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The title basically is the question.

Well, I know why Apple added chip to their cables and changed the connector for good measure. That's of course not a corporate greed but a loving care about their customers... and making sure it would cost them a fortune to leave.

But nowhere in the USB specification it requires anything but the wire. Of course high speed introduces strict limits to electrical characteristics etc. But still... it is JUST A WIRE! Billions of cables in the world work just fine without anything else.

However when it comes to magnetic cables, which are basically regular cables with connector attached to wires by some pins and magnets, they all have chips inside. The end result is that it is practically impossible to find cable which works with all USB devices in home. Each cable comes with a list of incompatible devices almost as long as compatible. Fast charging that used to be enabled by simple pull-up in micro connector now requires "improved smart chip" and does not work with half of the mobile phones.

The question is - why? Is there any reason I don't see? Unlike Apple products these cables are dirt-cheap, and if there is no increased cost then there should not be a reason for manufacturers to make all this mess out of perfectly simple and extremely useful idea.

Update:

Regarding suggestions to tear down some cable - yes, tearing down and analyzing them with DSO may provide an answer to compatibility problems. But the whole point of posting a question here was the hope that there could be an electrical engineer who knows an answer.

Also, according to (very limited) marketing info, some manufacturers include identification chip into plug portion, designed to trick Apple devices into thinking they connected to authentic cable and avoid annoying pop-ups. Legal issues aside, this has nothing to do with the question, since those cables still have more chips in the connector portion. Funny thing though, is that most devices on "incompatible" lists are Android micro-B / type-C devices.

Anyway, here are the theories presented so far:

  1. By @pjc50: To simulate the order of connection required by USB spec and guaranteed mechanically in regular cables. This is top contender at the moment, because it is applicable to all 3 types of cables on the market (non-reversible, electronically reversible and mechanically reversible).

  2. By @Ali_Chen: To provide additional ESD protection highly important here due to exposed pins. This is another strong argument for having chips in the cable, also applicable to all types of cables.

  3. By @dim: To avoid dangling pins in dual-row models, which might affect high-speed transmission. Not sure if this is critical for USB 2.0 speeds though.

  4. By @dim: To reverse outputs in single-row models. Yes, this certainly is the case. Note that many cables reverse power only, leaving you without data. Also note, that simple reversing should not result in selective compatibility issues.

  5. By @tom: To control the LED. Now, this is certainly a reason to have some circuitry inside. But if the price is loss of primary function with half of the devices then this is dubious reason, to say the least.

Update 2:

Is seems there is no one here with insider knowledge of what is really going on in those connectors.

So, I am willing to accept @pjc50 and @agent-l suggestions that those chips facilitate an order of connection required by USB specification, as something that is supposed to be inside from engineering point of view.

At the same time, considering how cheap those cables are, I'd like to point out @ali-chen suggestion that they are simple TVS as something that is actually there.

As a side note, very interesting idea came out of @ali-chen comments - that there are more reasons to have chips inside plugs than inside cables. I know that iPhone plugs have chips in them to simulate genuine Apple cable. But since those plugs expose USB interface pins to all the ESD around, having TVS in all plugs regardless of connector type strikes me as another must have.

Also for electrically-reversible cables the simplest solution to swap power lines would be to put diode bridge in the plug, instead of trying to somehow guess the insertion direction and manage power in the connector.

So, until someone tears down actual cable and tells us the truth I propose to consider this question answered.

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    \$\begingroup\$ Do you have a link to the sort of 'magnetic' cable you are talking about? \$\endgroup\$ – Jon May 26 '18 at 10:00
  • \$\begingroup\$ Just google "magnetic USB" and you'll get thousands of them \$\endgroup\$ – Maple May 26 '18 at 10:05
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    \$\begingroup\$ What makes you think that they all have a "chip" inside? We need an example to talk about. \$\endgroup\$ – Rev1.0 May 26 '18 at 10:21
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    \$\begingroup\$ Interesting. It looks like it's not just a wire, but it would be easier to discuss with an actual teardown. My guess is that it's monitoring the data line attachment and gating it somehow - look at a normal USB type A and you see the power and ground pins are longer, to make contact first and break contact last. Can't guarantee that with a flat magnetic connector. \$\endgroup\$ – pjc50 May 26 '18 at 10:52
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    \$\begingroup\$ And do those have electronic chips inside? And even with two rows, not switching D signals would require having stubs, which aren't good for high speed transmission lines. \$\endgroup\$ – dim May 26 '18 at 17:06
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Disclaimer: I am answering about USB-A and -B connectors, C are bit much more complicated.

