Serial protocol with physical decoding

On 5/4/2017 1:26 PM, Mark Storkamp wrote:
> In article <590acce3$0$58351$b1db1813$19ace300@news.astraweb.com>,
>  Clifford Heath <no.spam@please.net> wrote:
>
>> On 04/05/17 16:14, Don Y wrote:
>>> I want to push a bit stream through a series of *identical* devices
>>> and have (identical) software running on each determine if that
>>> particular device is being addressed, or not.
>>>
>>
>> Why daisy-chain? If all devices have to be listening anyhow,
>> why not send the same data to all in parallel, and get each
>> one to validate the checksum and determine if the message
>> is intended for them. That way you can use broadcast (groups),
>> and one software failure doesn't take down the chain.
>>
>> Otherwise you start getting into USB-style distribution trees
>> with all the complexity of enumeration, routing, etc. Avoid
>> that if you can, would be my advice.
>>
>
> If each device is identical, and running identical software, how do you
> get the devices to not all think the message is for them?

The message is received by a node and the address is decremented before 
being passed on to the next.  Only when the message address reaches zero 
does the node accept it as being for it.


> How about a hybrid approach where the data stream is connected in
> parallel, but the enable is daisy chained. On initialization, only the
> first device would see the enable, and it would receive and acknowledge
> a message setting its address, then it would set the enable for the next
> device in the chain, and the initialization continues until each has
> received a unique address. This would also solve the synchronization
> issue since one address could be reserved as an 'all listen' address
> that could be used to initiate an action.

Sounds good to me, but I bet there is an issue that hasn't been 
described yet.  When a new node is added to the system the nodes would 
need to be re-synchronized.

This makes me wonder what the purpose of such a network would be.  It 
would be very hard to manage a chain of any length and distance so that 
you knew which node was which.

-- 

Rick C

On Thu, 4 May 2017 10:52:40 -0700, Don Y <blockedofcourse@foo.invalid>
wrote:

>I'd like to come up with a scheme that I could apply to a variety of
>similarly "open ended" network configurations -- that have very
>different physical characteristics (e.g., a 10 ft long chain
>and a 300 ft long chain -- trading data rate for network size)


At some point you need to worry about reinventing Ethernet.  And many
MCU's are available with Ethernet for a very small incremental cost.
Although perhaps not at the very low end.  A huge advantage is that
you can punt much of the infrastructure to the customer ("It's
Ethernet, deal with it.").

On 5/4/2017 12:53 PM, Robert Wessel wrote:
> On Thu, 4 May 2017 10:52:40 -0700, Don Y <blockedofcourse@foo.invalid>
> wrote:
>
>> I'd like to come up with a scheme that I could apply to a variety of
>> similarly "open ended" network configurations -- that have very
>> different physical characteristics (e.g., a 10 ft long chain
>> and a 300 ft long chain -- trading data rate for network size)
>
> At some point you need to worry about reinventing Ethernet.  And many
> MCU's are available with Ethernet for a very small incremental cost.
> Although perhaps not at the very low end.  A huge advantage is that
> you can punt much of the infrastructure to the customer ("It's
> Ethernet, deal with it.").

Depends on the nature of the "messages" being passed -- their
content and frequency.

E.g., would you spring for the cost of a NIC, MCU capable
of supporting a TCP (or even just UDP) "stack" -- just to
command individual hammer drivers on/off at some sub-sonic rate?

OTOH, running a bunch of wires from *one* set of hammer drivers
(accessed "in parallel" from an MCU port) out to a bunch of
solenoids dramatically changes how you wire them, how you
deal with expansion, etc.

With Ethernet, you'd have to ensure each node had a unique address
so you could distribute the message to all and know that the
right node acted on it -- AND, that no OTHER node acted on it!

By interposing each node in a serial chain, a node need not
care about it's "address" -- it's *implied* (when the ID byte
is 0, the message is for you.  Otherwise, its for someone
DOWNSTREAM from you).  So, you can produce identical devices
and still treat them as individually unique.

