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'[PIC:] serial port dilemma - design advise reques'
2004\06\22@115806 by Alan B. Pearce

face picon face
>I have a PIC device that connects to my PC via serial port
>for constant communication.
>
>I would like to have over 10 PIC devices connected to my PC
>somehow for communication.
>
>However, i would not like to buy some sort of expensive
>multi serial port card for it.
>
>What sorts of things can i do to over come this problem?
>Potentially i would like to use over 10 of these PIC devices at once.


I used to work on computer terminals that used a multi-drop RS232 levels in
a polled terminal environment. They did this by having a resistor that went
from the TX data line to -12V, and then pulled the line to +12V using a PNP
transistor. This transistor is used in place of the normal MAX232 type
device. You then also need a suitable RS232 receiver at each PIC, but with a
higher input impedance than normal.

You then need to work out a protocol where the software in the PC calls up
each device in turn, telling it when to transmit the data to the PC.

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2004\06\22@121129 by tim_webb

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face
You might consider moving to LAN

http://www.lantronix.com/products/eds/xport/



-----Original Message-----
From: pic microcontroller discussion list
[spam_OUTPICLISTTakeThisOuTspamMITVMA.MIT.EDU]On Behalf Of Alan B. Pearce
Sent: Tuesday, June 22, 2004 8:59 AM
To: .....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU
Subject: Re: [PIC:] serial port dilemma - design advise requested


{Quote hidden}

I used to work on computer terminals that used a multi-drop RS232 levels in
a polled terminal environment. They did this by having a resistor that went
from the TX data line to -12V, and then pulled the line to +12V using a PNP
transistor. This transistor is used in place of the normal MAX232 type
device. You then also need a suitable RS232 receiver at each PIC, but with a
higher input impedance than normal.

You then need to work out a protocol where the software in the PC calls up
each device in turn, telling it when to transmit the data to the PC.

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.

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2004\06\22@125332 by Harold Hallikainen

face picon face
You could try "open collector RS232". Have a diode with the cathode
connected to TXD on each PIC. The anodes are all tied together. A single
pull-up resistor goes to the common anodes. Run this through an RS232
transmitter (like the MC1488 or equivalent). Each PIC can pull the common
line low to send a space. Each pic idles high when not transmitting. Have
an RS232 receiver (MC1489 or equivalent) receive data off the host PC.
This drives all the PIC RXD pins. The host sends commands to the PICs to
have them transmit. They only respond when requested, avoiding contention.

You can get fancier by packetizing the data, including to and from
addresses. Then both transmit and receive data could run on the same open
collector RS232 line (have the host also drive this line by having a diode
to the open collector data line after the RS232 receiver).

Harold

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2004\06\22@140248 by John J. McDonough

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face
----- Original Message -----
From: "Harold Hallikainen" <haroldspamKILLspamHALLIKAINEN.COM>
Subject: Re: [PIC:] serial port dilemma - design advise requested


> You could try "open collector RS232". Have a diode with the cathode

That was my knee-jerk reaction, but then I thought a minute and figured out
that I'm an old fogey.  Probably should be open drain in this century.  (But
hey, open collector still works, and it works well).

In the radio world, we use this to connect a PC to multiple radios.  Google
for "Icom CI-V" for examples.

72/73 de WB8RCR    http://www.qsl.net/wb8rcr
didileydadidah     QRP-L #1446 Code Warriors #35

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2004\06\22@154617 by David VanHorn

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At 02:00 PM 6/22/2004 -0400, John J. McDonough wrote:

>----- Original Message -----
>From: "Harold Hallikainen" <.....haroldKILLspamspam.....HALLIKAINEN.COM>
>Subject: Re: [PIC:] serial port dilemma - design advise requested
>
>> You could try "open collector RS232". Have a diode with the cathode
>
>That was my knee-jerk reaction, but then I thought a minute and figured out that I'm an old fogey.  Probably should be open drain in this century.  (But hey, open collector still works, and it works well).

Open Plate? :)

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2004\06\22@181051 by Peter van Hoof
picon face
What you sometimes see in industrial motorcontrollers is a simple echo
method
works as follows

-all motors are chained
-motor 1's rx is connected to the computers tx
-motor 2's rx is connected to motor 1's tx
... etc...
-motor n's rx is connected to motor n-1's tx
-motor n's tx is connected to pc's rx ad thus completes the circle.

-all motors echo their input to their output except if the command is for
this motor only (there are broadcast commands possible which would travel
thru the entire chain)
-every motor has its own address and responds only to commands with the
right address embedded
-a motor can reply to messages which will echo thru the chain till they
reach the pc
-There is a special command that sets the motor's address and since this
command is special only the first device that does not already have an
address assigned it will not be echoed to subsequent devices this way the
entire chain gets addressed by subsequent address setup commands.

If you can live with the delays in this method it wrks fairly well

Kind regards
Peter van Hoof

{Original Message removed}

2004\06\22@182921 by Richard Prosser

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Or - each card reads the address & decrements it before passing it on. Then
when the address reaches zero the card receiving actions the command. A
similar method with increment is used & decoded by the PC to determine
where data  origonates.  ??
The advantage is that the card does not need pre-adressing as its address
is set by the location in the chain.
Richard P





What you sometimes see in industrial motorcontrollers is a simple echo
method
works as follows

-all motors are chained
-motor 1's rx is connected to the computers tx
-motor 2's rx is connected to motor 1's tx
... etc...
-motor n's rx is connected to motor n-1's tx
-motor n's tx is connected to pc's rx ad thus completes the circle.

-all motors echo their input to their output except if the command is for
this motor only (there are broadcast commands possible which would travel
thru the entire chain)
-every motor has its own address and responds only to commands with the
right address embedded
-a motor can reply to messages which will echo thru the chain till they
reach the pc
-There is a special command that sets the motor's address and since this
command is special only the first device that does not already have an
address assigned it will not be echoed to subsequent devices this way the
entire chain gets addressed by subsequent address setup commands.

If you can live with the delays in this method it wrks fairly well

Kind regards
Peter van Hoof

{Original Message removed}

2004\06\22@185833 by Peter van Hoof

picon face
An option to consider if the pic's you have to talk to are in close
proximity:
Talk to one pic with rs232 in this area and let it talk to the other devices
with i2c or similar
this would be inexpensive (saves your rs232 level converters) and relatively
simple.

Peter
{Original Message removed}

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