JST GH 4 connector - no shielding acceptable?

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Eirik Lund

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Jun 11, 2017, 9:15:51 PM6/11/17
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Just found this forum,

I really see the value with a UAVCAN interface/specs/protocol implementations, thanks!


The JST GH connectors are small and "handy" on the PCB, but what about shielding?

We plan to integrate high sensitivity RF receivers in a fixed wing proto UAV (433MHx, 915 MHz) for telemetry and radio control.

Both receivers should have good preselection (filtering) on the front end, but having an "open" unshielded, not twisted (flat) cable, with a CANbus signal makes me a bit concerned about how this will affect the effective noise floor for these receivers. 
Digital noise from the CANbus could impact on (to mention some):
  1) Noise Floor, digital "semi" high-speed signals are broadband by nature
  2) Spurs entering into the receiver on the RF side
  3) Blocking (if poor received design or high gain enabled on LNA/front-end)
  4) Signal entering into the power lines for the LO (phase noise)
  5) Signal entering into the IF or chain of A/D converters (also on the A/D sampling side on a "zero IF receiver")
  ...

I understand the H//L signals are symmetrical "inverted" around CAN_ground, but with the JST GH 4 connector recommended, all wires in the cable are in parallel on the flat cable used (sorry if I got this wrong).

Any comments or real experience with noise from any of the CAN devices, or if any evaluated other connectors?

If this protocol with it recommendations are intended to be used in a high reliability /mission critical applications, I think this could be a "show stopper".


Sorry, I do not want to be negative here, just a bit concerned ...
















Pavel Kirienko

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Jun 12, 2017, 12:49:02 PM6/12/17
to Eirik Lund, UAVCAN
Hello Eirik,

Normally the wires CAN H and CAN L should be twisted, otherwise, long buses may exhibit frame losses (most of the commercially available UAVCAN-compatible cables, however, tend to be flat, but they work okay for light UAV systems nevertheless).

If you're looking to improve your EMI situation at a low cost, you should just use a shielded twisted pair with JHT GH. Standard USB 2.0 cables work great for that because they already incorporate a single signal twisted pair alongside a 0.5 A capable non-twisted power pair, which is exactly what is needed for UAVCAN.

Lately, I was living with the idea of extending the Hardware chapter of the UAVCAN specification with an alternative connector option derived from the CiA 303 document. The physical layer specification proposed there is a de-facto industry standard for many CAN bus applications, so using that is a big win from the standpoint of the ease of adoption. There are lots of types of compatible DB-9 connectors and cables available commercially that cover a wide selection of requirements: there are shielded connectors, waterproof connectors, vibration-resistor connectors, etc.

Inline image 1

Actually, I'm going to hijack this thread to collect comments to the above-proposed idea. Please respond here.

Pavel.

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Eirik Lund

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Jun 12, 2017, 1:44:16 PM6/12/17
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Thanks Pavel for diligent respond,

 

Yes, it could be an idea with USB 2.0 cable, but I think it is designed to have a 90 ohm impedance. Maybe not that critical (vs 120 ohm), but I would not trust/use that, unless I measured the impedance to check.  Also a lot of these USB cables are made under not controlled environment/loose specs, I do not want to use that in my UAV.

 

I guess my point is that the connector recommended here : http://uavcan.org/Specification/8._Hardware_design_recommendations/

 

(in my opinion); is not that well suited for UAV’s.

 

  1. It is not made to withstand vibrations
  2. There should be 5 pins (or even 6), one (or two) for the shield(s), it may or may not be connected to ground (*)
  3. It is not too easy to add shielding (it can be done)
  4. It is simply not intended for critical applications (at least I can’t find hard specs, vibration etc.)

 

(*) Also to add better ESD protection (it can be a lot if static developed in the air, dry air over the wings, not to mention lighting..

 

If I develop some hardware, I do not want a design/PCB with a very critical component not up to specs.

 

I think that using all 4 connectors ( 2 x primary + 2 x secondary) in a fault tolerant setup, will simple be defeated by the JST connector.

 

From many years of field (army) experience,  with missing critical system, I know that connectors are one of the single most point of failure…

 

I consider to use the Harwin Gecko (1.25mm pitch, or Harwin L-Tek or J-Tek). Yes they are very expensive, for a good reason…

 

/eirik

 

 

I have looked a bit into the various connectors. I do not think the current recommended JST will suit my need:

 

- it only has 4 pins, should  have at least 5 to allow separation between GND and shield

- It is not well suited for shielded cables

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Pavel Kirienko

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Jun 12, 2017, 4:45:49 PM6/12/17
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Hi Eirik,

Your criticism of JST GH is not entirely unwarranted. When the DroneCode foundation converged on this connector type, they had to evaluate other aspects besides the ones you listed; most notably cost and availability. It is true that this connector type isn't designed for mission-critical applications, but it is probably still a near-optimal choice for light civilian UAV, which aren't (yet?) subjected to strict reliability requirements.