Of course, this makes binding addresses to physical nodes
a separate issue:  you either carefully document the
physical (electrical?) order of the nodes at deployment
*or* introduce a "configuration step" in which you
empirically derive the (address,node) mapping.

     "I just activated node 1.  What do you see as having
     changed in the field?  OK, I'll write that down.  Now
     proceeding to node 2..."

This would be tedious *if* the field configuration changed,
often.  But, if it is largely static and only updated as
needs evolve (or, in response to failures), then its almost
assumed that some degree of manual intervention will be
required (i.e., someone has to swap out the defective node
or splice a new node into the chain <somewhere> -- which
may not be at the *end*)

Am 04.05.2017 um 08:14 schrieb Don Y:
> I want to push a bit stream through a series of *identical* devices
> and have (identical) software running on each determine if that
> particular device is being addressed, or not.
> 
> It seems like the simplest way to do so is to require each device
> to examine an address field in the packet and, if '0', assume the
> data that follows is intended for its consumption.  If not,
> decrement the address and pass the packet along.

Aha, so this week you're re-inventing cable position based addressing. :-)

Depending on the number of nodes, the likelihood of equal data being
sent to more than one node, and the length of said data,
encoding the target selection as a bit pattern that you shift, instead 
of a number that you decrease, might be more efficient.

It can be be even better to have two modes of operation for the whole
chain: either the network is learning its own layout, or it's working 
normally. Only the learning phase really needs the extra complication of 
e.g. an "take the first word of this message to use as their own address 
from now on, remove it from the packet, then transmit any remainder 
further down the daisy chain" mechanism.  Normal operation can then use 
a more straighforward addressing scheme.

Don Y wrote:
> I want to push a bit stream through a series of *identical* devices
> and have (identical) software running on each determine if that
> particular device is being addressed, or not.
>
> It seems like the simplest way to do so is to require each device
> to examine an address field in the packet and, if '0', assume the
> data that follows is intended for its consumption.  If not,
> decrement the address and pass the packet along.
>
> (I.e., the protocol would require a SERIAL_IN and SERIAL_OUT
> signals)
>
> Of course, this means a failure of any device renders those that
> follow it inaccessible.  That's acceptable.
>
<snip>

It's way simpler to just have a 422/485 bus.

-- 
Les Cargill

On 5/4/2017 4:47 PM, Les Cargill wrote:
> Don Y wrote:
>> I want to push a bit stream through a series of *identical* devices
>> and have (identical) software running on each determine if that
>> particular device is being addressed, or not.
>>
>> It seems like the simplest way to do so is to require each device
>> to examine an address field in the packet and, if '0', assume the
>> data that follows is intended for its consumption.  If not,
>> decrement the address and pass the packet along.
>>
>> (I.e., the protocol would require a SERIAL_IN and SERIAL_OUT
>> signals)
>>
>> Of course, this means a failure of any device renders those that
>> follow it inaccessible.  That's acceptable.
>
> <snip>
>
> It's way simpler to just have a 422/485 bus.

Then you have to have a way of making each device "unique".
This typically means a configuration activity that is probably
non-trivial.  And/or stocking different "part numbers" for
what are essentially the same component but with different
addresses, etc.

And, protocols to handle the case where two devices are
NOT unique (i.e., duplicate address).

Plus all the bus contention/arbitration possibilities, etc.
(if you want to get data *back* from the devices)

For example, with the cascaded scheme, you can wire all
of the lamps on the playfield of a pinball machine to power
and ground; then daisy chain a data line through a "smart
lamp driver" located adjacent to each individual lamp.

That *one* "lamp bus" signal would allow the processor to
turn any lamp on or off (dim, flash, whatever) regardless
of the number of lamps on the playfield.  The system
software would only need to know the lamp "order" of the
prototype as all future playfields (for that model) would
be wired in the same daisy-chain order.

When a lamp driver fails, it could be replaced with a
new component without any need for reconfiguration,
testing for duplicate bus addresses, etc.

