Hi again.

I use a single MAX3223 (or maybe it's MAX2332, I'm never sure which without looking it up). In any case, you only need Tx, Rx, and GND. Tx from the host computer ties to Rx on the AT89C51Rx2 and Rx on the host ties to Tx on the chip (both via the appropriate receiver or driver on the MAX). You don't need any of the other signals. I know. This seems wrong. It seemed wrong to me too the first time. After all, I was ignoring all of those all important handshaking signals, right? Well, it's just easier to take care of the handshaking between firmware and software, much the way USB (Universal Serial Bus) protocols dictate. In any case, you can incorporate all of those other lines in hardware if you wish, but I don't recall a recent example of it being done.

Are you familiar with the bootloader firmware and the autobaud routine incorporated in the Atmel µ-controller? If you are prototyping this together yourself, and if it isn't too much of a distraction, you might want to set up the hardware and then use Hyperterminal to communicate with the µ-controller while watching the Tx and Rx lines on an ocsilloscope. Do the autobaud routine and then type a few commands manually just to learn the details of what happens. Obviously this is not a good way to actually develope a real application or project, but it is incredibly instructive if you are new to this kind of thing.

There's my two cents worth. Good Luck.