Author Topic: Q on board to board connections  (Read 284 times)

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Offline Jonathon_Doran

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Q on board to board connections
« on: June 17, 2021, 03:01:06 am »
I am inexperienced in PCB design, but have been a hobbyist for decades.  I am finally giving in to the idea of having someone else manufacture a board for me.  (I tried the do-it-yourself thing with a laser printer about 20 years ago, with mixed results).

For my current project, it is a bit big (600x150mm).  PCBShopper shows two vendors, with the US supplier the cheapest!

I have more options if I were to chop the board into two chunks, but the cost goes up.  That makes sense, setup takes time/money.  That isn't the problem.  This is a hypothetical, because I don't have enough problems right now.

If I needed to chop boards into pieces for fabrication, would it be feasible to create board-to-board connections if the signal frequency was low?  What would I need to take into account?

Would some headers and a ribbon cable work?  What if I put in Bus/Line drivers?

My current project is running low frequency, 10-20Hz, low voltage (3-5V).
 

Offline rx8pilot

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Re: Q on board to board connections
« Reply #1 on: June 17, 2021, 03:10:20 am »
It sounds like pretty much anything will work from what you are describing when you are pretty much DC low voltage and current.

What 'line drivers' or 'bus' are you talking about? I design 12Gbps line driver boards and that is a very serious challenge. If you are talking about RS-232 - it is a totally different thing.

I tend to choose the easiest to work with connectors that can still maintain signal integrity. Through hole headers are common and nice, but end up being manually assembled parts. There are SMT B2B connectors that are pick and place friendly, which can eliminate the hand assembly. That may or may not be a factor for you.

Lots and lots to choose from.
Factory400 - the worlds smallest factory. https://www.youtube.com/c/Factory400
 

Offline evb149

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Re: Q on board to board connections
« Reply #2 on: June 17, 2021, 03:32:37 am »
At 20Hz frequency and correspindingly low edge rates (rise time, fall time) you can do anything you want for connections and as far as signal integrity goes it will behave mostly like DC.
You have to mind the ampacity of the interconnect if you're sending more than modest numbers of 10s-100s of milliamps around for power purposes obviously sending 1A aross a long 30AWG ribbon cable isn't really the most ideal thing because of connection resistance etc.

But for connectors, yeah, ribbon cable and headers is just fine for the signals and even for a bit of power if you mind the cable rating and installation environment, temperature rise, the possible oxidation / fraying of connections over time, etc.  So maybe double or quadrapule up the power feeds and their return currents.

For signal integrity is is good practice to have one signal wire associated with one return current wire (e.g a differential Signal+, Signal- pair or a SignalX, SignalX_Ground unbalanced pair).
But at slow millisecond to second edge rates you don't even have to mind that quite so much and you can have a few single ended signals per signal ground return conductor.

The point is to avoid large inductance loops, so the separation of a signal wire and the return conductor for that signal shouldn't form too much of an area of a loop depending on the way you assign the signals and returns in a connector / cable.

The other things that are possibly problems which line drivers / receivers might help with are :
1: ESD -- self explanatory, surges, spikes, static discharge -- protect the fragile ICs with either discrete transient protection or adequate integrated protection in line drivers / receivers, or both.  If you have something that is rated to withstand several kilovolts of ESD then that's reasonably good for casual use across small distances in like office / home environments.

2: Ground potential differences -- this can happen if you have different grounds connected externally to the system between different points.
e.g. Board A has a RS-23 cable to a PC-A which is ground referenced to that PC and its electrical outlet.
Board A and Board B are linked by whatever cable interconnect they have.
Board B has a audio cable on a RCA jack going to stereo system B which is grounded according to wherever that is plugged in.

And the "ground" isn't exactly the same location / source between some number of these different connectors and power supply feeds and signal connections so maybe there is ground noise or ground potential difference somewhat between these different grounds.  And that noise / potential difference eats up your line driver / receiver signal tolerance margin particularly for single ended signals.  Differential / balanced lines and drivers / receivers help combat a larger amount of ground potential shift since they're differential and not just ground referenced.

RS-422, RS-485, RS-232 are all kinds of line drivers / receivers that can be reasonably handy for low speed use across modest distances in office / home environments.

