Searching \ for 'PIC & RS485' in subject line. ()
Make payments with PayPal - it's fast, free and secure! Help us get a faster server
FAQ page: www.piclist.com/techref/microchip/ios.htm?key=rs485
Search entire site for: 'PIC & RS485'.

Truncated match.
PICList Thread
'PIC & RS485'
1999\05\28@084716 by John Esposito

flavicon
face
Hello all:

For the next phase of my PIC project (home security), I want to have remote
keypads communicating (only) with the master control unit.  I originally
wanted to use RS232, but was under the impression that RS232 is difficult
to deal with when using long runs (i.e. through a house) and multiple
interface nodes.  For future expandability of the "network", I am leaning
toward RS485, using a Maxim interface chip.  However, I will not run it as
a multi-drop configuration, but as a star-type configuration, with the
keypads running (throughout the house) directly to the master.

My question is:  is my impression of RS232 unfounded?  If so, why?  Has
anyone used RS485 in the way I want to use it?  If so, did you use a
terminating resistor as each node to minimize reflections?  Any other
information you could provide would be very helpful


Regards,

--John

1999\05\28@121314 by Carlos L. M.

flavicon
face
PICLIST@MITVMA.MIT.EDU wrote:

> Hello all:
>
> For the next phase of my PIC project (home security), I want to have remote
> keypads communicating (only) with the master control unit.  I originally
> wanted to use RS232, but was under the impression that RS232 is difficult
> to deal with when using long runs (i.e. through a house) and multiple
> interface nodes.  For future expandability of the "network", I am leaning
> toward RS485, using a Maxim interface chip.  However, I will not run it as
> a multi-drop configuration, but as a star-type configuration, with the
> keypads running (throughout the house) directly to the master.
>
> My question is:  is my impression of RS232 unfounded?  If so, why?  Has
> anyone used RS485 in the way I want to use it?  If so, did you use a
> terminating resistor as each node to minimize reflections?  Any other
> information you could provide would be very helpful
>
> Regards,
>
> --John

You can use the RS232 for long distance but you have to make an amplifier to
use it without problems. I saw one circit sometime, that could use the RS232 in
a ddistance of 100m from the computer. Maybe, if you are intereted in this
circuit, I can search it and send you.

Carlos L.M.

1999\05\28@123906 by Darren Logan

picon face
Hi,

       I've not actually used RS485 in a star connection although I
       use it all the time as multi-drop.

       At baud rates <4800 i find the terminating resistors    unnecessary,
even when there are many nodes with
       the most cheapest of cable (including just wires!!).

       You cant use a terminating resistor for each connection
simply because the total parallel resistance of each would
       be so small that the RS485 lines would effectively be
       short-circuited.
       You could however experiment and put one terminating    resistor at
the master end only but personally, i'd leave 'em
       out altogether. I almost guarantee you wont have any    problems as
long as you keep the baud rate to say 1200.

Regards,
Darren

1999\05\28@132304 by John Esposito

flavicon
face
Carlos:

What about using RS232 in a multi-drop or star configuration?  I though
RS232 was point-to-point.

--John





> My question is:  is my impression of RS232 unfounded?  If so, why?  Has
> anyone used RS485 in the way I want to use it?  If so, did you use a
> terminating resistor as each node to minimize reflections?  Any other
> information you could provide would be very helpful
>
> Regards,
>
> --John

You can use the RS232 for long distance but you have to make an amplifier
to
use it without problems. I saw one circit sometime, that could use the
RS232 in
a ddistance of 100m from the computer. Maybe, if you are intereted in this
circuit, I can search it and send you.

Carlos L.M.

1999\05\28@142935 by Jamil J. Weatherbee

flavicon
face
Recommendation is if you just want a star configuration RS232 that will
work reliably at long distance use RS422 (5-Wires, Full Duplex)
This is same interface used on Macintosh serial ports.  RS485 is an
improvement on RS422 that allows multimaster, which you don't need in a
star configuration.

For RS422 you can basically just use CAT5 Cable, and terminate both ends
with 100ohm resistors, see the national semiconductor application notes.


On Fri, 28 May 1999, John Esposito wrote:

{Quote hidden}

1999\05\28@143316 by William K. Borsum

flavicon
face
At 12:49 PM 5/28/99 -0400, you wrote:
>Carlos:
>
>What about using RS232 in a multi-drop or star configuration?  I though
>RS232 was point-to-point.
>

It usually is.  For multi-drop, try RS485.  There is an IEEE standard (1118
I think) that defines an address and packet system for talking to multiple
black boxes.  Basically, the first byte is 9-bits and contains the address,
and the subsequent 8-bit bytes contain the data.  As soon as a pause in the
data stream occurs, or the packet is complete, the receivers must switch
back to watching for the 9-bit address again.  Supposedly the UARTS in the
newer PICs support this.  We are looking at doing a system with 100+ data
loggers spread down an 18,000 foot long structure with a repeater every few
thousand feet.  The wire will end up costing more than the electronics :_p.