If you're looking for a mission-critical high-reliability solution, you should consider hi-rel D-Sub connectors I mentioned earlier, e.g. http://www.ckswitches.com/media/1844/a2_23apr13.pdf. There are loads of them on the market, and since they all are mutually compatible, you're free to choose whatever suits your application while retaining compatibility with 3rd party hardware.

Pavel.

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Lorenz Meier

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Jun 12, 2017, 4:56:41 PM6/12/17
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Hi Eirlund,

The Harwin series you mentioned isn’t offering shielding either - have you considered micro-D?

That said its much bigger (factoring in backshells) than the Harwin series. But you can buy IP-rated variants if you really care about maximum ruggedness. The Harwin offering certainly is attractive, but the question is: If someone feels JST GH is not robust enough, are the Harwin connectors cutting it for a case-to-case connection or is it much rather a shielded, IP-rated part that is required?

-Lorenz


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Pavel Kirienko

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Jun 12, 2017, 5:07:36 PM6/12/17
to Lorenz Meier, eir...@online.no, UAVCAN
The post submitted by Lorenz reminded me about another consideration that I forgot to include in my previous answer. The widely popular D-Sub connectors may not be ideal from the standpoint of their mechanical dimensions and weight, but their huge advantage is that there is a lot of hardware parts out there that are readily compatible with them. So I would advise to try and avoid sticking to the referred Harwin and Molex connectors, if possible, in favor of any brand of the standard D-Sub DB9, which are not vastly different in size and weight anyway, in order to avoid unnecessary fragmentation of the ecosystem.

Pavel.

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Eirik Lund

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Jun 12, 2017, 5:17:48 PM6/12/17
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Yes Pavel,

 

That’s just my two cents 😊

 

Just trying to evaluate/suggest what could possible add value also on the HW side.

 

I really like the UAVCAN idea, and maybe it could be an idea to suggest also some other connectors then?

 

I think this could add value to the  UACCAN project, in particular where reliability is needed. As today most long endurance system use fuel motors, they vibrate a lot. Also the more “long endurance” UAV systems are needed, the more issues like reliability, redundancy, single point of failure, and specs. on vital parts  become prevalent.

 

To have a redundant bus system with the JST GH, without dedicated shielding pin, seems like a candidate for improvement (or to have a “Hi-Rel” recommendation)

 

Product development effort (investment) considered,  I think the UAVCAN deserves a better connector to “match” the software and protocol standard.

 

Also if the community plans to develop several HW modules (PCB’s/product), it could perhaps be wise hint about this, again maybe a ““Hi-Rel” recommendation.

 

It could also be left to the developer, but some kind of standard would/could be nice..

 

 

I think the Hi-rel Micro-D Connector also could be a candidate, just a bit worried about the weight and size..

 

https://www.digikey.no/en/ptm/h/harwin-inc/gecko-micro-miniature-connectors/tutorial

 

According to Harwin, the Gecko has a 45% smaller footprint and offer a weight saving up to 75% compared to the Micro-D.

 

https://youtu.be/Sk5bunbM5lI?t=30

 

/eirik

 

 

 

From: Pavel Kirienko [mailto:pavel.k...@gmail.com]
Sent: Monday, June 12, 2017 22:45
To: eir...@online.no
Cc: UAVCAN <uav...@googlegroups.com>
Subject: Re: JST GH 4 connector - no shielding acceptable?

 

Hi Eirik,

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Eirik Lund

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Jun 12, 2017, 5:40:02 PM6/12/17
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Hello Lorenz,

 

Yes I did consider the micro-D from some different vendors.

 

According to Harwin, the Gecko has a 45% smaller footprint and offer a weight saving up to 75% compared to the Micro-D.

 

https://youtu.be/Sk5bunbM5lI?t=30

 

The Gecko also have very impressive hi-rel (Space) specification’s, they come in board-to board and board-to-cable variants, with several options for locking.

 

https://www.digikey.no/en/ptm/h/harwin-inc/gecko-micro-miniature-connectors/tutorial

 

Regarding shielding, I am in search for a circular shielded cable, then I would make the casing (printed carbon?) with a tab that closes up the missing shield on the Gecko. The micro-D would be  better from a shielding point of view, but I do not like the added size and weight. It’s kind of a trade-off.

 

The Gecko connector with is very expensive tools for making cables/wiring hardness will be well suited for a shielded twisted pair, just the connector itself that twill not be shielded.

 

/eirik

 

From: uav...@googlegroups.com [mailto:uav...@googlegroups.com] On Behalf Of Lorenz Meier
Sent: Monday, June 12, 2017 22:57
To: Kirienko Pavel <pavel.k...@gmail.com>
Cc: eir...@online.no; UAVCAN <uav...@googlegroups.com>
Subject: Re: JST GH 4 connector - no shielding acceptable?

