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PICList Thread
'2 Pics on one crystal'
1997\11\12@164731 by Patrick Hanson

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A couple of quick questions to  the PIC Guru's  on the list.

(1)   Is it possible to operate  2  PIC16C84's  on the same  pcb from one
crystal ? (XT mode). The data sheet shows how to use an external clock input
but I am uncertain how to obtain this clock from the other PIC without
loading the  oscillator circuit.


(2)  Somebody on the list  recently indicated that it was  easy to convert
from RS232 to RS485, I have no problem at the PIC level but without
purchasing an expensive RS485 Card for my PC which is  intended  to be the
master controller in a multi-drop system I am having difficulty in enabling
the RS485 driver from a standard PC RS232 port while transmitting  and then
disabling the driver and still be able to listen  for a reply on the PC port.

Thanks in anticipation

Pat Hanson

1997\11\12@165540 by Andy Kunz

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face
>(2)  Somebody on the list  recently indicated that it was  easy to convert
>from RS232 to RS485, I have no problem at the PIC level but without
>purchasing an expensive RS485 Card for my PC which is  intended  to be the
>master controller in a multi-drop system I am having difficulty in enabling
>the RS485 driver from a standard PC RS232 port while transmitting  and then
>disabling the driver and still be able to listen  for a reply on the PC port.

Try B & B Electronics on the web, or send e-mail to somebody at
http://www.carmacon.com because I know they have one, I just don't know if
they sell them.

Andy


==================================================================
Andy Kunz - Montana Design - 409 S 6th St - Phillipsburg, NJ 08865
         Hardware & Software for Industry & R/C Hobbies
       "Go fast, turn right, and keep the wet side down!"
==================================================================

1997\11\12@173851 by David W. Duley

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In a message dated 97-11-12 17:27:02 EST, you write:

<<
(2)  Somebody on the list  recently indicated that it was  easy to convert
from RS232 to RS485, I have no problem at the PIC level but without
purchasing an expensive RS485 Card for my PC which is  intended  to be the
master controller in a multi-drop system I am having difficulty in enabling
the RS485 driver from a standard PC RS232 port while transmitting  and then
disabling the driver and still be able to listen  for a reply on the PC
port.

Thanks in anticipation

Pat Hanson >>
Hello Pat,
Most RS232 - RS485 converters use the RTS line as a read/write line.  You
have to drive the RTS line just before writing to the bus.
BlackBox and Inmac have RS485 conversion stuff.  They range from $99 to
several hundred.  The $99 converter needs the RTS line to control direction,
the more expensive version has a micro that takes care of that.  I made my
own with a MAX233, a MAX487 and a PIC.
Omega also carries them

Dave Duley

1997\11\12@204835 by Mike Keitz

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On Thu, 13 Nov 1997 08:46:52 +1100 Patrick Hanson <spam_OUTp.hansonTakeThisOuTspamUWS.EDU.AU>
writes:
>A couple of quick questions to  the PIC Guru's  on the list.
>
>(1)   Is it possible to operate  2  PIC16C84's  on the same  pcb from
>one
>crystal ? (XT mode). The data sheet shows how to use an external clock
>input
>but I am uncertain how to obtain this clock from the other PIC without
>loading the  oscillator circuit.

Yes, this works, though not recommended by Microchip.  One PIC chip has
the crystal connected the usual way and does the oscillator function.
The X2 pin of this chip connects to the X1 pin of the other PIC.  Leave
the X2 pin on the other PIC open.  Locate the PICs close together on the
board so the oscillator signal doesn't have to travel a long way.  Also
you may need to reduce the capacitor connected from X2 on the first PIC
to ground to compensate for the extra capacitance from the second PIC and
the wiring.

It may work a little better to couple the signal through a relatively
large (0.01 uF) capacitor between the PICs.  This lets the second PIC
bias itself to its proper DC threshold.  If both PICs are the same type
and supply voltage (and lot number most likely), it shouldn't be
necessary.
>
I am having difficulty in
>enabling
>the RS485 driver from a standard PC RS232 port while transmitting  and
>then
>disabling the driver and still be able to listen  for a reply on the
>PC port.
>
Build a box which has a RS-232 transceiver (2RX, 1TX) and a RS-485
transceiver.  Use the RTS line from the PC to enable (or maybe disable)
the RS-485 driver.  Most likely this exists already in pre-built form.

The RTS line is controlled by one of the bits in the 16550 chip so the PC
can turn the driver on and off.  I don't think there's any hooks though
to coordinate the RTS control with the actual sending of data though, so
it may get a little dicey synchronizing the software with the hardware
buffers.  It may be best to not use them.

1997\11\13@042046 by wwl

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On Thu, 13 Nov 1997 08:46:52 +1100, you wrote:

>A couple of quick questions to  the PIC Guru's  on the list.
>
>(1)   Is it possible to operate  2  PIC16C84's  on the same  pcb from one
>crystal ? (XT mode). The data sheet shows how to use an external clock input
>but I am uncertain how to obtain this clock from the other PIC without
>loading the  oscillator circuit.
Yes - connect oscout on the crystal to oscin on the secons PIC. Keep
track as short as poss. Adding a K or so series resistance may be
useful to reduce loading, and you may need to reduce the cap on oscout
to compensate for the extra track length and Cin of the second PIC

    ____                                                           ____
  _/ L_/  Mike Harrison / White Wing Logic / .....wwlKILLspamspam@spam@netcomuk.co.uk  _/ L_/
_/ W_/  Hardware & Software design / PCB Design / Consultancy  _/ W_/
/_W_/  Industrial / Computer Peripherals / Hazardous Area      /_W_/

1997\11\17@135925 by Karel Hladky

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In article <346ac599.290867spamKILLspamsmtp.netcomuk.co.uk>, Mike Harrison
<.....wwlKILLspamspam.....NETCOMUK.CO.UK> writes
>>(1)   Is it possible to operate  2  PIC16C84's  on the same  pcb from one
>>crystal ? (XT mode). The data sheet shows how to use an external clock input
>>but I am uncertain how to obtain this clock from the other PIC without
>>loading the  oscillator circuit.
>Yes - connect oscout on the crystal to oscin on the secons PIC. Keep
>track as short as poss. Adding a K or so series resistance may be
>useful to reduce loading, and you may need to reduce the cap on oscout
>to compensate for the extra track length and Cin of the second PIC

Or you could use one of the dedicated crystal oscillator chips such as
the Harris HA7210. Makes things much easier, if your budget can stand it
(about UKP 1 each).

Karel
--
    ++++++++++++++++++ KH Design & Development ++++++++++++++++++
    Electrochemical Corrosion Measurement and Control Consultancy
                  http://www.khdesign.demon.co.uk/

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