USB standard does not include magnetic connectors. Therefore, those can't be formally called "USB cables".

How should you think about such "cable" is that it's USB-A-to proprietary and then proprietary-to-USB-B device. The "device" part is crucial here, because the magnetic connector imposes own problems, like current limitation, reliability of connection, exposed pins and wrong order of making contact.

There is a crucial part of USB spec that makes simple magnetic connector impossible. USB defines plugging as a process. The ground and voltage pins are supposed to make the contact first, only then the data pins can made contact. So the device can power up and begin negotiation as soon as data gets connected. That's why data pins in USB plugs are shorter, so as you put the plug in, they make contact last. If I were making a magnetic connector (which, makes all contacts simultaneous), I'd insert a chip that would delay connecting of data pins to replicate the required behaviour.

I believe that the compatibility tables are mostly "made-up". They don't guarantee that the cable will be incompatible with other devices, they merely guarantee that the cable will be compatible with listed devices. This is most likely because the magnetic connectors are bulkier than regular connectors. Compatibility table is merely a manufacturer securing themselves against you returning the cable because "it doesn't work with my device".

On the other hand, there are also devices that use the voltage negotiation over old USB plugs. Eg my Samsung S7 asks the voltage to be bumped to +9V and the bundled charger delivers. It could fry a Charger Doctor, if I'd try to use one. So, it's reasonable that manufacturers don't want you to use a cable that's limited to 5V (by the chip) on S7. Either the cable chip can be smart enough to detect such negotiation and disrupt it or it'll fry. Thankfully, USB-C has it covered.

Fast charging that used to be enabled by simple pull-up in micro connector now requires "improved smart chip" and does not work with half of the mobile phones.

I don't know anything about that, can you point me to the spec regarding micro plugs? AFAIK there is no spec for resistors in connectors (if we exclude OTG and USB-C). Resistors are in the charger and the device determines max current by those resistance + voltage drop. Having extra connector on the way (which is weak and with small contact surface) surely adds some voltage drop, so the most primitive implementations (high currents @5V) would certainly be very limited.

Bottom line: I believe that limits on fast charging are mostly due to physics of magnetic connector itself. The order-ensuring chip has to sense a device being connected, so it most likely monitors current, and that most likely add a bit of resistance on +5V too, further limiting current-carrying capacity.

Some regular cables and USB gadgets also prevent fast charging, so magnetic cables are no exceptional in this regard at all. I wouldn't put the blame on the chip, but rather on stretching USB to it's limits.

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It is usually very difficult to short out a regular USB cable. Both male and female connectors have covers and recessed pins.

This is not the case for magnetic connectors so they will require a microcontroller, or at least an IC, to determine whether a connection has been made and therefore whether it is safe to deliver power and data. They also appear to have fewer pins than a regular connector, so I assume some data conversion is required at either end.

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    \$\begingroup\$ Umm... they all have at least 4 pins, same as about 50% of the regular cables on the market. I agree with the need for short circuit protection bit. However that can be done without introducing any communication and/or charging problems. \$\endgroup\$ – Maple May 26 '18 at 10:26
  • \$\begingroup\$ @Maple Charging problems could be down to the pins not having the same surface area as a standard connection hence not being able to deliver as much current. Communication problems can arise when the internal conversion is not capable of the higher speeds or latency requirements. \$\endgroup\$ – Ferdia McKeogh May 26 '18 at 19:07
  • \$\begingroup\$ There are many videos on youtube where people measure and compare internal resistance of these cables. While some of them fail miserably the rest are on par with regular cables, about 0.15 Ohm/m. And I still don't understand what conversion you are talking about. At most they should have some simple gates. If they do something else I want to know a reason, which is exactly the point of original question. \$\endgroup\$ – Maple May 26 '18 at 20:00
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These are some mock-up "improvements" to charging USB cables. They have a Type-C plug, and then a magnetically-coupled power cable. Other "cables" have a Micro-B plug interchangeable with Type-C plug.

