High-density connector recommendations

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Hi all,

 

I would like to ask, whether someone could recommend a decent high-density connector. I would need at least 12 lines (GND, +3.3V, 12V, I2C, plus 6 general-purpose I/O lines) in a small form-factor. My criteria are the following:

 

1. Small footprint (10, max. 15 mm in width)

2. Cheap (I need something like 1000 pieces, so even a couple of dollars would make a huge difference)

3. Standard, so that inexpensive matching cables can be found.

 

USB-C would be perfect, except that I have recently learned that in USB-C, either not all braids are implemented, or there is a chip that assumes that the device using the cable is intelligent, and is capable of negotiating current consumption, protocol, etc. I need a simple pass-through cable without any gimmick.

 

In any case, I would appreciate any hints.

 

Zoltán

 

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Well, 1 + 2 don't usually go together, as smaller usually means more expensive, also

1 + 3 can be difficult to find as well.  

You have not said how long your cable is, but I2C is usually meant for on same circuit board connections but can be used off board if kept short.

Why would 2x6 ribbon cables with IDC connectors not work?  Standard 0.1" x 6 would be ~14-16mm?    You could use 0.050 but would cost more.

 

Jim

 

 

Edit, correct size in mm

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Last Edited: Wed. Jun 14, 2017 - 05:58 PM
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ki0bk wrote:

Well, 1 + 2 don't usually go together, as smaller usually means more expensive, also

1 + 3 can be difficult to find as well.  

 

Well, USB-C does fulfill these requirements. The connector itself is cheap (costs less than a dollar), however, I have to admit, the cable itself is a bit pricier (approx. 10 USD). I definitely see your point.

 

ki0bk wrote:

You have not said how long your cable is, but I2C is usually meant for on same circuit board connections but can be used off board if kept short.

 

Right. Shorter than a metre.

 

ki0bk wrote:

Why would 2x6 ribbon cables with IDC connectors not work?  Standard 0.1" x 6 would be ~14-16mm?    You could use 0.050 but would cost more.

 

If there are no alternatives, then I guess, I will have to resort to that.

 

Zoltán

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Alternatives:  https://www.digikey.com/products...

 

Lots to choose from!

 

Jim

 

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ki0bk wrote:

Alternatives:  https://www.digikey.com/products...

 

Lots to choose from!

 

Jim

 

 

Thanks, Jim, very useful list. Do you happen to know if these connectors can be unplugged? I should probably have mentioned that I want to connect two or more separate boxes, and not simply two boards. I am trying to construct a modular system, where the user can arbitrarily change sub-units, so it is important that the cables can be removed a number of times.

Zoltán

 

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 How about an HDMI connector, 19 pins, you can find HDMI to HDMI cables in different lengths.  I thnk there are mini versions as well, don't know how many connections those have...

 

Jim

 

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ki0bk wrote:

 How about an HDMI connector, 19 pins, you can find HDMI to HDMI cables in different lengths.  I thnk there are mini versions as well, don't know how many connections those have...

 

Jim

 

 

This is a very exciting suggestion! The mini version has 19 pins, and measures 11 mm in width. The micro is some 6 mm wide, but has only 9 pins. It is not even so difficult to solder as the USB-C, because the pads are in a single row.

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A DIN connector is another possibility, they can be found with up to 13 pins.  

 

Jim

 

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Don't you need to tell how much current is going through those power lines?  And perhaps the speed of the signals along with the environment with respect to noise, so the corresponding cable can be appropriate?  Shielding, twisted pairs, ...

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theusch wrote:

Don't you need to tell how much current is going through those power lines? 

 

A couple of hundred mA, not more. In fact, since the HDMI cable has a couple of spare pins (19 versus the required 12), the current should really not be an issue.

 

theusch wrote:

And perhaps the speed of the signals along with the environment with respect to noise, so the corresponding cable can be appropriate?  Shielding, twisted pairs, ...

 

The fastest signal is I2C, the rest are in the kHz range, or even slower. I think the mini HDMI cable that Jim suggested should do. Micro would be fantastic, but that has only 9 lines:(

 

 

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zoltanvoros wrote:
A couple of hundred mA, not more.

That is a lot, isn't it, for "high density" which implies tiny contact points and tiny wires in the mating cable?

 

Perhaps not so bad after all, with the short run.  A quick Google search indicates that 30 gauge wire, single conductor in free air, is rated at a few amps.  Derating factor in a bundle is 0.5 or greater indicating roughly an amp or so.

 

On the other hand, another table http://www.powerstream.com/Wire_... indicates 140mA for 30 gauge.  Now I need to find a third...

 

http://www.engineeringtoolbox.co... doesn't give a number for smaller gauges.

http://cablesondemandblog.com/wo... has the same table as the 140mA above.

 

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Last Edited: Wed. Jun 14, 2017 - 08:07 PM
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theusch wrote:

zoltanvoros wrote:
A couple of hundred mA, not more.

That is a lot, isn't it, for "high density" which implies tiny contact points and tiny wires in the mating cable?

 

Perhaps not so bad after all, with the short run.  A quick Google search indicates that 30 gauge wire, single conductor in free air, is rated at a few amps.  Derating factor in a bundle is 0.5 or greater indicating roughly an amp or so.

 

The HDMI micro can carry 0.5 A per contact.

