Steve is my favorite rs485 guru, but here's my shot, hope that helps

To *really* do the 9bit scheme, if you have to use some custom driver (either windows or linux).
As you are using the standart serial port, you have to convert rs232 signal levels to rs485 both for input or for output. That means: Pc <-> max232 <-> max485 <-> cable line <-> max485 <-> 8051.
(course you aren't tied to maxim, my rs485 driver is from Texas, for example)

If you're on half-duplex:
In any 485 driver that i have knowledge there's a pin called 'driver enable' (or like that) that tells the line driver to xmit anything that is on the TX pin.
To control this driver enable pin you have to use some other pin that is availabe on the rs232 interface. Albeit Steve used CTS (clear to send) i found more useful to use the RTS (request to send), as lots of windows drivers use this pin instead of CTS

a bit of theory (you can jump this part :) ):
Those pins (RTS/CTS) among others are used to control devices like modems, and they are used like this: when the computer has something to send it pulls the RTS to tell the device his wish, if it's all good, the devices pulls CTS to tell this, after that data is exchanged. That's why i used RTS after all, the pc *is* xmiting something - but ANY output pin can do the job.

I use Delphi/WinNT to handle the client application, and as i'm *not* (yet) using 9bit scheme, i'm not worried about custom drivers, but for VCL stuff i use AsyncPro as they handle 485 in NT and 95/98 out-of-the-box (again: it isn't a custom driver), and works just fine.

Links-Links-Links-Links-Links:
TurboPower: www.TurboPower.com

Willies Computer Software: www.wcscnet.com, paid custom driver that handles 9bit scheme and rs485, for windows - NT and 98/95. Most code examples are C++, but there's one pascal 7 import unit for the DLL's.

Device Drivers International: www.ddiusa.com, free custom driver that handles the RTS issue for you.

Serial-How-To:
http://www.linuxdoc.org/HOWTO/Serial-HOWTO.html, describes the linux serial port interface.

Sorry about my superb english :)

[]'s
Bruno