But nowhere in the USB specification it requires anything but the wire.

This is incorrect, for Type-C connector the USB Specifications do require a chip to be embedded into the Type-C connector, on CC lines. It is called "electronic marker". This chip is required to inform the power recipient that the cable can carry higher current, 3 A or 5 A, so the device can engage the "fast charging" internal circuitry without risk of smoking out inappropriately thin cables. Without this IC the consuming device won't charge at maximum rate.

There seems to be an unlimited creativity for aftermarket "charging" cables, with magnetic locks (pioneered by Apple I guess), with lighted shrouds, etc., so the compatibility likely depends on who makes the fancy cable.

EDIT: If the cable is the "legacy assembly Type-A to Type-C", it would be illegal to advertise the cable as having more than 500 mA capability. So ICs pictured in the the provided link "magnetic USB" are less likely to be the "electronic markers". Since the magnetic joint exposes signal pins without typical protection by shield/shroud, these ICs are likely just additional ESD suppressors.

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  • \$\begingroup\$ Aha, finally something concrete. I admit, not owing any type-C devices I did not look into its specification past basic pinout diagram. And since all these cables have type A on the PC side and only 4-5 pins on the plug side, I assumed they simply use USB-2.0 pair of the C spec without any support for C mode (even with C plug attached). Do you know if it is possible to negotiate fast charging with type-C device over 2.0 pins? Also, does this "electronic marker" describe cable only or power source on the other side? If the latter than it is of no use for USB-A cable. \$\endgroup\$ – Maple May 27 '18 at 17:31
  • \$\begingroup\$ @Maple, the "electronic marker" only describes the cable, pddnet.com/article/2016/06/… To get higher voltage from power source, additional round of negotiations is required using Power Delivery portocol(s). \$\endgroup\$ – Ale..chenski May 27 '18 at 17:44
  • \$\begingroup\$ I see. Found another interesting info from chip maker. However exciting all this new technology is, I don't think it is applicable to magnetic cables we are discussing, since they all have A downstream plug and cannot (must not, in fact) make any assumptions on the host capabilities beyond USB 2.0. They can only embed type-C negotiating logic into plug part (since connector only has 5 pins) and I don't think they do. \$\endgroup\$ – Maple May 27 '18 at 18:12
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    \$\begingroup\$ @Maple, "all this new technology" is in brutal violation of all ESD protection engineering. The signal pins are exposed to electrostatic discharge over air. If you put extra TVS devices at each end of this magnetic coupling, the signal integrity will suffer tremendously. Have you seen any of this "exciting" cables to have USB-IF certification logo? But this is a good marketing and acceleration of replacement cycle, since the phones, if used with this exiting new technology will likely have the USB interface killed much earlier. \$\endgroup\$ – Ale..chenski May 27 '18 at 18:54
  • \$\begingroup\$ I was referring to type-C specification, which IMHO is great leap forward after DOA "Micro-B super speed" brain fart. As for ESD protection, the quick search I did on type-C after your first post revealed that all chips used in type-C cables have ESD protection in them. It does not stop them from working much faster than these magnetic cables, which are only USB 2.0. But you are 100% correct about increased exposure to ESD with magnetic plugs attached. They are basically lighting rods wired directly into USB interface! Yikes! \$\endgroup\$ – Maple May 27 '18 at 22:17
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Disclaimer: This answer is mostly a guess

The device you link to in the comments has a built in LED in the connector, which illuminates when the device is attached and charging. As such it will require some form of circuitry to detect device presence and illuminate the LED.

Having said that, the only picture you provide a link to is a cartoon graphic showing the mystical ICs. Who knows what if anything is actually in the cable, and what if anything it does. Without a proper tear down and photos of the guts of a real cable, it's impossible to answer.

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  • \$\begingroup\$ Yes, it is a cartoon in most cases. However as I pointed out before, the LED indicator, short protection etc. do not explain compatibility tables. Look for example at this one. Samsung S2 S3 S4 S6 supported, S7 and Tab S2 aren't. LG G4 yes, HTC M8 no. All these phones are perfectly happy with any regular USB cable. \$\endgroup\$ – Maple May 26 '18 at 17:13

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