Perhaps I'm old school, but this sounds overly complex to me. Probably the most important thing I learned in engineering school (besides where the beer and amiable consorts were) was KISS. The more complex, the less likely the project is to complete.
So I pose the question: Do we need a bus like this at all to meet the basic goals? The suggestion of a main board with input/output modules that plug into the main board sounds much simpler. If the IO needs to be modified, you build a separate module. If you think there are going to be parameters that will be switched on/off/adjusted/whatever on that module, put a few io pins from the main controller on the connector to the module. May never get used, but if someone wants to rework the i/o module, you're all set. Many of the popular controllers can put an I2C bus or a serial port on any pin pair. Libraries for both are common for the controllers I'm familiar with. Example: How much complexity do you need in an input module? What are the features you might like to manipulate there? Would it really take more than, say, 8 pins? The discussion about opto isolation and/or differential pairs and the like seems like overkill as well. My vision is that this thing fits into a single box. If you need that much isolation or are worried about that much noise on a bus inside a box, either you are pushing some serious speed or operating in an environment that's way more hostile than I would like to be sitting in and observing the front panel. With that said, I also concur that any module should be easy to test in a standalone mode. Not necessarily useful to many folks outside of the opencontroller project, but easy to test. That's why, for example, I suggested that the core counter module be usable with nothing more than TTL inputs/outputs. An input module then might be nothing more than appropriate signal conditioning. Final design, will, of course, dictate pcb sizes. Bob On Wed, Dec 22, 2010 at 9:05 PM, Chris Albertson <[email protected]> wrote: > On Wed, Dec 22, 2010 at 3:32 PM, Poul-Henning Kamp <[email protected]> > wrote: > >> I used a shared opto-isolated async bus. You need two optocouplers >> per microcontroller, and one place you power the shared bus, and >> you're all set. >> >> I have yet to see an microcontroller without an async port. > > Opto-isolater? Why not just use fiber cable between cards. I know > it sounds exotic but also seems to have half the parts count. those > s/pdif jacks are so cheap and I bet you can use them as pretty much > drop in replacements for opto-isolators. Would s/pdif jacks work as a > physical layer? > > Really I just used I2C in my write-up as a place holder. I you say > nothing no one ever says a blank paper is wrong and suggests something > better. > > s optical isolation required when all the modules are sharing a common > power supply? Does this means all the coax connectors need to be > isolated. I guess some one better step up and propose a grounding > scheme. That's not going to be me. > ===== > Chris Albertson > Redondo Beach, California > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
