Yeah, but you still have to turn off your transmitter when you receive.
That's the whole point off the 2-wire half-duplex link.

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

----- Original Message -----
From: M. Adam Davis <adavis@UBASICS.COM>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Monday, August 21, 2000 8:59 AM
Subject: Re: [PIC]: Commx with half duplex RS485


> You can leave the rs-485 converter open both ways (don't turn off the
receiver
> when you send).  Your program will receive the data it is sending on the
> network, and you can test for the last byte sent.  Once you've received
it, you
> know the tx register is empty.  But since you're leaving both tx and rx
open on
> the line all the time anyway, it's not important to worry about switching
it.
>
> -Adam
>
> Ruben Jvnsson wrote:
> >
> > Hello,
> >
> > I am working on a project where I have PIC devices on a RS485 half
duplex (2 wire) network with a master slave protocol, where the master is a
PC connected to an external RS232/485 converter. Data direction for the
converter is controlled with the DTR signal on the PC com port.
> >
> > I have made a DOS program handling this master/slave protocol without
any problems (mainly because I am using a self-developed  interrupt driver
for the serial port).
> >
> > I am now converting this program to Windows and since I haven't got the
time (or the knowledge yet) to redevelop a serial port interface for
Windows, I purchased the Commx Active X component from Green leaf software.
> >
> > However, I can't get the half duplex RS485 communication to work
properly. The main problem is that Commx has an event for TX buffer empty,
but not for TX shift register empty in the UART. It seems that the event for
the TX buffer empty is fired after the last Transmit Register (FIFO) Empty
Interrupt from the UART when Commx hasn't got anything more to write to the
TX FIFO (when the FIFO is ready to receive new data but the last byte is
still in the TX shift register). Thus, using this event for controlling data
direction of the RS232/485 converter switches the converter from transmitter
to receiver a couple of bits into the last character of the TX string which
means that it isn't read properly by my receiving devices on the network. I
have used the technique to append a dummy character (0xff) after my last
significant character to make sure the last significant character is seen by
my network devices and got the communication to work. But since the
information about when!
>  the TX
> > shift register is empty is a
> >
> > Has anybody successfully got this to work? Are there anybody else using
CommX with Visual C++?
> >
> > The doc (helpfile) says that the port handle, accessible through CommX,
is the very same handle returned from the CreateFile() Windows API function
call used inside the PortCtl object. Can this handle be used to access
registers from the UART directly?
> >
> > --
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>
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