More "From Engine to Caboose"
Wes Johnston email (remove spam text)
<x-flowed>a few pointers:
what happens when one of the modules in the middle of the train
breaks? You will need to either have a sort of tracert function to count
the number of "hops" back the info stops coming... OR place them on a bus
that runs the entire length of the train - that each module is capacitively
coupled to. If the output driver in one module fails, and it is DC coupled
to the bus, you'll loose communications with the entire chain.
Rather than query each car... why not just have them all come on at regular
intervals and ID... reemmeber, any car that hears something on one serial
port will rexmit it on the other port.... if each is connected to it's
neighbor.... then there is little chance for a lost packet due to
collision. The thing that makes this work is that it is very unlikely that
any two cars will be turned on at the same time... and if each one keeps
some sort of counter going... it could be used to generate a pseudo rand
num that ca be used to calculate ID intervals +/- some number of seconds.
now in dealing with collisions(packets, not trains)... consider that a
packet could be travelling in EITHER direction... and the next car in line
must be ready to accept this packet. SO some scheme of "hey are you there"
and wait for a "yes" before actually forwarding the packet. make sure that
each serial port (the front and rear serialports) have different delays,
and different back off calculations for retries. each car can generate a
busy ACK on one of its serial ports when its other serial port is busy.
the engine behaves just like any othe car.... but has 3 serial ports. one
up and downstream, and one to copy all bus activity to.
in addition to random id's, each car must carry a query packet from one
port to another... each packet should have a time to live word (64k
counts). that way the engine(s) can send out queries with various TTL's and
wait for them to return.
At 01:11 PM 10/12/99 -0400, you wrote:
Stupidity should be painful
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In reply to: <199910121711.NAA96732@mail.rpi.edu>
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