The same power/ground could be distributed to all of the
"smart hammer drivers" sited adjacent to each of the
solenoids/actuators on the playfield.  That *one*
"hammer bus" signal would allow the processor to turn
any solenoid on or off (including supporting a pull-in
coil, maximum duration timer, shorted/open coil detector,
etc.) regardless of the number of drivers on the playfield.

[The lamp driver and hammer driver could be similar save for
current handling capacities and other "use related" issues
in their software; you probably don't care if a lamp
stays lit indefinitely -- but, a coil often has a duty cycle
limit!]

Likewise, you could have a "switch driver" that is distributed
around the playfield to sense contact closures arising from
the ball's motions.

You could do this with *other* busses -- including those that
require explicit addressing -- but with considerably more pain.

On Thu, 4 May 2017 13:17:18 -0700, Don Y <blockedofcourse@foo.invalid>
wrote:

>On 5/4/2017 12:53 PM, Robert Wessel wrote:
>> On Thu, 4 May 2017 10:52:40 -0700, Don Y <blockedofcourse@foo.invalid>
>> wrote:
>>
>>> I'd like to come up with a scheme that I could apply to a variety of
>>> similarly "open ended" network configurations -- that have very
>>> different physical characteristics (e.g., a 10 ft long chain
>>> and a 300 ft long chain -- trading data rate for network size)
>>
>> At some point you need to worry about reinventing Ethernet.  And many
>> MCU's are available with Ethernet for a very small incremental cost.
>> Although perhaps not at the very low end.  A huge advantage is that
>> you can punt much of the infrastructure to the customer ("It's
>> Ethernet, deal with it.").
>
>Depends on the nature of the "messages" being passed -- their
>content and frequency.
>
>E.g., would you spring for the cost of a NIC, MCU capable
>of supporting a TCP (or even just UDP) "stack" -- just to
>command individual hammer drivers on/off at some sub-sonic rate?


I'm not sure I can answer that definitively, even for myself, but as a
first order approximation, if it's a separate box, the first choice of
interconnect should be Ethernet or one of its wireless cousins (WiFi,
Bluetooth), if a wireless connection is required.  And UDP+IP does not
impose much of a burden, even on a OS-less MCU.

Which is not to say that there aren't going to be exceptions.  But I'm
pretty sure things are at the point where you need to justify *not*
using Ethernet on an external connection.


>OTOH, running a bunch of wires from *one* set of hammer drivers
>(accessed "in parallel" from an MCU port) out to a bunch of
>solenoids dramatically changes how you wire them, how you
>deal with expansion, etc.


Direct drive stuff is one obvious exception.  But you're asking about
devices you're sending packets to.


>With Ethernet, you'd have to ensure each node had a unique address
>so you could distribute the message to all and know that the
>right node acted on it -- AND, that no OTHER node acted on it!


Any device claiming to be Ethernet should have the process for
attaching a unique MAC address well established.  Use the MAC number
as the device serial number (pre-pend some of your own digits for
sanity, on the sticker you attach to the outside of the device).


>By interposing each node in a serial chain, a node need not
>care about it's "address" -- it's *implied* (when the ID byte
>is 0, the message is for you.  Otherwise, its for someone
>DOWNSTREAM from you).  So, you can produce identical devices
>and still treat them as individually unique.
>
>Of course, this makes binding addresses to physical nodes
>a separate issue:  you either carefully document the
>physical (electrical?) order of the nodes at deployment
>*or* introduce a "configuration step" in which you
>empirically derive the (address,node) mapping.
>
>     "I just activated node 1.  What do you see as having
>     changed in the field?  OK, I'll write that down.  Now
>     proceeding to node 2..."
>
>This would be tedious *if* the field configuration changed,
>often.  But, if it is largely static and only updated as
>needs evolve (or, in response to failures), then its almost
>assumed that some degree of manual intervention will be
>required (i.e., someone has to swap out the defective node
>or splice a new node into the chain <somewhere> -- which
>may not be at the *end*)


The implied address approach doesn't really fix the problem.  Sure
someone attached *something* to port 7 of controller 13, but did they
do that correctly?  IOW, is that water valve on 13/7 actually attached
to the cold supply to the motor cooling system, or did someone attach
it to the hot water line to the reactor heater?  Or is what they
plugged into 13/7 actually attach the electrical power switch for the
ammonia pump motor?  Or heck, maybe it *is* an actual accelerometer,
of the type you wanted, but did they plug the X axis one into 13/7
like the should have, or did they get the Y or Z axis one?

