piclist 2002\01\25\022814a >
Thread: 2-wire comms Was Replace old chips ?
picon face BY : Russell McMahon email (remove spam text)

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> So has anyone bright ideas for 2-way 2-wire comms using the
> Vcc line ?

The circuit that you have shown could be used as two way half duplex as is.
If ALL the PICs on the circuit have a transmitting transmitter as per the LH
one in the diagram and all have a receiving connection as per the rh one
then all can send or receive - as long as only one is sending at a time.

The circuit wants to be a little more complex than that shown. The supply
should be substantial;ly above Vcc so each can derive Vcc from it using a
series regulator. The receiver sense has to be able to handle the Vsig > Vcc
without causing problems (but let's not venture into the simple resistor
RS232 receiver argument :-) ).

If the use of two wires is more valuable to you than the cost of some extra
circuit complexity then you can achieve full duplex by shunting the circuit
with partial and full loads and using comparators or similar to establish
which of the 4 possible states the circuit is in.
eg if you want a master to send full duplex to N slaves then you could have
the master drop the supply to 75% of full level on tx. A slave loads the
circuit to drop the level  to half typically. so you get

Slave    Master             Bus
1                1                 100%
1                0                   75%
0                1                   50%
0                0                   37.5%

You are still not finished as:

- You need enough high time to supply Vcc to the salves in bursts.
- The slave level drop will vary depending on where it is on the circuit and
the resistance of the line relative to the feed resistor.
- As periods with low or lower Vcc get longer slaves will recharge at higher
currents when full supply again arrives and the current spike may affect
apparent digital level depending on how much current is drawn during
recharge surges.

A possible alternative is to place all slaves in series in a loop with each
"agreeing" to draw a constant current. What they don't use internally is
dissipated by a regulator. For a resistive power feed, If one slave goes
short circuit the voltage across all other slaves and the feed resistor will
vary. Similarly, all slaves could agree to drop a fixed voltage and the
current would then vary with differing numbers of slaves in the loop.
In each case one could look at approximations to constant current and
constant voltage feed to see what makes most sense.

I suspect that half duplex is liable to be fine given the probable target
markets for this sort of system. A simple collision detection / repeat
protocol would be needed.

       Russell McMahon

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