> your limit per serial wire depends on how you approach it.

That's certainly true.
But I am mystified as to why so many people "hardcode" addresses into
firmware -- that requires a custom version of the code for every chip
on the serial network.
I am also mystified as to why so many people "use a dip switch", which
requires a bunch of pins to read the dip switch.

Why don't these people use "scalable" networking protocols that scale
up to an unlimited number of chips?
Protocols that work fine with identical firmware in every chip, and
only require a maximum of the "serial data in" pin plus 2 or so other
pins on the master and each slave, even if there are dozens of slaves?

Off the top of my head, here are 3 such protocols:
(Pick whichever one protocol is the easiest for you to implement).
(Please tell me about other "scalable" "simple" networking protocols,
especially non-proprietary open standards).

* daisy-chain serial SPI: (4 wires per slave; automatic baud rate
synchronization) approach used by, among other things, JTAG scan.

* decrement packet address: (2 wires per slave) each chip has a
serial-in and a serial-out pin. The host serial-out pin is connected
to the first slave serial-in pin. Each slave serial-out pin is
connected to the serial-in pin of the next slave in the chain. (The
last serial-out pin is typically connected to the serial-in pin of the
master, for loopback test purposes). When a chip receives a packet of
data on the serial-in line, it checks the destination address. If the
destination is address "00", it accepts that data as a command, turns
on lights, spins motors, activates the death ray, etc. etc. If the
destination address is any other address, the chip forwards the packet
to the next chip almost exactly. Every byte of the message is copied
the same, except for the address byte(s), which are decremented. If
the received address was "99", the chip sends "98"; if the received
address was "01", the chip sends "00". (With a bit of clever
programming, each chip does not have to wait until the entire packet
is received, but instead can start transmitting at near-wire-speed as
soon as the address byte is received). After forwarding the entire
packet, the slave immediately forgets it (in particular, it does *not*
accept that packet as a command).

* daisy-chain select, common data: (3 wires per slave) The host
serial-out pin is directly connected to every slave serial-in pin. In
addition, each chip has a select-in and a select-out pin. The host
select-out pin (which the host normally holds high) is connected to
the first slave select-in pin. Each slave select-out pin (which the
slave normally holds high) is connected to the select-in pin of the
next slave in the chain. When the slave-select line of a slave is
high, it "sleeps", ignoring the serial data line. When a slave sees
its select-in pin pulled low, it wakes up, accepts first command on
the common serial line (even if the command is "no-operation"), and
then pulls its own select-out pin low and goes back to sleep, ignoring
further commands until it is woken up again. When that slave sees its
select-in pin pulled hi, it immediately drives its own select-out pin
high and goes back to sleep. At some later time, when the slave sees
its select-in pin pulled low, it (as always) wakes up and accepts
exactly one command on the common serial line, then pulls its own
select-output line low. (Even while sleeping and ignoring most
commands on the common serial line, perhaps the slave might wake up on
the special "On my mark ... NOW!" synchronized start command addressed
to every slave).

Perhaps one of these protocols could be used to automatically assign
addresses whenever slaves are added, removed, or re-arranged, even if
some global broadcast net (transmitting addressed packets) were used
to transfer most data.

The "daisy-chain SPI" protocol has a name; do these other protocols have a name?
Is there a general name for this entire category of protocols, a
better name than "scalable" "simple" networking protocols?

--
David Cary
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