that note at the bottom (which I almost missed), how would I know that video output is not supported?  At the moment, I have no specific need for video output, but if I were to buy a new external monitor (with USB-C), I presume I'd need a USB-C cable that was capable of carrying video ... or am I misunderstanding something here? 

[Aaron44126]

“USB 2.0” is a sure sign that it will not support display output.  (Not sure how people are supposed to know this.)  They just have the four USB2 pins connected and no more capabilities than a regular USB2 cable, just a different end/connector.  A USB3/5Gbps cable should support most everything except Thunderbolt and >100W power delivery, I believe.  There could be some value in the lower-end cables, though, the high speed ones will be generally “fatter”.

[Steerpike]

Good catch on the 'USB 2.0' reference.  

I since found this site, which is fairly comprehensive - What You Need to Know About USB Connectors and USB Cables | CUI Devices , which has a separate page just for 'power only' connectors - An Introduction to Power-Only USB Type C Connectors | CUI Devices .  

 

One thing that I'm still confused by is power 'negotiation'.  My basic understanding is, the way that universal power supplies (capable of delivering a range of voltages and therefore power levels) work is that they 'negotiate' with the plugged-in device to find out what voltage they need, and then supply that voltage. This way, a device only needing 5V doesn't get blown up by a power supply capable of supplying 20V.   And if no negotiation is possible ('dumb' device, inappropriate cable) then only the lowest 5V/1A is supplied. 

 

So the question is, how does this 'negotiation' occur?  The new USB-C cable/connector spec includes 24 pins (detailed here), with a mix of data and power conductors (16 data, 8 power/ground) - so plenty of conductors available for 'negotiation'. But, in reading about dedicated 'power only' cables, the 'power only' article above says that a typical 100W 'power only' cable has only the 8 power/ground conductors present - pins A1, A4, A9, A12, B1, B4, B9, B12.  So how does 'negotiation' occur with only the power/ground conductors?  The article says "USB PD requires a digital device handshake to achieve these higher ratings. If the requisite chips are not available and the handshake does not occur, the system will revert to the 5 V/1 A standard" and "PD can also work with devices that do not transfer data, utilizing USB purely for power. It does require separate communication lines for power negotiation and therefore is slightly more complicated to design and manufacture than many non-USB formats". 

 

So how does a 100W USB-C PD charger connected by a 'Power Only' USB-C PD cable know that my laptop wants 20V / 5A, if the power supply will default to 5V/1A without digital handshaking? Perhaps the negotiation takes place over the power lines?  Some further reading suggests two things; 1) there is talk of negotiation occurring on the Vcc (power) lines of the USB-C cable, and 2) there is talk of using the 'CC1' or 'CC2' pins (pins A5, B5 in USB-C connector). CC1/2 are 'configuration' channels, so that makes sense.  Looking back at the 'power only' cable description above (An Introduction to Power-Only USB Type C Connectors | CUI Devices ) , they actually describe a 6-pin and an 8-pin power-only cable, with the 6-pin version using the CC1 and CC2 pins - which would thus support negotiation over CC1 and 2, but they also describe the 8-pin version that does NOT include CC1/2, but uses more power / ground conductors. 

 

I'll continue reading and update here as I make discoveries ... 

 

[Aaron44126]

I forgot about "power only" cables when making the post above.  I haven't run into this yet with USB-C, but I have some power-only "micro-USB" cables (just two pins I think?) and I was confused when I first encountered one of these and tried to use it with a game controller or something.  Sure would be nice if there was a standard way to have these labeled or something.

[Steerpike]

13 minutes ago, Aaron44126 said:

I forgot about "power only" cables when making the post above.  I haven't run into this yet with USB-C, but I have some power-only "micro-USB" cables (just two pins I think?) and I was confused when I first encountered one of these and tried to use it with a game controller or something.  Sure would be nice if there was a standard way to have these labeled or something.

Further, I believe even some 'micro-USB' cables can handle variable power. From my reading today, micro-USB introduced a 5th conductor (so you get Vcc, Gnd, D1, D2, and 'ID'), and that fifth conductor 'ID' was introduced to support 'USB OTG' - 'on the go' - which can also be used as a general-purpose 'signaling' conductor, used for power negotiation. Not that you need a 5th conductor, necessarily, as you can modulate communications over the Vcc / GND pair. 

 

And further complicating matters, you have non-USB standards like the Qualcom QI standards, which allow for another form of variable power delivery.  Apparently QI is more widespread than USB-PD, despite being proprietary (hopefully changing now that USB-PD is getting more exposure). 

 

I've sent off some questions to a few manufacturers to see if they'll give me more info on the USB-PD stuff. 

[Steerpike]

I just found this interesting page - Designing a Type-C electronically marked cable - Part 1  - which explains the concept of 'EMCA' as applied to USB-C PD - "Electronically Marked Cable Assembly". In order for a USB-C PD cable to be able to carry more than 3A, the cable has to have the "EMCA" capability added. This is an active chip in the cable 'head' itself, which carries information about the cable characteristics. Without EMCA, a USB-C PD cable is limited to 3A.  Thus, there appear to be at least two categories of power cables in this realm: 

 

1. 'Type-C only' - The USB Type-C specification allows for up to 15 Watts of power to be transferred from DFP to UFP on the VBUS and Ground signals.  This 15 Watts of power can only be transmitted at 5 Volts when a “Type-C Only” solution is used. 
2. 'Type-C PD' - When you add the USB Power Delivery specification to a “Type-C Only” system, you create a “Type-C PD” system and can raise the VBUS voltage above 5 Volts to a maximum of 20 Volts and raise the VBUS current to a maximum of 5 Amps.

 

But I think this latter category - Type-C PD - has two sub-categories - with or without the EMCA. If the cable doesn't have an EMCA chip, it is limited to 3A (but can still go up to 20 V, and thus, has a max power of 20x3=60W). If you add the EMCA, you can now go to 5A, which means the max power is 20x5=100W.

 

This is confirmed in this article: WHAT THE TECH? USB-C AND POWER DELIVERY EXPLAINED – Goal Zero - "Power Delivery starts at the 5V setting and is configurable up to 20V. Using a standard USB-C cable, it can handle up to 60W, and will go up to 100W using a designated EMCA cable."

  

As bizarre as this sounds, I am 99% sure I found some cables on offer that were 60W and some that were 100W, so this would make sense!  Part 2 of the document seems to have gone missing ...