There are also diagnostic advantages to being able see the entire
network from one place.  It also offers some opportunities for
redundancy (controller 13 appears to be unresponsive, have the
monitoring system send a predefined safeing command to all the devices
13 was supposed to be controlling).  If you need higher levels of
redundancy, having two Ethernet ports on devices is also easy, and
having two parallel sets of Ethernet switches, with a few
interconnects, leaves you with two links to any device.

Certainly the setup and verification procedures will be *different*. I
see modest-to-large advantages in a fair number of situations for the
Ethernet approach, and small-to-modest advantages for the custom
approach in (fewer others), and a majority where it's a wash.

On 5/4/2017 10:30 PM, Robert Wessel wrote:
> On Thu, 4 May 2017 13:17:18 -0700, Don Y <blockedofcourse@foo.invalid>
> wrote:
>
>> On 5/4/2017 12:53 PM, Robert Wessel wrote:
>>> On Thu, 4 May 2017 10:52:40 -0700, Don Y <blockedofcourse@foo.invalid>
>>> wrote:
>>>
>>>> I'd like to come up with a scheme that I could apply to a variety of
>>>> similarly "open ended" network configurations -- that have very
>>>> different physical characteristics (e.g., a 10 ft long chain
>>>> and a 300 ft long chain -- trading data rate for network size)
>>>
>>> At some point you need to worry about reinventing Ethernet.  And many
>>> MCU's are available with Ethernet for a very small incremental cost.
>>> Although perhaps not at the very low end.  A huge advantage is that
>>> you can punt much of the infrastructure to the customer ("It's
>>> Ethernet, deal with it.").
>>
>> Depends on the nature of the "messages" being passed -- their
>> content and frequency.
>>
>> E.g., would you spring for the cost of a NIC, MCU capable
>> of supporting a TCP (or even just UDP) "stack" -- just to
>> command individual hammer drivers on/off at some sub-sonic rate?
>
> I'm not sure I can answer that definitively, even for myself, but as a
> first order approximation, if it's a separate box, the first choice of
> interconnect should be Ethernet or one of its wireless cousins (WiFi,
> Bluetooth), if a wireless connection is required.  And UDP+IP does not
> impose much of a burden, even on a OS-less MCU.

Why the "separate box" criteria?  Should car manufacturers replace
CAN with Ethernet?  Should PC manufacturers use ethernet to connect
their floppy disk drive to the CPU (on the other end of the
motherboard)?  See my pinball machine example, elsewhere, this thread.
Should the pushbutton controls for my garage door opener use ethernet
frames to pass "open" and "close" commands to the actuator head?
Or, individual keystrokes from the external keypad to the controller
in the actuator?

> Which is not to say that there aren't going to be exceptions.  But I'm
> pretty sure things are at the point where you need to justify *not*
> using Ethernet on an external connection.

Should my doorbell use an ethernet connection?  My thermostat to
talk to the furnace?

As I said, it depends on the nature of the data ("messages") being
exchanged and their frequency.  My CCTV cameras are IP based -- because
they move a fair bit of data, continuously, and I need (choose) a bit of
processing power AT the camera.  It's easier to run CAT5 with a PoE
injector than to run a separate power connection and coaxial video
cable back to <something>.  Esp if I want to be able to add more
cameras without having to upgrade an N-port "camera card" in a PC...
(N = 4 or 8)

>> OTOH, running a bunch of wires from *one* set of hammer drivers
>> (accessed "in parallel" from an MCU port) out to a bunch of
>> solenoids dramatically changes how you wire them, how you
>> deal with expansion, etc.
>
> Direct drive stuff is one obvious exception.  But you're asking about
> devices you're sending packets to.