If anyone has implemented this protocol in a 16C7xx or similar, I would
sincerely appreciate a copy of the code or a point to it.

Kelly

****************************************************************************
********
All legitimate attachments to this email will be clearly identified in the
text.
William K. Borsum, P.E.
OEM Dataloggers and Instrumentation Systems
<spam_OUTborsumTakeThisOuTspamdascor.com> & <http://www.dascor.com>

1999\05\28@150028 by Vic Lopez

flavicon
face
Carlos, I'd be interested in that circuit. Thanks. Vic Lopez
-----Original Message-----
From: Carlos L. M. <.....clmKILLspamspam@spam@PLANET.COM.BR>
To: PICLISTspamKILLspamMITVMA.MIT.EDU <.....PICLISTKILLspamspam.....MITVMA.MIT.EDU>
Date: Friday, May 28, 1999 9:09 AM
Subject: Re: PIC & RS485


>EraseMEPICLISTspam_OUTspamTakeThisOuTMITVMA.MIT.EDU wrote:
>
>> Hello all:
>>
>> For the next phase of my PIC project (home security), I want to have
remote
>> keypads communicating (only) with the master control unit.  I originally
>> wanted to use RS232, but was under the impression that RS232 is difficult
>> to deal with when using long runs (i.e. through a house) and multiple
>> interface nodes.  For future expandability of the "network", I am leaning
>> toward RS485, using a Maxim interface chip.  However, I will not run it
as
{Quote hidden}

to
>use it without problems. I saw one circit sometime, that could use the
RS232 in
>a ddistance of 100m from the computer. Maybe, if you are intereted in this
>circuit, I can search it and send you.
>
>Carlos L.M.
>
>

1999\05\28@153657 by Bob Drzyzgula

flavicon
face
On Fri, May 28, 1999 at 08:39:44AM -0400, John Esposito wrote:
...
> My question is:  is my impression of RS232 unfounded?  If so, why?  Has
> anyone used RS485 in the way I want to use it?  If so, did you use a
> terminating resistor as each node to minimize reflections?  Any other
> information you could provide would be very helpful

FWIW, the latest issue of Circuit Cellar (June, 1999)
contains an article by Jan Axelson entitled "Designing
RS-485 Circuits". She does a pretty good job of explaining
the issues surrounding signalling & slew rate, termination,
and cable length, and how they affect attenuation,
reflections, etc. Well worth the cover price.

--Bob

--
============================================================
Bob Drzyzgula                             It's not a problem
bobspamspam_OUTdrzyzgula.org                until something bad happens
============================================================

1999\05\28@155636 by John Mitchell

flavicon
face
On Fri, 28 May 1999, Bob Drzyzgula wrote:

> FWIW, the latest issue of Circuit Cellar (June, 1999)
> contains an article by Jan Axelson entitled "Designing
> RS-485 Circuits". She does a pretty good job of explaining
> the issues surrounding signalling & slew rate, termination,
> and cable length, and how they affect attenuation,
> reflections, etc. Well worth the cover price.

Her articles were the sole reason to pick up The MicroComputer Journal --
always interesting, relvant, and useful.

Check out her site: Lakeview Research -- http://www.lvr.com/


- j

1999\05\28@191110 by paulb

flavicon
face
John Mitchell wrote:

>> FWIW, the latest issue of Circuit Cellar (June, 1999) contains an
>> article by Jan Axelson entitled "Designing RS-485 Circuits".

> Her articles were the sole reason to pick up The MicroComputer Journal
 Never was particularly fond of PICs though...  Strictly 8051
--
 Cheers,
       Paul B.

1999\05\29@030257 by Russell McMahon

picon face
I wouldn't recommend RS232 for long runs if it can be avoided and it
is definitely outside the formal specification

BUT

at 300 baud it will run with essentially 100% reliability over very
large distances on wet string (well, maybe the water needs to be
salty :-)).
Think about it - you have a nominal +/- volt swing - say +/- 5 in
practice. At 300 baud you have 3ms per bit. With 1 mile/1600 metres
of wire you probably have say 500 ohms max resistance and 100 nF
capacitance. Time constant of this is about 50 microsecond or about
1/60th of a bit time. Faster baud rates will give narrower margins
but I'd expect reliable operation around a house at baud rates
significantly above 300 baud. That said, it's outside the spec - look
at the results on a 'scope and decide for yourself.

