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PICList Thread
'RS232 Bus'
1999\03\24@122919 by Clark, John

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Has anyone ever implemented an RS232 bus using CTS/RTS & some addressing
scheme?  Something like I2C is what I am thinking about...


John Clark, Software Engineer
spam_OUTJohnCTakeThisOuTspaminter-intelli.com
(317) 715-8175 (voice & fax)

Interactive Intelligence, Inc.
3500 DePauw Blvd., Suite 1060
Indianapolis, IN  46268-1136
http://www.inter-intelli.com

1999\03\24@141311 by Gerhard Fiedler

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At 12:26 03/24/99 -0500, Clark, John wrote:
>Has anyone ever implemented an RS232 bus using CTS/RTS & some addressing
>scheme?  Something like I2C is what I am thinking about...

i think i don't understand what exactly you're up to. if you want to put
multiple devices on the bus, rs232 is not a good choice -- the drivers are
designed for point-to-point.

OTOH, if you just mean the serial async encoding, many such schemes have
been implemented with rs485 driver (or, simpler, oc outputs) and some kind
of software protocol.

or there are long distance driver for the i2c bus, if you just want to go
long distance with it. (i know philips has one.)

was this far off?

ge

1999\03\24@154030 by Clark, John

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Yes, put multiple devices on an RS232 bus.

