1. Synchronous dial up connection - most dial up connections are asynchronous - they use the PC's serial port that can only do asynchronous. Whilst the actual modem data transfer may be synchronous, the modem takes care of the conversion. As for sending/receiving at the sme time - that is simplex/duplex not synch/asynch. The critical thing is to be able to grab the incoming characters, sending the characters can be done at a more leisurely pace.

2. Why would you want to cpature only a fixed number of characters? Especially when doing PPP/TCP. You might want to include SLIP just so there's another acronym. The common approach to receiving chars via serial is to use a circular buffer. The serial rx isr puts the incoming chars into this buffer and your main line code retrieves the chars when it wants to.

No use having a 'good speed' of 56k if you can't process the incoming chars fast enough. There's usually 10bits per char using async, this equates to 5600 chars/sec or 178uS per char. Using a 12clocker cpu at 3.686MHz equates to around 54 machine cycles per char. At a quick guess, you're cpu won't be doing much else apart from grabbing the rx chars and shoving them into a buffer.


On top of all this you have the TCP/IP protocol code. There's plenty floating around the web for the 8051. Just make sure you got a good slab of code and ram space.

When doing all of this, be careful not to us an AM radio nearby - The received noise could be interpreted as aliens speaking in tongues.