This is a fine solution if you want the computer to snoop the RS-485 bus all
the time, but if you want it to become an addressable node on the network,
you need to design a RS232 to RS485 converter according to your network
protocol specifications.
That's what I've done to use dumb terminals as network nodes for monitoring
and configuring our devices.



Thiago Turchetti Maia
Di-elitrons Eletrtnica Industrial Ltda.
http://www.di-eletrons.com.br/




-----Mensagem original-----
>De: John Maud <maud@IAFRICA.COM>
>Para: PICLIST@MITVMA.MIT.EDU <PICLIST@MITVMA.MIT.EDU>
>Data: Quarta-feira, 29 de Setembro de 1999 22:15
>Assunto: Re: Addressable serial port - 9 data bit RS485
>
>
>>Paul,
>>
>>We have successfully completed a similar project very similar to what
>>you describe, using the 'F877.  We have 32 PIC-based slave nodes on the
>>RS-485 bus, with the PC as the master.  The 9 bit address wake-up
>>feature of the PIC is worthwhile in reducing overhead, and works a
>>treat!
>>
>>In fact, all UARTs operate in "9-bit" mode, that is 8 data bits plus 1
>>parity bit.  If you examine the timing requirements for the PIC you will
>>see that 9th bit used for address wake up sits in the "parity" position
>>on the serial bit stream.  Thus addressing a PIC from the PC simply
>>requires setting the parity bit of the PC UART to "mark" for the address
>>byte, thereafter setting the parity bit to "space" for subsequent data
>>bytes of the message to that addressed PIC, and so on.
>>
>>We have provided an external isolated RS-232 to RS-485 convertor plugged
>>into the COM port on the PC side.
>>
>>Hope this helps.
>>
>>BTW, we are running at 19 200, 'twas fine for our application.
>>
>>John Maud
>>
>>Paul Brown wrote:
>>>
>>> I am interested in using a feature that Microchip is promoting as a way
>to
>>> simplify programming of serial comms. for PICs located on a multi-drop
>>> network.The 16F87x devices will generate an interrupt when a byte is
>>> received with the ninth bit set.  The idea is to use the USART in 9 data
>>> bit mode where the ninth bit is used to indicate an address.  This would
>>> greatly simplify programming networked PICs.  You just check each
address
>>> byte to see if it is yours.  If it is, just read in the subsequent
bytes.
>>> Otherwise, just ignore all bytes.
>>>
>>> In my application, I want to network some number of PICs (at least six
to
>>> start with) and communicate to them with a PC.  The question is: who
>makes
>>> an RS485 card with a USART that has 9 data bits?
>>>
>>> Paul A. Brown
>