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should i use rs485 or other ?



Jan 1, 1970

I need a group of PIC circuits (upto 32 in a group) to send data back
to a 'master circuit' located elsewhere (they dont need to receive
data, only send). Lets call this group a 'bank'.
Communications must be reliable, and only a slow data speed is
necesary, not a lot of data will be transfered, but there is a chance
that some circuits will try to send at the same time.

Each node in the group would be close (under 5 meters away from each
other), but the master would be upto 300 meters away.

I think RS485 would be the best thing to implement here, because of
its signal range on a single twisted pair.
So i could have a single twisted pair cable coming from the master to
the first node, then to the second and so on.

Should i be looking at something other than RS485 ?
I have considered a standard TCP LAN, but the costs per-circuit for
the interface, plus the extra configurations etc, all seems total
I have read that i can have upto 32 standard nodes using RS485.
should i be using standard rs485 or some other variation which allows
Each node must check the line-state before transmiting (high
using a standard max485, is there any chance of data collisions ? this
must be very reliable!

Do i need to use a max485 chip for each node, because they are only
transmitting, can i not just use some simple parts to emulate the
RS485 levels easily enough ?
Its very difficult to get parts where i am, and i would go out and buy
some max485's if i could, which i will order online - but for the
prototype i am hoping i can just use a few transistors/resistors to
get it going.

For the master, i will probably use some kind of rs485 hub, which then
converts to rs232 into a standard PC, because i may have multiple

I would like to find a RS485 hub which will accept 8 inputs (each of
32 nodes), and 1 rs232 output. Anyone know of any ?
A bit of googling has come up with 'one to many' hubs, ie: the master
transmits, and all nodes will get the signal.
I am after the opposite, where each node will send independently, and
the master will get all incoming signals.

I dont want to have to make one, im sure it must exist, or maybe they
all do this, and its just far too trivial for them to specify it in
their product specs :). Just want to be sure.

Also, it would be nice to get one with memory. IE: the PC gets shut
down, crashes, and the RS485 hub will store all incoming
communications (upto a curtain amount) until the PC comes back online
to receive them.
Could be done easily enough (sbc + rs485hub), but again, must exist
already ?

Thanks for any help!