Re: Positional/physical addressing

On 6/25/2025 10:40 AM, Ian wrote:

On 2025-06-23, Don Y <blockedofcourse@foo.invalid> wrote:
A thought...
 If you have a chain, you get the "downstream failure" issue.

Yes.  But (reading ahead), note that this can also manifest as a
failure during the configuration phase.

If you have a bus, each device nees an address.

And, each device can "jam" the bus -- another failure mode.

How about having both. Use the chain to assign addresses at power-up
or whenever, then use the bus for real communication. If the devices

I had considered this -- and a "bypass" that effectively shorted
the "input" to the "output" thereby turning it into a bus
as it also allowed the "failed" device to remove itself.
E.g., design a simple switch controlled by a watchdog mechanism
that closes the switch when the device appears to be malfunctioning,
thereby removing it from the chain.
Of course, you still have to ensure physical removal of the device
effects the same change!

have some NV storage, the address assignment can be a 1-off, so the

I control power to the "display" so can always cycle power and reinitiate
a discovery phase.  I.e., there is no need for a NV store.  "Discovery"
can be designed to ensure there are no duplicate addresses created.

whole chain only needs to work "once" (assuming a dead device
just keeps quiet and doesn't kill the bus).

The last is the assumption that has to be examined:  What are
the *likely* failures and how will they manifest?

------            -----     -----     -----     -----
       |          |     |   |     |   |     |   |     |
 ctrlr |--------->| Dev |-->| Dev |-->| Dev |-->| Dev |-----> Chain
       |          |     |   |     |   |     |   |     |
       |           -----     -----     -----     -----
       |             ^         ^         ^         ^
       |             |         |         |         |
       |             v         v         v         v
       |<---------------------------------------------------> Bus
       |
------

  ------            -----       -----     -----      -----
        |          |     |    |     |    |     |    |     |
  ctrlr |-------+->| Dev |-+->| Dev |-+->| Dev |-+->| Dev |--+---> Chain
        |       |  |     | |  |     | |  |     | |  |     |  |
        |       |   -----  |   -----  |   -----  |   -----   |
        |       |          |          |          |           |
        |       +----/ ----+-----/ ---+-----/ ---+-----/ ----+  Bus
        |
        |          Bypass "switches", one per device
I am increasingly convinced that I need to just see if I can
find a (dirt cheap) MCU with built in (factory) serial number
and incrementally poll for S/Ns during network discovery.
Having the *factory* (manufacturer) guarantee the uniqueness
of these identifiers means I'd never have to worry about
duplicates or assigning identifiers, etc.
I'd still have to address likely failures that could compromise
this shared bus as it would render the entire mechanism useless.