USB as product-internal interconnect system?

From: Chris Graham (chrisgr_at_shaw.ca)
Date: 01/20/05 

Date: Thu, 20 Jan 2005 04:48:37 GMT

I'm designing a product that includes a number of interchangable boards that
give various capabilities, each with its own small microcontroller and
sensors/actuators. Any modules in a system would be connected to a more
powerful main board (basically the reference design of a PDA). It must be
easy for the user to open the box and plug in new modules, or plug them into
sites on the outside, and also for third parties to provide modules that
also plug in.

I was designing my own interconnect standard that included power and
bidirectional data lines (ideally at least1mbps), maybe based on RS485, and
a way for devices to announce their type and capabilities. As I worked on
the design I started to realize I was reinventing USB. I've been looking at
the details of USB, and it certainly does all I need and more.

Also, fortunately, the central processor board is based on a PDA that has a
USB host capability (and HID driver, which my modules could use), and I
could use small USB enabled processors in the modules. I guess I'd also
have to provide a hub based on one of the available hub chips.

I was hoping this group could give me a sanity check before I proceed to
design a system based on USB.

- Is anyone aware of other products using USB in this way?
- Any other approaches to plug-and-play or mix-and-match smart modules in a
system?
- To keep the costs down I would not want to use standard USB connectors
internally - just headers (but still shielded cable). It this likely to
cause EMI problems? I would have to use the 12mbps version of USB to keep
latency low.

Thanks for reading all the way to the end :-)

- Chris Graham

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