Design of remote monitoring and 'pluggable'  PCB standards - Page 1
Forum section: electronics / projects

author: philbx1
author_rank: Regular Contributor
timestamp: October 06, 2011, 08:15:29 am
content: Hi all,Just me thinking again, and thought I might ask if there is a miniature 'standard' in pluggablePCB's that is more prevalent in compact portable applications.I've seen mention of the Eurocard and PXI standards in the Open Source multimeter discussionbut have yet to find a 'standard' that uses minimal space.Basically what I'm after would have (for instance) a backplane of maybe 60 x 30 mm, a horizontal 'main board' oblique to this of maybe 25W x 50L mm, and various vertical module PCB's being25H x 50L mm.These 'modules' (IE. battery monitoring, FET, gyro, temperature etc.) can use any standard protocol, but I'll be leaning towards I2C as I think (maybe wrongly) that it's pretty much a microcontroller standard now.If this is all a bit confusing I might just try using Sketchup to show you what I mean, although that'sa painful area to go to after trying it out for a while now..Anyway, your thoughts are much appreciated.

author: daedalus
author_rank: Regular Contributor
timestamp: October 10, 2011, 10:04:40 pm
content: I2C is fine, but there are a few things you need to consider if you want to have redundancy, failover, and / or hot-plugging. Have a look at some of the TI I2C hub and buffer chips, the datasheets cover problems like what to do if a daughter board has a fault on i2c pins, how to handle correct start-up on hot-plug, etc. Its worth a browse even if you don't use their chips.http://focus.ti.com/paramsearch/docs/parametricsearch.tsp?family=analog&familyId=1651&uiTemplateId=NODE_STRY_PGE_T

author: philbx1
author_rank: Regular Contributor
timestamp: October 11, 2011, 07:48:55 am
content: Hey there Daedalus,Thanks much for the reply. I was thinking this was all a bit too much to ask from our forumbut you've given me some great info.BTW, I'm currently looking at Samtec Edge Card connectors etc. which look pretty neat.And yes, your thoughts on I2C protection will mean a lot with my design.Again, thanks.Quote from: daedalus on October 10, 2011, 10:04:40 pm

author: ejeffrey
author_rank: Super Contributor
timestamp: October 13, 2011, 09:54:58 pm
content: Standard is hard to reconcile with compact, especially if you want rugged, flexible, simple, and cheap. Increased integration and lowered cost of fabrication has also reduced the demand for such a thing: The cost of a backplane + connectors + multiple PCBs + an enclosure that supports pluggable modules has to be weighed against the cost of just building everything you might want on one PCB, and rolling a new revision if you discover a new sensor you want. You don't even need to populate chips you don't want to use, just put a footprint in.So you generally only see backplane standards where end-user customization is absolutely necessary, and/or the per-module cost is fairly high (see PXI). Otherwise you usually just see ribbon cables or flat-flex cables running serial protocols between boards when necessary, or people roll their own with only the pins you need.Possibly the closest thing to a standard in the small(ish) form factor is the arduino stackable shields. Not all shields play nice, but if you stick to I2C/SPI + power you shouldn't have any problem.

author: philbx1
author_rank: Regular Contributor
timestamp: October 15, 2011, 06:29:57 am
content: Hi Jeff,Thanks for the input, again much appreciated.Although I was thinking as a 'product' simple boards could be added for whatever purpose and have the base-boardavailable without too many board upgrades.Maybe an arduino 'mini-board' spec will appear in future?You are right though. Maybe too much thought and probably not enough action :-)Quote from: ejeffrey on October 13, 2011, 09:54:58 pm

author: Unknown
author_rank: Unknown
timestamp: October 31, 2011, 12:42:42 pm
content: Hi HissyfetWish I have seen this post earlier. I've been looking at many of the form factors out there to find out which ones are suitable for open source development.Unfortunately there aren't many that are not proprietary.There is CoreExpress for example which is 58 mm × 65 mm but in order to use it and claim compatibility you need to pay for membership and agree to NDA.Conditions for PXI and most others are similar. Frankly I don't see a reason to bother with any of these unless you are commercial and want to have a share of the market. A form factor is merely a pcb dimension and a bus slapped together, definitely not worth the royaties.One notable exception is maybe PC/104. As far as I am aware (but I may be wrong) they do not require membership or NDA in order to use their form factor. PC/104 was used for CubeSat(s) which are very small satellites designed by students and subsequently launched into space. So presumably it can be used for amateur work and is both robust and lightweight albeit a bit bigger than what you asked for at 90 x 95 mm. One advatage it has is that it is a stackable form factor so it does away with the backplane.A while ago I drew one of them in SketchUp to see how it looks like. The file is attached. It is quite superficial so check carefully if using for serious design work.So far my impressions arePros:+ Stackable does away with expensive backplane (PXI backplane runs around 700$!!)+ Alredy used in amateur work and even launched into space+ Small and lightweight+ Provision for connections on three sides of the pcb+ Some modules already available commercially (but not cheap)+ Can implement either PCI or PCIe or even bothCons- Specification not open source (maybe in the future)- Single source for the PCIe connector.- One of the mounting holes is misaligned to make the board polarized. This somewhat limits the possible mounting configurations- No provision for stacking analog boards which do not have big PCI or PCIe connector on them. If these boards are stacked they obscure the connector.- The spec does not say how the power will be distributed if you do not utilize the full PCI connector on analog boards. A possible couse for conflict.I've been mailing thehttp://www.pc104.org/people to ask if their specifications can be used for open source hardware. They kindly answer my mails however the issue is still moot. My guess is that even if they wanted to release their spec open source they still do not own the rights to PCI and PCIe so this may create problems for them.