Even just using CMOS / TTL lines interfaced through bus drivers like 74HC244, 74HC245, etc. can be "ok" for a few feet of ribbon cable driving in good environments but you will still not have nearly as good ESD protection or edge rate control or noise immunity as you'd have with lower speed more robust line drivers / receivers like RS-xxx ones.  Your choice.

If you're particularly worried about faults on the cable or ESD or such you could even use optocouplers or signal isolators of other kinds for logic level or similar digital signals.  But there is no requirement for that in the cases you mentioned unless harsher environments, longer distances, or more robustness against accidents / spikes / power supply errors etc. is required.



 
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Offline evb149

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Re: Q on board to board connections
« Reply #3 on: June 17, 2021, 03:48:31 am »
It has been too long since I used this stuff a lot to have handy bookmarks for application notes about basic signal communications at low speeds and short distances
like you need, but here's a few links to pages which include some representative links to application notes, tutorials / articles, some transceiver parts, etc.
No particular suggestion, just to familiarize yourself with the kinds of interfaces / transceivers / standards that exist, some of the chips, etc.
then you can make a more informed decision for now and future growth / use.

https://www.maximintegrated.com/en/design/technical-documents/app-notes/3/3884.html
https://www.maximintegrated.com/en/products/interface/transceivers/rs-485-rs-422-transceivers.html
https://www.maximintegrated.com/en/design/technical-documents/tutorials/7/723.html
https://www.maximintegrated.com/en/products/interface/transceivers/MAX3380E.html
https://www.maximintegrated.com/en/design/technical-documents/tutorials/2/2020.html
https://www.maximintegrated.com/en/design/technical-documents/app-notes/8/882.html
https://www.ti.com/product/TRS3243E

TI, Maxim, National (TI now owns) all had some good application notes and line driver / receiver parts since the 1990s so there's probably
all the background information you need if you look at some of the PDFs they have about signal transmission over XX meter distances at XX speeds and X level of ESD protection / noise immunity etc.

It is hard to go wrong with short distances and such low speeds & edge rates.  ESD is probably the biggest worry.
 
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Offline evb149

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Re: Q on board to board connections
« Reply #4 on: June 17, 2021, 04:01:54 am »
Re: connectors / cables:

* Ribbon cable or FFC -- good for compact, small size connectors with lots of wires/pins.

* Modular cables / connectors  like RJ45, 8P8C (similar to ethernet) good for just a few signals / wires e.g. 1-8 wires / connector, but very attractive for cheap easily available cable that is very easy to make / run / terminate with low cost tools and techniques.

* Old style RS-232 cables usually in the range of 4-25 conductors plus possibly a shield.  Ideally twisted pair but not always made that way.  Good for RS-xxx type interfaces (choosing the appropriate cable for the interface type / connector and requirements).  More expensive cables.  Harder to self make.  Very low cost cables are probably not that good quality.  But excellent rugged cables for indoor or outdoor use exist.  Connectors relatively easy to work with on a PCB and wire up / terminate if needed but not as easy as ribbon cable / modular jacks.

* Terminal blocks and discrete wires from pre-made round cables... good solution, competitive options exist with typical kinds of RS-xxx cables.  2 conductors ... dozens.  Pluggable headers, screw terminal blocks, spring latch terminal blocks, etc. all make wire to board termination very simple with high quality modest cost PCB terminal connectors readily available.  Ability to terminate wires or ferrules / pins crimped to wire ends easily is nice.  Probably the easiest to re-wire / modify.

* There are also IDC based "push in" wire to board connectors that are typically used to terminate a small number of wires which can be signal or power to a PCB easily.

 
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Offline Jonathon_Doran

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Re: Q on board to board connections
« Reply #5 on: June 18, 2021, 08:00:39 pm »
Rather than use one large board, could I use two or more adjacent to each other with some connection. I was thinking of driving signals from one board to another.

I was checking to see if this was a reasonable thing to do, and if I needed to do anything special.
 

Offline Jonathon_Doran

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Re: Q on board to board connections
« Reply #6 on: June 18, 2021, 08:06:08 pm »
Thanks for a great deal of information, that was exactly what I needed.

I don't believe that I need to down this road now, but possibly in the future.
 


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