> 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.

This is a common problem with standard UART drivers on PCs.  I don't see
what the Commx library is buying you here.  You would get the same treatment
from the native Win32 calls, which is probably what the Commx library
accesses.  Unfortunately, the transmitter empty information is not surfaced
to the Win32 layer either.  You therefore have to customize the existing
device driver, or create a new that only does the things you need.  This can
be done, but messing with OS internals is not something the average
programmer should try at home.  There may be folks out there that provide
"enhanced" device drivers that include this feature, but I don't know of any
off hand.  I have seen commercial products for other COMM devices, like the
paralell port, so it might be worth looking around bit.  Sorry for the
unwelcome news.

This may not help you much, but I did something almost like this once.  The
only difference is that the PC was embedded and not running any form of
Windows.  This allowed direct access to the UART from assembler, which does
at least provide a bit indicating transmission is complete.  Could the PC be
embedded where you can control the HW directly.

Another thought is to have the PC talk to a PIC via RS-232, then the PIC
drive the RS-485 on behalf of the PC.  You could still use DTR to signal
that transmission from the PC will be over after the next pause, then have
the PIC do the actual turn around sequencing and timing.


*****************************************************************
Olin Lathrop, embedded systems consultant in Devens Massachusetts
(978) 772-3129, olin@cognivis.com, http://www.cognivis.com

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