RS232 multidrop

Archiver

Hi all,

I have seen many emails rergarding multiple stamps on one rs232
line. i have seen a cool implementantion of this. In nutshell this is
how it works

each node on the bus has a unique address above decimal 127 the
processor in each node(box) listens to its rs232 recive line for its
address. if it's address comes allong the bus it enables it's rs232
transmitter (some boxes even uses a relay to connect it's rs232
transmitter to the bus. the bus is normally in the highz state where
no rs232 transmitters are connected to the bus) and transfers it's
data with host the only thing is if you need to send data that has a
decimal value greater then 127 from the host to the controlled box
you will have to send the data as two bytes the first byte with most
signifgant bit turned off and then the least signifgant bit of the
second byte turned on (acting as the most sigfigant bit of the first
byte). when the box and the host are done with their data transfer
the box disables it's rs232 transmitter and then the whole thing can
start again.

This might not make any sense to anybody but i can answer some
questions(very busy) privatly or on the list.

All you guys have great and merry Christmas,

Michael Fowler
mikef@l...
Lectrosonics, Inc
581 Laser Road, Rio Rancho, NM 87124 USA
PO Box 15900, Rio Rancho, NM 87174 USA
fax (505) 892-6243 - tel (505) 892-4501
toll free phone in USA and Canada (800) 821-1121
www.lectrosonics.com

Archiver

> Date: Thu, 21 Dec 2000 10:31:41 -0600
> From: "Michael Fowler" <mikef@l...>
> Subject: RS232 multidrop
>
> Hi all,
>
> I have seen many emails rergarding multiple stamps
> on one rs232
> line. i have seen a cool implementantion of this.
> In nutshell this is
> how it works
>
> each node on the bus has a unique address above
> decimal 127 the
> processor in each node(box) listens to its rs232
> recive line for its
> address. if it's address comes allong the bus it
> enables it's rs232
> transmitter (some boxes even uses a relay to connect
> it's rs232
> transmitter to the bus. the bus is normally in the
> highz state where
> no rs232 transmitters are connected to the bus) and
> transfers it's
> data with host the only thing is if you need to send
> data that has a
> decimal value greater then 127 from the host to the
> controlled box
> you will have to send the data as two bytes the
> first byte with most
> signifgant bit turned off and then the least
> signifgant bit of the
> second byte turned on (acting as the most sigfigant
> bit of the first
> byte). when the box and the host are done with
> their data transfer
> the box disables it's rs232 transmitter and then the
> whole thing can
> start again.
>
> This might not make any sense to anybody but i can
> answer some
> questions(very busy) privatly or on the list.
>
> All you guys have great and merry Christmas,
>

With the RS-232 outputs tied together, from what I see
in the schematic, as long as you don't xmit from one
of the outputs it is in a hi-z state.

I run a loop something like this (psudocode):

Wait_For_Select:
' the first stamp is A, subsequent
' stamps are B, C, etc.
serin s_in, baudrate, [noparse][[/noparse]w2]
if [noparse][[/noparse]w2] = "A" then Select_Me
goto Wait_For_Select

Select_Me
' routine to do whatever needs
' to be done, including all i/o.
' then just return to above loop
goto Wait_For_Select

I've used this successfully with 5V open-drain rs-232
from the regular i/o pins. I'm expecting it to work
when I connect the rs-232 pins together as well.








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