Why can't I send packets to solenoids?  Again, see the pinball example.

One wall of the house is glass.  Blinds on each window.  Do I run
ethernet to each window?  Or, do I install a "blind controller"
<somewhere> and pass messages over a low speed serial interface
to tiny motes mounted on the motors for each of the individual
blinds:
     3OPEN
causes the 4th motor "from here" -- "external" to "here" -- to open
while:
     2CLOSE
causes the 3rd motor to close.

>> With Ethernet, you'd have to ensure each node had a unique address
>> so you could distribute the message to all and know that the
>> right node acted on it -- AND, that no OTHER node acted on it!
>
> Any device claiming to be Ethernet should have the process for
> attaching a unique MAC address well established.  Use the MAC number
> as the device serial number (pre-pend some of your own digits for
> sanity, on the sticker you attach to the outside of the device).

But, then they aren't IDENTICAL devices!

I can rephrase my problem and claim I'm using MCU's with serial
numbers laser etched into their silicon from the factory.  Now,
I have a means of uniquely identifying each device -- even if I
talk to them using CAN or any other transport layer.  No need
to rely on wiring "position" to identify a device!

Remove the MAC address and solve the problem with ethernet.
I.e., you have to ADD the MAC address back in -- or, some other
"unique identifier".

Apples-apples.

>> By interposing each node in a serial chain, a node need not
>> care about it's "address" -- it's *implied* (when the ID byte
>> is 0, the message is for you.  Otherwise, its for someone
>> DOWNSTREAM from you).  So, you can produce identical devices
>> and still treat them as individually unique.
>>
>> Of course, this makes binding addresses to physical nodes
>> a separate issue:  you either carefully document the
>> physical (electrical?) order of the nodes at deployment
>> *or* introduce a "configuration step" in which you
>> empirically derive the (address,node) mapping.
>>
>>     "I just activated node 1.  What do you see as having
>>     changed in the field?  OK, I'll write that down.  Now
>>     proceeding to node 2..."
>>
>> This would be tedious *if* the field configuration changed,
>> often.  But, if it is largely static and only updated as
>> needs evolve (or, in response to failures), then its almost
>> assumed that some degree of manual intervention will be
>> required (i.e., someone has to swap out the defective node
>> or splice a new node into the chain <somewhere> -- which
>> may not be at the *end*)
>
> The implied address approach doesn't really fix the problem.  Sure
> someone attached *something* to port 7 of controller 13, but did they
> do that correctly?

Sure, someone bought a box of 50 model 123 Ethernet valve controllers.
And, I can now "see" the controller that they just cabled to my system.
Did they attach the correct device?  Did they wire it to the correct
valve?  Or, is it wired to a light bulb, instead?

Using a different transport protocol doesn't tell you anything
about the outside world.

> IOW, is that water valve on 13/7 actually attached
> to the cold supply to the motor cooling system, or did someone attach
> it to the hot water line to the reactor heater?

Is the water valve on 12:34:56:78:9A:BC port 7 actually attached
to the cold supply to the motor cooling system, or did someone attach
it to the hot water line to the reactor heater?

Or, to the hand towel dispenser in the men's room?

> Or is what they
> plugged into 13/7 actually attach the electrical power switch for the
> ammonia pump motor?  Or heck, maybe it *is* an actual accelerometer,
> of the type you wanted, but did they plug the X axis one into 13/7
> like the should have, or did they get the Y or Z axis one?

How is any of this made BETTER or mode robust/reliable by using Ethernet?

> There are also diagnostic advantages to being able see the entire
> network from one place.

Why can't I see the daisy-chained devices?

> It also offers some opportunities for
> redundancy (controller 13 appears to be unresponsive, have the
> monitoring system send a predefined safeing command to all the devices
> 13 was supposed to be controlling).

How do you *talk* to those devices if the controller that handles them
is unresponsive?  Why does ethernet to an unresponsive controller
give you that ability but daisy-chain to an unresponsive controller
doesn't?

That's a separate design issue.  And, you have to decide how much you
want to invest in your solution to address those conditions.