 

Hi Eirlund,

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Pavel Kirienko

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Jun 13, 2017, 4:59:20 AM6/13/17
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Eirik,

I don't think that the options suggested so far are viable competitors to the classic D-Sub. The moderate advantages in the size and weight that they offer are unlikely to outweigh the disadvantages of the increased cost and reduced availability. A quick look around on DigiKey reveals that shielded Micro-D options tend to be 2~7 times more expensive than equivalent D-Subs (these are not hi-rel though, but I expect a similar cost ratio for hi-rel as well).

We should be looking at the whole picture rather than focusing exclusively on the tech specs.

Pavel.

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Eirik Lund

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Jun 13, 2017, 9:48:16 AM6/13/17
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Hello,

 

Hmm.

 

For our use, a normal size D-type seems too big. I will try to illustrate it below;

 

Compare: D-Sub, Amphenol, L17SM209SB64T (9 pin SMD), USD $5

https://www.digikey.no/product-detail/en/amphenol-commercial-products/L17SM209SB64T/L17SM209SB64T-ND/3464957

Note that we only consider connectors with high reliability, it will be the most important “value added” to the hobby grade connectors, this connector at $5.6 is only rated as a “commercial product”.  If we consider a redundant system with two buses (as recommended), and both with 2 connectors (also as recommended) the total size will be:

4 connectors x 39.4 mm wide  x 12.7 mm depth on each board, rounded up to about (4*40*13) = +- 2 000 mm2 (!), or 45 x 45 mm.

L17SM209SB64T Amphenol Commercial Products | L17SM209SB64T-ND DigiKey Electronics

Please also note that the D-sub series only goes down to 9 pins.

 

Compared to: Gecko type, Harwin G125-FS10605L0P (6 pin SMD), USD $8

https://www.digikey.no/product-detail/en/harwin-inc/G125-FS10605L0P/952-2617-ND/4952760

4 connectors x 6.3 mm wide x 5.8 mm depth on each board, rounded up to about (4*7*6) = +- 170 mm2), or 13 x 13 mm.

G125-FS10605L0P Harwin Inc. | 952-2617-ND DigiKey Electronics

Mating, pins to be inserted making a cable.

 

To me that is a huge difference (2000 mm2 vs 170 mm2)!

 

The main idea/thesis is that it only makes sense to use this (vs. the JST GH 4) in a critical system, where it will be 4 connectors required.

 

Benefits:

  • Small foot print
  • Light
  • Very high reliability (and fully specified for traceability)
  • Separation between GND and SHIELD (*)
  • Low cost (comparing to other MIL/space connectors)¨

 

Disadvantages:

  • Proprietary design, souring could be a problem
  • Expensive tools (tools to make cable will cost about USD $ 1500

 

If using a 6 pin connector we would also separate the CAN_GND, POWER_GND and SHIELD, so it will be 6 pins (with the CAN_H, CAN_L, POWER)

 

(*) Again is (here) important to separate between GND and SHIELD, not only because of EMI, but also ESD. An electro static discharge (as it will be in the air) into the shield, at any place, will directly impact the GND, and could cause all CPU/MPU (also they redundant one) on the boards to boot or just lockup. The use of two buses and any redundancy/back up will be defeated.

 

So, based on the above, I do not think we can use neither the JST GH, nor the (normal( D-type. The D-sub was introduced in 1952, it was small then, but not for an UAV today.

 

I think the micro-D, could be an option, and they are also specified for high-reliability, and they would offer the best shielding. But  again they only goes down to 9 pin.

 

Compared to: Micro-D, Molex (9 pin TH), USD $12

https://www.digikey.com/product-detail/en/molex-llc/0836119006/WM2142-ND/1001287

4 connectors x 19.94 mm wide x 10.41 mm depth on each board, rounded up to about (4*20*10) = +- 800 mm2), or 28 x 28 mm.

0836119006 Molex, LLC | WM2142-ND DigiKey Electronics

Advantage:

  • Available from other sources
  • Very high reliability (and fully specified for traceability), expensive (USD $100+)
  • Separation between GND and SHIELD (*)

 

Disadvantage

  • No SMD?
  • Heavy compared to the Gecko
  • Size (800 mm2 vs 170 mm2), remember only goes down to 9 pin

 

Based in this we will for now based our design on using the Harwin connector, but that could change 😊

 

This is about how much time I can spend on this connector issue, take it for what it’s worth.

 

It was good anyway to have this reviewed with all you feedback, thank you again.

 

/eirik

 

 

 

 

 

From: uav...@googlegroups.com [mailto:uav...@googlegroups.com] On Behalf Of Pavel Kirienko
Sent: Tuesday, June 13, 2017 10:59
To: eir...@online.no
Cc: uav...@googlegroups.com
Subject: Re: JST GH 4 connector - no shielding acceptable?

 

Eirik,

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