To send data to a slave:
~~~~~~~~~~~~~~~~~~~~~~~~
(master TX) Address Token  |  Data Tokens  |  Stop Token

The master would send an address broadcast and data to all devices on the
RS232 bus.  The master would be passed with a stop token.  The bus would be
ready again.

To fetch data from  a slave:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
(master TX) Address Token  |  Request Token  |  Stop Token
(slave TX) Data Tokens  |  Stop Token

The master would send an address broadcast and request to all devices on the
RS232 bus.  The addressed device would then transmit back whatever was
requested with a stop token.  The bus would be ready again.

The RS232 bus is not inherently better or worse than other devices for bus
implementation.   Using the RTS/CTS lines should provide all the negotiation
necessary.

I want RS232 as it is the only port on my PC104 to control various devices.


John Clark, Software Engineer
.....JohnCKILLspamspam@spam@inter-intelli.com
(317) 715-8175 (voice & fax)

Interactive Intelligence, Inc.
3500 DePauw Blvd., Suite 1060
Indianapolis, IN  46268-1136
http://www.inter-intelli.com

> {Original Message removed}

1999\03\24@170301 by Gerhard Fiedler

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At 15:39 03/24/99 -0500, Clark, John wrote:
>Yes, put multiple devices on an RS232 bus.

rs485 drivers have a "transmitter enable/disable" function (something like
a three state logic output), which leaves the line free for other
transmitters. rs232 drivers don't have that, since they're meant for
point-to-point. how would you control a line with multiple rs232
transmitters connected to it?


>To send data to a slave:
>~~~~~~~~~~~~~~~~~~~~~~~~
>(master TX) Address Token  |  Data Tokens  |  Stop Token
>
>To fetch data from  a slave:
>~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>(master TX) Address Token  |  Request Token  |  Stop Token
>(slave TX) Data Tokens  |  Stop Token

that's a common protocol on rs485 busses. if you include a start token, you
make your life a bit easier. and some block check (crc or xor) before the
stop token maybe. (actually a unique start token and a length information
is better in most cases than a stop token and eliminates the need for one.)
and a "repeat message" mechanism with timeout (in case of transmission errors).

there are quite a few protocols like this in use, check the plc
manufacturer (eg. modbus and the like). it's often good to use such a
definition as a starting point (because it's usually a protocol that works
reasonably :).


>I want RS232 as it is the only port on my PC104 to control various devices.

you can use a 232-to-485 converter, which usually use one of the modem
control lines (eg. DTR) for enable/disable of the transmitter.

ge

1999\03\24@180730 by Antonio L Benci

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A few years ago some of my digital design students devised a simple
method of daisy chaining RS232 ports on several PC's and developed a a
simple CSMA/CD LAN.

The trick was to ensure that multiple TXD connections did not over load
the lines, this was solved by a diode and resistor isolation network on
each port so that TXD and RXD could be connected together.

Transmission format could only be half duplex but it enabled the sending
system to monitor for collisions and take appropriate action. The
network could cope with 20 PC's max before line loading took its toll.

The actual cct details escape me at the moment but if pressed i could
dig them up... The comms protocol was 9bit packet based with CSMA/CD.

Gerhard Fiedler wrote:
{Quote hidden}

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1999\03\24@180954 by JAM [Naish]

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If you want some info about i2c, i've got some. Particularly i have more
info for 16CXXX


"Aprended de los maestros, pero evitad sus faltas"
__________________________________
e-mail: EraseMEnaishspam_OUTspamTakeThisOuTeuskalnet.net
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visit my homepage: http://fly.to/naisaint

1999\03\25@021224 by w. v. ooijen / f. hanneman

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For WISP I use rs232 as a 3-wire multi-drop bus (master sends on one, all
slaves send on the other).
The addressing is based on using the break condition as atention signal.
Wouter.

----------
{Quote hidden}

1999\03\25@034759 by Gerhard Fiedler

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At 07:58 03/25/99 +0100, w. v. ooijen / f. hanneman wrote:
>For WISP I use rs232 as a 3-wire multi-drop bus (master sends on one, all
>slaves send on the other).
>The addressing is based on using the break condition as atention signal.

what kind of driver do you use for the slave transmitter? rs232 with active
low and active high?

ge

1999\03\25@044738 by Dr. Imre Bartfai

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Hi,

sounds quite interesting. Could you share us the details?
Imre

1999\03\25@081006 by Clark, John

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I had envisioned:

Master RTS      slaves listen to master's TX bus
Master CTS      slaves permitted to transmit on master's RX bus
Master TX
Master RX

Beyond this, I had not given it too much thought.  Perhaps some sort of
winking on the CTS & RX bus by the slave a la I2C would reduce slave
transmit collisions.  This would support full duplex.


John Clark, Software Engineer
spamBeGoneJohnCspamBeGonespaminter-intelli.com
(317) 715-8175 (voice & fax)

Interactive Intelligence, Inc.
3500 DePauw Blvd., Suite 1060
Indianapolis, IN  46268-1136
http://www.inter-intelli.com

{Quote hidden}

1999\03\25@113206 by w. v. ooijen / f. hanneman

picon face
The 'driver' is a PIC output pin with a small series resistor and diode and
a relatively
large resistor from the line to ground.
Definitely not reliable under all circumstances, but enough to daisy chain
a few programmers on my desktop with roughly zero components.
schematics etc: http://www.xs4all.nl/~wf/wouter/pic/wisp
Wouter.

----------
> From: Gerhard Fiedler <listsEraseMEspam.....HOME.COM>
> To: EraseMEPICLISTspamMITVMA.MIT.EDU
> Subject: Re: RS232 Bus
> Date: Thursday, March 25, 1999 09:35
>
> At 07:58 03/25/99 +0100, w. v. ooijen / f. hanneman wrote:
> >For WISP I use rs232 as a 3-wire multi-drop bus (master sends on one,
all
> >slaves send on the other).
> >The addressing is based on using the break condition as atention signal.
>
> what kind of driver do you use for the slave transmitter? rs232 with
active
> low and active high?
>
> ge

1999\03\25@150921 by Gerhard Fiedler

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At 08:08 03/25/99 -0500, Clark, John wrote:
>I had envisioned:
>
>Master RTS      slaves listen to master's TX bus
>Master CTS      slaves permitted to transmit on master's RX bus
>Master TX
>Master RX

you definitely have to use some sort of oc driver (and not rs232 :) and
then you can use the cts as a synchronizing mechanism for the slave
transmissions. you'd probably get =very= few collisions.

ge

1999\03\25@151136 by Gerhard Fiedler

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At 17:24 03/25/99 +0100, w. v. ooijen / f. hanneman wrote:
>The 'driver' is a PIC output pin with a small series resistor and diode and
>a relatively large resistor from the line to ground.

and i thought you guys were talking about rs232... :)

ge

1999\03\25@154040 by Wagner Lipnharski

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Oh my, a multidrop RS232... :)

Gerhard Fiedler wrote:
>
> At 17:24 03/25/99 +0100, w. v. ooijen / f. hanneman wrote:
> >The 'driver' is a PIC output pin with a small series resistor and diode and
> >a relatively large resistor from the line to ground.
>
> and i thought you guys were talking about rs232... :)
>
> ge

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