If I sense a water leak in the house, I have no way of knowing
WHERE it is.  Perhaps a toilet is "running on".  Or, maybe a
hose to the washing machine has ruptured.  Or, someone left a
faucet partially open.  (all have happened, here)

In my case, I can shut off water TO THE HOUSE.  But, not to the
toilet, individually.  Or, the faucet.  So, the magnitude of the
leak weighs into how I attempt to protect the house.  If it's a slow
leak, chances are, its a toilet running on or a faucet that wasn't
turned off completely.  Send a message to the occupants -- even if
they are away on vacation.  If no response, maybe let the leak
continue (running up the water bill for this month) *or* shut off
the house supply -- until you *need* to turn it back on (e.g., to
water the yard).

If I wanted more options in handling this "failure", I'd spend
more resources on better isolating the source of the leak AND
limiting the damage that it could potentially cause.

If I want to be able to turn off ALL the solenoids because I
suspect one solenoid controller may be crashed with its
solenoid engaged (which could cause it to burn up), then I'd
add a disconnect in the power feed to the "solenoid bus".
Or, a special disconnect for just that one solenoid.

> If you need higher levels of
> redundancy, having two Ethernet ports on devices is also easy, and
> having two parallel sets of Ethernet switches, with a few
> interconnects, leaves you with two links to any device.

So, the lights on the pinball machine's playfield should have *two*
ethernet connections?  The irrigation valves should similarly have two?

> Certainly the setup and verification procedures will be *different*. I
> see modest-to-large advantages in a fair number of situations for the
> Ethernet approach, and small-to-modest advantages for the custom
> approach in (fewer others), and a majority where it's a wash.

I'd be interested in the modest-to-large advantages you see in
controlling individual irrigation valves.  Or, lights/actuators on
a pinball machine playfield.  Or, the lighted pushbuttons on a
large broadcast studio video mixer:
<https://www.shutterstock.com/video/clip-12972671-stock-footage-broadcast-tv-studio-production-vision-switcher-broadcast-video-mixer-pan-right.html>

(Do you really think there is one *giant* PCB hiding behind all of those
LPB's and a gazillion "output drivers" wired, in parallel, to service
all those lamps/buttons?  Or, do you think, perhaps, they are organized
in IDENTICAL subassemblies hiding behind the top cover and wired
together using some sort of message passing bus?  Do you think it is
Ethernet based?  Think about what the messages being passed around,
there, are likely to contain.)

On Wed, 3 May 2017 23:14:09 -0700, Don Y <blockedofcourse@foo.invalid>
wrote:

>I want to push a bit stream through a series of *identical* devices
>and have (identical) software running on each determine if that
>particular device is being addressed, or not.
>
>It seems like the simplest way to do so is to require each device
>to examine an address field in the packet and, if '0', assume the
>data that follows is intended for its consumption.  If not,
>decrement the address and pass the packet along.
>
>(I.e., the protocol would require a SERIAL_IN and SERIAL_OUT
>signals)
>
>Of course, this means a failure of any device renders those that
>follow it inaccessible.  That's acceptable.
>
>It also means there is a latency proportional to the device address
>inherent in the protocol.  Also acceptable (the data source can
>take that into consideration when creating the message stream).
>And, the incremental latency can be made pretty small.
>
>Another downside is the cost of updating data on *all* (multiple)
>devices as this requires N messages.
>
>An alternate message format (or, a cleverer encoding of the above)
>could be used to allow a single COMPOSITE message to be propagated
>in which a node encountering a '0' address field strips the data
>that is intended for its use and then reconstructs a message
>bearing a '0' address field with the BALANCE of the data that it
>then passes to the next device, in series.  In effect, letting
>each subsequent device strip off "its" data before passing the
>remainder along.
>
>[When no data remains, the message dissipates]
>
>As such, you could "address" any set of contiguous addresses
>by creating an initial message of the form:
>     <first_target_ID> <first_datum> ... <last_datum>
>where the number of data present in the message implicitly defines
>the *last_target_id* relative to the first_target_id.
>
>Any problems with this sort of kludged encapsulation?

