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'[EE]: Controlling multi-boards with RS232'
2001\04\11@021805 by Bill Westfield

face picon face
   I am thinking of first connecting PC to the first board thru Tx/RX, the
   first board to the second board thru Tx/Rx, the second board to the
   third board, and so on. So the system looks like daisy chain
   configuration. Since I am going to control each board one by one, I may
   have to assign an ID for each of them. So when I want to control the
   20th board, since the PC is connected to the first one, the program
   checks until it finds #20 board and sets the register.

You mean like this?


+->Rx.PC1.Tx---->Rx.PC2.Tx--->Rx.PC3.Tx--->Rx.PC4.TX---...-->Rx.PC36.Tx--+
|                                                                        |
|                                                                        V
+------------<--------------------<-------------------<------------------+

Congratulations!  You've reinvented the ring network.  I did that too, back
in college, and am still (again?) thinking of using it for some extendable
controller setups I'm working on (ok, a sprinkler controller.)  I don't see
anything wrong with a scheme like this (and it has a bunch of advantages),
but it's beginning to bother me that I've never seen it used anywhere.

I supposed that it would be worthwhile to read up on the history of token
ring networks (like - why do they need a token in the first place?) to see
why all the complexity got added to such a simple idea, but I've never
gotten around to it...

BillW

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2001\04\11@052506 by Chunhee Song

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face
Thank you for all PIC members who read my email.
Now I am encouraged to design my control system as I planned.

Yes, Bill. My system will look like this.



+->Rx.PC1.Tx---->Rx.PC2.Tx--->Rx.PC3.Tx--->Rx.PC4.TX---...-->Rx.PC36.Tx--+
|                                                                        |
|                              __________                                V
+------------<----------------| Main PC  |-<-------------------<---------+
                              __________

I am trying to write a control program that will control a multivision system, so called cubes.
So I need to adjust brightness, contrast, tint and all bunch of stuff with PC.
Thank you.

Chunhee Song

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2001\04\11@083247 by Olin Lathrop

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> +->Rx.PC1.Tx---->Rx.PC2.Tx--->Rx.PC3.Tx--->Rx.PC4.TX---...-->Rx.PC36.Tx--+
> |                                                                        |
> |                                                                        V
> +------------<--------------------<-------------------<------------------+
>
> Congratulations!  You've reinvented the ring network.

That should work, but you've got to be really careful about ground loops and
think about noise.

Here's a slightly different solution to this problem.
I've got a current project that includes connecting up to a few hundred
advertising signs to be driven from a single COM port.  I used a 17C PIC
with two UARTs as a communications controller on each sign.  Each unit has
an upstream and downstream port, with the upstream port of the first unit
connected to the PC COM port.  The protocol has been designed to support
nodes with more than one downstream port (tree structure instead of linear
chain), but so far we have no hardware that does this.  All communication is
wrapped in packets with checksums, and use ACKs and retries to get reliable
delivery.  At wakeup, the host software assigns each node a unique address
based on the topology.  It can then send messages to specific nodes or do
broadcasts.  So far we've tested this with ten nodes and all is working very
well.

> I supposed that it would be worthwhile to read up on the history of token
> ring networks (like - why do they need a token in the first place?)

The short answer is so that only one node at a time is allowed to inject a
message onto the ring.  The other nodes are only allowed to pass on the bits
from upstream to downstream.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, .....olinKILLspamspam.....embedinc.com, http://www.embedinc.com

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2001\04\11@084526 by Ken Walker

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And what happens if one of them dies.......a lot of trouble....I'd use
425...all connected in parallel so if one goes down it does not take down
the rest. At least then u could request an answer from each in turn and the
one that does'nt answer is the problem, The problem i see with your schem
below is if you ask 1 for data and say 36 is dead or locked then how do u
find which one it is,,,,if 2 goes down and you ask say 27 for data which one
do u point to as the problem

just my tuppence..

Kenny baby,
One of Two in a vast wilderness.


{Original Message removed}

2001\04\11@145055 by Olin Lathrop

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> > That should work, but you've got to be really careful about ground loops
and
> > think about noise.
>
> Why noise?  The signal is digitally reconstructed at each node...
> Forwarding is done packet by packet (or after N bytes of delay to check
for
> the address.)  (except when a node is off and the signal travels through
> NC relay contacts or something...)

I didn't see the original message, but I was assuming a bunch of nodes would
be connected that could be some distance apart.  Noise can get into long
signal runs in various ways.

> Well, yes, but I'm not sure why THAT was considered necessary?  ie - is it
> only required to acheive the predictability that TR afficiandos used to
> claim was so important?  Theoretically in an ABCD ring, A can send to B
and
> C can send to D simultaneously without interfering with each other at all.
> Further, if you have a "master node", then can its messages be a sort of
> "implicit token"?

Assume a ring of 26 nodes with A sending to B, B sending to C, ..., Z
sending to A.  In theory, A could send to B independently of C sending to D.
However, this gets very cumbersome to manage.  What if C decided to send to
D, but B was trying to send to E?  How is B supposed to know that it can't
right now?

Another thing to consider is that each node forwards bits, not messages.
Therefore a signal from A shows up at Z after 25 logic delays and cable
propagation delays.

The token in a token ring is really just the permission to send.  You give
up the potential efficiency of having multiple messages being transmitted at
one time when all the conditions are right.  In return you get good use of
the bandwidth at saturation with relatively simple management.  When a node
receives the token, it passes it on immediately if it has nothing to send.
If it has something to send, it sends one packet and then passes on the
token.  Nearly 100% of the ring bandwidth is used as long as any node has
something to send, without degredation when lots of nodes want to send.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olinspamspam_OUTembedinc.com, http://www.embedinc.com

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