Summary: For around a house and SLOW data rates it will work. If you
can use differential drivers (RS422) then you'll get better
reliability.

(Being picky AFAIR RS422 is mono-drop and RS485 has the spec
extensions to cover multi-drop)


regards


       Russell McMahon

From: John Esposito <    >
>For the next phase of my PIC project (home security), I want to have
remote
>keypads communicating (only) with the master control unit.  I
originally
>wanted to use RS232, but was under the impression that RS232 is
difficult
>to deal with when using long runs (i.e. through a house) and
multiple
>interface nodes.  For future expandability of the "network", I am
leaning
>toward RS485, using a Maxim interface chip.  However, I will not run
it as
>a multi-drop configuration, but as a star-type configuration, with
the
>keypads running (throughout the house) directly to the master.
>
>My question is:  is my impression of RS232 unfounded?  If so, why?
Has
>anyone used RS485 in the way I want to use it?  If so, did you use a
>terminating resistor as each node to minimize reflections?  Any
other
>information you could provide would be very helpful

1999\05\29@061218 by paulb

flavicon
face
Russell McMahon wrote:

> BUT
> at 300 baud it will run with essentially 100% reliability over very
> large distances on wet string (well, maybe the water needs to be
> salty :-)).

 Sounds fine to me.  RS-232 *is* after all differential - it operates
on the difference between the TXD and GND wires.  If you use *real*
RS/ EIA-232 with *real* +/- 10V drive, it should work quite a long
distance at quite high baudrates.

 Accordingly, I see just three problems with it.

 1} The single-ended protocol is quite susceptible to bias from ground
returns, so it may well need an isolated receiver (just like that
recommended for RS-422/ 485!).

 2} You must *use* RS-232, not "imaginary RS-232" with 5V CMOS logic
levels, which is really "asking" for threshold problems.

 3} Many/ most receivers include termination because - that is
specified in the standard.  Paralleling these for "multi-drop" may well
result in out-of-spec termination, in which case again it would be
deceitful to say "RS-232 is not good enough".
--
 Cheers,
       Paul B.

1999\05\29@073301 by ranguelo
flavicon
face
Hi !

John Esposito wrote:
>
>
> For the next phase of my PIC project (home security), I want to have remote
> keypads communicating (only) with the master control unit.  I originally
> wanted to use RS232, but was under the impression that RS232 is difficult
> to deal with when using long runs (i.e. through a house) and multiple
> interface nodes.

For both RS485 and RS232 you can increase the maximum distance by
decreasing
the speed (boudt rate). I expect for interfacing a keyboard or card
reader
you wouldn't need more then 1200 bps.

For future expandability of the "network", I am leaning
> toward RS485, using a Maxim interface chip.  However, I will not run it as
> a multi-drop configuration, but as a star-type configuration, with the
> keypads running (throughout the house) directly to the master.

If you want to do the termination right, you can use at your control
unit
one transmitter and receiver driver for each line and make the
connection of
all signals on the TTL side of the drivers. So physically every line
will have
a driver circuit (and a terminator for RS485) on each end.

It costs more, but imho it makes sense for a security system. If someone
attacks one of the lines and blows the driver circuit away with a high
voltage applaied to the cable you can manage it to keep the other
keypads
working.

>
> My question is:  is my impression of RS232 unfounded?  If so, why?  Has
> anyone used RS485 in the way I want to use it?
>

Haven't tried it myself. There are some ideas :

- Terminate only one side of the line (at the transmitter). Then a
reflected
wave travels back from the end to the transmitter and is absorbed by the
terminator and doesn't affects the next transmission.

- You can terminate only the AC part of the signal by putting a C in
series
with the terminator. So the voltage wouldn't drop if you connect
parallel
more terminated cables.

- If not terminating at all you can use diodes to clamp the positive and
negative overshoot of the reflected wave. Such diodes are build in most
ICs.


If cost is a problem you can try it first by reducing the speed and by
using
a check sum in the transmission protocol to make it immune to errors.
(That's
the way the one-wire bus by Dallas works: no termination, slow speed and
a CRC check.)


Just my 2 cents ...
St.


'PIC & RS485'
1999\06\02@023620 by Zdenek Bohm
flavicon
face
Carlos, I'd be interested in that circuit. Thanks. Zdenek - @spam@bohmKILLspamspamgacc.cz

More... (looser matching)
- Last day of these posts
- In 1999 , 2000 only
- Today
- New search...