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'[PIC]: Paralleling USART outputs.'
2002\05\13@135717 by Larry McKibben

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Hi Folks,

I'm a PIC newbie.  This is my first question to the PIC list.  I signed up and
have been reading the list for about a month and a half.

I want to use a 16F76 as a master for a burglar/fire home alarm system.  I want
to have maybe 4 16F628's around the house for remote access.  Using the USART
functionality, the 4 remote transmitters would be electrically be in parallel to
the 16F76's receiver input.  The data sheet does not state the 16F628 USART/TX
(RB2 - pin 8) as open drain.  Will paralleling them be a problem?

I don't believe there is a concern in the opposite direction, that is 4 16F628
RX inputs electrically in parallel from the 16F76 TX.

I realize there is the RA4 pin on both IC's that are open drain, but wanted to
use the USART function.

About me.
As this is my first question, I thought I would introduce myself.  I have been
retired 2 years now, 27.5 years of electronic field service.  Although the
majority of my work was hardware related, I enjoyed programming.  In the 80's, I
did some machine language programming on a DEC PDP11-44.  Later I wrote BASIC
programs to emulate and debug RS232 protocol communications.  I found I missed
that part of the job so have gotten involved with PICs.  My first project was to
take a published article to sound a speaker in morse code and modified it to
flash an LED array with different texts selectable by jumpers, like GO MARINERS
and HAPPY HOLIDAYS.  It's mounted on top of my tower.  This project will be from
the ground up and I expect it will take months to implement.

If you read this whole mail then maybe you are like me and have too much time on
your hands.  ha  ha

Thanks all for making this list very informative.  I often paste together many
responses of a thread and keep it in a file for reference.
--
Happy Trails....  Larry

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2002\05\13@145843 by Olin Lathrop

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> I want to use a 16F76 as a master for a burglar/fire home alarm system.  I
want
> to have maybe 4 16F628's around the house for remote access.  Using the
USART
> functionality, the 4 remote transmitters would be electrically be in
parallel to
> the 16F76's receiver input.  The data sheet does not state the 16F628
USART/TX
> (RB2 - pin 8) as open drain.  Will paralleling them be a problem?

Yes.  You should check out RS-485, which is basically what you are trying to
do except that the electrical details have been worked out.  There are
driver chips available the pretty much "convert" between the PIC TX and RX
lines and RS-485 given an additional direction control input.

> I don't believe there is a concern in the opposite direction, that is 4
16F628
> RX inputs electrically in parallel from the 16F76 TX.

Correct, but you should consider issues like noise immunity, protection from
inductively or capacitively coupled spikes, and line termination.

> I realize there is the RA4 pin on both IC's that are open drain, but
wanted to
> use the USART function.

You should be able to do what you want to do with RS-485 although you will
have to figure out the low level protocol details yourself.  Since this is a
single-master system, you could use two RS-485 busses.  One for sending by
the master and the other for sending by all the slaves.  You could also do
this with a single RS-485 bus, but have to get a little more fancy in the
protocol so that everyone on the bus agrees who is talking.

Another option is CAN.  Some 18 series PICs come with CAN built in.  CAN
takes care of the lowest protocol levels directly in the hardware.


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2002\05\13@152926 by David P. Harris

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There are schemes to use RS232 as a net -- using diodes to acheive the isolation.  I
don't have any specific links, sorry.
David

Larry McKibben wrote:

{Quote hidden}

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2002\05\13@162253 by Dwayne Reid

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At 10:55 AM 5/13/02 -0700, Larry McKibben wrote:
>Hi Folks,
>
>I'm a PIC newbie.  This is my first question to the PIC list.  I signed up and
>have been reading the list for about a month and a half.

Glad to have you on board!

>I want to use a 16F76 as a master for a burglar/fire home alarm system.  I
>want
>to have maybe 4 16F628's around the house for remote access.  Using the USART
>functionality, the 4 remote transmitters would be electrically be in
>parallel to
>the 16F76's receiver input.  The data sheet does not state the 16F628 USART/TX
>(RB2 - pin 8) as open drain.  Will paralleling them be a problem?

Yes - tying the TX pins directly together is a problem.  What data (baud)
rate do you want to use?  How far between nodes and what is the total path
length?  What kind of stresses (ESD, transients, etc) will the nodes be
subject to?

>I don't believe there is a concern in the opposite direction, that is 4 16F628
>RX inputs electrically in parallel from the 16F76 TX.