You would need two fields, one the 'Target Address' and a 'Address
Counter'

The Target Address is the device to  be controlled.
The Counter address is the Address that each device increments and
passes to the next device.

But I don't see why a 8 pin address header is a bad idea. You can
still do a pass thru serial chain. plus the devices dont need to
enumerate.

I've done a 2 device serial pass thru chain, but they were hard coded
addresses.

Cheers

On 5/6/2017 9:14 AM, Martin Riddle wrote:
> On Wed, 3 May 2017 23:14:09 -0700, Don Y <blockedofcourse@foo.invalid>
> wrote:
>
>> I want to push a bit stream through a series of *identical* devices
>> and have (identical) software running on each determine if that
>> particular device is being addressed, or not.
>>
>> It seems like the simplest way to do so is to require each device
>> to examine an address field in the packet and, if '0', assume the
>> data that follows is intended for its consumption.  If not,
>> decrement the address and pass the packet along.
>>
>> (I.e., the protocol would require a SERIAL_IN and SERIAL_OUT
>> signals)
>>
>> Of course, this means a failure of any device renders those that
>> follow it inaccessible.  That's acceptable.
>>
>> It also means there is a latency proportional to the device address
>> inherent in the protocol.  Also acceptable (the data source can
>> take that into consideration when creating the message stream).
>> And, the incremental latency can be made pretty small.
>>
>> Another downside is the cost of updating data on *all* (multiple)
>> devices as this requires N messages.
>>
>> An alternate message format (or, a cleverer encoding of the above)
>> could be used to allow a single COMPOSITE message to be propagated
>> in which a node encountering a '0' address field strips the data
>> that is intended for its use and then reconstructs a message
>> bearing a '0' address field with the BALANCE of the data that it
>> then passes to the next device, in series.  In effect, letting
>> each subsequent device strip off "its" data before passing the
>> remainder along.
>>
>> [When no data remains, the message dissipates]
>>
>> As such, you could "address" any set of contiguous addresses
>> by creating an initial message of the form:
>>     <first_target_ID> <first_datum> ... <last_datum>
>> where the number of data present in the message implicitly defines
>> the *last_target_id* relative to the first_target_id.
>>
>> Any problems with this sort of kludged encapsulation?
>
> You would need two fields, one the 'Target Address' and a 'Address
> Counter'

No.  The "address counter" serves dual purpose:  when the count
attains '0', the message is intended for the current device.
I.e., it indicates the number of devices *upstream* from the
targeted device that should be skipped.

[Of course, you can define other semantics]

> The Target Address is the device to  be controlled.
> The Counter address is the Address that each device increments and
> passes to the next device.

The only advantage, there, is that it then *tells* the device what it's
position (address) happens to be.  If such a capability was needed,
you could kludge that capability into this structure -- incurring that
overhead only when needed.

> But I don't see why a 8 pin address header is a bad idea. You can
> still do a pass thru serial chain. plus the devices dont need to
> enumerate.

Depends on the number of devices, the distance between them, the
nominal data rates, etc.  E.g., if you have to handle a network
spanning hundreds of feet, you'd want to minimize the number of
conductors (and associated line drivers/receivers).  If you wanted
to handle a network with hundreds of "devices" (note that a device
is an arbitrary construct -- a physical device could, potentially,
consist of 30 devices, if you opted to treat them as such!), then
the width of the address field could exceed a fixed size.

The logical idea of increasing the width of the address field then
penalizes smaller networks as well as impacting maximum data rate
(more symbols to xmit with "no" information content!).  So, you could
encode the "address"/ID in a manner that allows for it to expand to suit
the needs of the particular network/message -- without unduly burdening
short messages/nearby targets or long messages/distant ones.

> I've done a 2 device serial pass thru chain, but they were hard coded
> addresses.

So far, the problems that I've identified are related to error analysis
(and recovery) along with distributed clock synchronization.  Hard failures
are (relatively) easy to address:  fix the damn thing!  :>