 

And somehow I made a mistake earlier in the thread: The micro HDMI has 19 pins altogether, so that is definitely a strong contender.

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Jim, theusch, many thanks for your suggestions and comments! The HDMI is a most satisfactory solution. As a bonus, with a type-A-type-D cable one can daisy-chain the modules, because type A can be assigned to the output, type D to the input. In each module I have an I2C repeater, and that is why the configuration is not entirely symmetric.

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ki0bk wrote:

Alternatives:  https://www.digikey.com/products...

 

Lots to choose from!

 

Jim

 

 

Pretty good list Jim, thanks!

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That's the beauty of finding an industry standard and going with it.  You can get extensions and gender-benders and whatever all for not much money.

 

I started reading this thread thinking "VGA" but HDMI is better.

 

S.

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I too started with VGA, but having just purchased a Rpi3 to play with, HDMI was on my mind!

Jim

 

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Here is a warning and guarantee. If you build a device that uses a standard connector such as a USB connector in a non-standard way you can guarantee that eventually someone is going to plug a USB device into it. Bad results will soon arrive. For all that is good in electronic design, make sure you can't plug a cable into the wrong connector.

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Hafta admit I'm kinda curious where you get connectors that are NOT industry standard.  I've built a gazillion systems with DE-9 connectors, both male and female, and the only ones that got a serial port plugged into them were the ones intended for it.  Nobody ever tried to plug an EGA monitor or a serial mouse into one (at least, that they admitted).

 

I do agree with you in principle, and at least on standalone systems I generally try to use different connectors (either by size or gender or both) wherein mixing up the cables is a) easy and b) catastrophic, but if the user won't follow the labeling and something goes bang, that's their ass.  Making them responsible is a great way of making them careful.

 

Yes, things do go bang occasionally, too.  I give them about the same respect as they would get from sticking a fork into an electrical socket.

 

As is true of pretty much all engineering, it's a tradeoff.  If you want distinct unique connectors for everything, you can have it - for a screaming cost.  For something designed for the International Space Station, maybe that's appropriate (not really.  My ISS design used DB-25s).  For something cheap and cheerful like an egg-timer or a teabag dunking machine, I'll go with the standard with the cheapest cabling costs - And those tend to be the mass-market standards, because every cable manufacturer in China is cranking them out by the container load.

 

It's also worth noting that little of what I make goes out into the wild public - Almost all of it is custom-made for a single customer who gets instructions on what to connect where.  Even then sometimes they get it wrong, and I get to tell them their warranty don't cover that.

 

S.

 

PS - not sure anyone does this anymore, but an old dirty trick was to pull a certain pin from the male connector(s), then plug up that corresponding hole in the female socket.  Pull different pins from different connectors, and there's your unique connectors.  Of course, some simian would just mash them home anyhow, and then wonder why a) it went bang, and b) why there's all these bent pins on their connectors...  S.

 

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PS - not sure anyone does this anymore, but an old dirty trick was to pull a certain pin from the male connector(s), then plug up that corresponding hole in the female socket.  Pull different pins from different connectors, and there's your unique connectors.  Of course, some simian would just mash them home anyhow, and then wonder why a) it went bang, and b) why there's all these bent pins on their connectors...  S.

 

I make my connectors fool proof that way.  Well, fool resistant anyway.  It's apparently good enough to keep me from doing it wrong.  My Intel motherboard does this on the 2x5 USB headers.  Let's put it this way.  The more intelligent folks do it.  smiley

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A couple of hundred mA, not more

Right. Shorter than a metre.

A quick Google search indicates that 30 gauge wire, single conductor in free air, is rated at a few amps

Call it 0.2 A.  Call it a metre.  Call it 30 AWG.

 

Voltage drop per metre will be:

E = IR

  = 0.2 * 0.3386

  = 0.06772 V

 

With a 1 metre cable, that's actually 2 metres of conductor (+V and GND), so your +3.3V feed will actually drop to 3.16V, or -4%.  Will your gear on the other end tolerate that?  Note also that GND will be above 0V as a result.  More so if you're also pulling substantial current from the +12V line.  This can negatively impact signalling on the other lines.

 

As you've noted, you can mitigate some of this by using multiple conductors in parallel for the GND, +3.3V, and +12V.  However, you should do the math and engineering to predict the behaviour of the gear at the other end and the system as a whole before settling on a solution.

 

Not all cables are equal.  A quick Google search suggests that 'cheap' HDMI cables are 28 AWG, and better quality cables are 24 AWG or even 22 AWG, so the arithmetic above may not apply.  Know what you're getting before handing over any shekels.

 

 

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I sometimes use 2mm squeeze on connectors that use 1mm ribbon cable.  They are somewhere between the 0.1 inch and the 0.05 inch stuff, but maybe less common.

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Another thought would be to send a much higher voltage (48?) down the cable and use DC-DC convertors to make the required voltages at the other end.  Actually, the 12V would probably do if your system isn't sensitive to the 12V rails.  12V - 3.3V DC-DC convertors like this: http://www.cui.com/product/resou... can be had for $4ish at Digikey.  Probably cheaper at Jameco.

 

S.

 

* Repaired the url. Ross *

 

Last Edited: Tue. Jun 20, 2017 - 11:35 AM