You are correct - no problem at all.

dwayne


Dwayne Reid   <spam_OUTdwaynerTakeThisOuTspamplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
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2002\05\13@172321 by Jinx

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Just about all you'd ever need to know to make RS485 nodes.
Mainly about implementation of hardware (driver-receiver
node pairs) solutions. For a reasonably simple system under
your control you may be able to use passive components, but
I have no reference for that. One benefit of using purpose-built
RS485 ICs is the ability to hang long cables off them

==================================================

Bob Perrin (Circuit Cellar 7/99)

http://www.circuitcellar.com/library/ccofeature/perrin0799/index.asp

==================================================

Jan Axelson (Circuit Cellar 6/99)

http://www.edtn.com/embapps/emba062.htm

==================================================

http://www.piclist.com/techref/microchip/rs485.htm

==================================================

10 Ways To Bulletproof RS-485 Interfaces (AN1057)

http://www.national.com/an/AN/AN-1057.pdf

==================================================

Interface Circuits For TIA/EIA-485 (Texas)

RS485 at bottom of page. Particularly useful and in-depth interface
circuits and "How it works" descriptions. Details on many many
other comms accessible from this page

focus.ti.com/docs/analog/interfacehomepage.jhtml?familyId=361&templat
eId=5&path=templatedata/cm/overview/data/interface

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2002\05\13@193819 by Kelly Kohls

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Larry,

>Will paralleling them be a problem?

If you leave the transmitter portion of the remote devices disabled when
they aren't transmitting, then you should be OK.  Clearing the TXEN bit will
place the transmit pin into a high impedance state. BTW, some datasheets
have this tidbit of information (16C76) and others don't (16F876).  However,
all of the devices that I've used (16C76, 16F876) behave this way.  As Olin
and others have mentioned, if the distance between your master and remote
devices is very long, you could have noise problems.  Probably better to go
the RS485 route.  Hope this helps.

Kelly Kohls, N5TLE
Dallas, TX
http://www.qsl.net/n5tle/
There's never enough time to do it right, but plenty of time to do it over.

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2002\05\14@120340 by Larry McKibben

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Hi Folks,
Thanks to those that responded.  I'm not sure I want to add the complicity of
RS485.  I realize it would probably be the best approach for the comm. link.
Remember this is my first 'ground up' PIC project.  I want to keep the parts
count down.  I do believe in KISS (keep it simple stupid).  The furthest wire
run between the master and remote will be about 50 feet.  I think I will use a
lo Z current loop.  The wiring in my house is multi-conductor cables that I have
the alarm system in as well as some phone circuits.

Reconsidering, I will abandon the USART.  I am going to use the open drain of
RA4 (16F628) for the TX and an ordinary input for RX.  All I need to send
between the PICs is three bytes, a control byte, a security byte and maybe a
error byte. Or I may just send the control & security byte twice and compare
them for a match.  The BPS will be slow, maybe 4800 or 9600.  I'll run the PICs
at 10 MHz.

My programmer mostly does the '16' PICs and I only want to use the flash
versions.

Thanks again for the feedback and ideas.


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2002\05\14@121433 by Alan B. Pearce

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>Reconsidering, I will abandon the USART.  I am going to use the open drain
of
>RA4 (16F628) for the TX and an ordinary input for RX.  All I need to send
>between the PICs is three bytes, a control byte, a security byte and maybe
a

I fear you are cutting off your nose to spite your face.

Use an external transistor on the TX pin of the uart to give yourself the
open drain equivalent - saves having to generate/debug a software uart
transmitter (assuming you are still going to use the hardware uart to
receive. hardware cost is one transistor and two resistors, less the
resistors if you use one of those so-called digital transistors that have
the resistors built in.

Then on your master PIC you will just need an inverter with a pull up
resistor on the input. This could also be a single transistor if you select
the resistor values right.

It would pay to have a similar arrangement on the transmit from the master
to the slave devices as well, mainly to keep noise spikes from destroying
your PIC's, you are likely to have enough problems with spikes mashing your
messages without worrying about destroyed chips as well.

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2002\05\14@123530 by Larry McKibben

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Alan,

Great point in putting a transistor on the USART output.  In hind sight, why
didn't I think of that.  ha

Thanks.  Larry
==================

"Alan B. Pearce" wrote:

{Quote hidden}

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2002\05\14@140227 by Bob Ammerman

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If you are going to go with a lo-Z current loop you might want to take a
page out of the old-timers book....

Wire all your current loop outputs in series (perhaps using optoisolators).
Have the master provide wetting for the loop. The default state of each
driver is such that the loop current is allowed to flow. The one station
that is attempting to transmit breaks the loop current.

Each station's connection to the loop could be via two optoisolators. The RX
optoisolator's LED and the TX optoisolator's transistor would be in series
with the loop.

Bob Ammerman
RAm Systems


{Original Message removed}

2002\05\15@105442 by Mike Singer

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Larry McKibben wrote:

>  I'm not sure I want to add the complicity of RS485.
> .
> .
> .
> Reconsidering, I will abandon the USART.

  Hello, Larry.

  Why not to use the idea of Microchip ICSP interface in this context.
Plus for more reliability I'd recommend star topology instead of bus.
Plus for the same reason I'd recommend a sort of MAX232 stuff as
bus drives. (I bought ICL232CPE for less then 1$)
  What do I mean. PIC16F8XX has a lot of I/O pins. Set two lines
(DataIn and DataOut) among central chip and each peripheral chip.
Add two common lines (Reset and Syncro) from central to periferal.
You'll get a fully static system with easy programming and debugging.
  The idea is quite clear. After "Reset" you send commands in, say, 4 bits,
synchronizing each bit by "Syncro" front. Then you send and receive data
simultaneously going on with the synchronization.

  Good luck.
  Mike.

PS: Microchip ICSP has DataIn and DataOut in one line. The above
      idea looks more like an Atmel's SP.

----------------------------------------------------------------------------
  Thanks to James Newton, Dale Botkin & others for supporting this forum.

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2002\05\15@122442 by Harold M Hallikainen

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       As you suspect, there will be a problem tying two UART transmit outputs
together. You can fix this by putting a diode between the transmit output
and your bus (cathode towards the tx output). Then, put a pull-up between
the bus and +5V. I'd also put a current limit resistor between the bus
and the UART input pin since the bus is leaving the box. I used this
approach for the remote handle control/display on the Phase II and
Saphire  at http://www.denmat.com/solsheet/virtuoso/info_phase2.htm and
http://www.denmat.com/solsheet/sapphire/info.htm .

Harold

On Mon, 13 May 2002 10:55:22 -0700 Larry McKibben <KB7ZTRspamKILLspamARRL.NET>
writes:
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2002\05\15@125741 by Larry McKibben

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Opps, had to send this again because I didn't pay attention to the reply address.

Thanks Mike and Bob Ammerman for the thoughts.  For now I'll use the USART with an
open collector (as Alan suggested) or try the diode (as Harold suggested).  I've
always enjoyed  the journey more more so that the destination.  So trying different
methods is enlightening and fun.  I've ordered the parts, will start the program
development in a day or so.  With all the good comments and suggestions, I have
several alternatives if my present path doesn't work out.

Thanks again to all who responded.  What a great list.

Happy Trails........ Larry



Mike Singer wrote:

{Quote hidden}

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2002\05\15@181630 by Mike Singer

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  Larry McKibben wrote:
> For now I'll use the USART with an open collector (as Alan suggested).

  Alan suggested:
> It would pay to have a similar arrangement on the transmit from the
> master
> to the slave devices as well, mainly to keep noise spikes from destroying
> your PIC's, you are likely to have enough problems with spikes mashing
> your messages without worrying about destroyed chips as well.

  Larry, why do you want problems? The cost of home-made bus driver will be
much higher then the cost of a standard driver. Transistor as receiver needs
a lot of components to be added in order to achieve a fair data transmission
reliability.
  As for me, I prefer star bus topology with no need of paralleling outputs.
I'm not old enough to be an expert in PDP, but I think, the idea of the PDP
bus with open collector transmitters and terminators on both endings would
not work in this case. You'd better use RS232C(+30v-30v) standard for PDP
terminals.

  Good luck.
  Mike.

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2002\05\16@081533 by Olin Lathrop

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> Great point in putting a transistor on the USART output.  In hind sight,
why
> didn't I think of that.  ha

Just remember that a transistor in the common emitter configuration, as
suggested, will invert the signal.


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2002\05\16@083543 by Alan B. Pearce

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>> Great point in putting a transistor on the USART output.  In hind sight,
why
>> didn't I think of that.  ha

>Just remember that a transistor in the common emitter configuration, as
>suggested, will invert the signal.

Which is why I explicitly pointed out he would need an inverter on the
master receive input, but I did not point out why. :)

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