1. What is the protocol should I use ? Is it RS-323, RS-485, VT-100, TCP-IP, or else, or should i design a new one due to no avaiable well suited protocol at the present.

RS232 is for short distances up to 15 meters. RS485 is for distances up to 1200 meters.

2. As I said I'm a rookie in Serial Programming in VB.
Is there anyone know a short cut how/where in the net should I learn VB, especially in 'serial programming' ?

If you check www.amazon.com there is a book from a author called Grier (I am not sure about his name), who is dealing with VB and serial and parallel ports. There are some examples for RS232, RS485 and even examples for GPS, NMEA etc.

3. How to make a differential data line with the optocoupler-ed current loop, so that it can transmmit in KM range? (or there is any other way to achieve it ?) What is the type of Op-amp good for this purpose ?

If you are talking about differential data lines you are talking about RS485. A friend of mine has developped a Master/Slave solution with opto-couplers, written in assembly for the AT89C2051 (Atmel, a 8051 derivate). His name is San Bergmans and you should find a link to his site on www.wisclub.net He has put some information about his SB-Bus on his homepage. The sources will be there soon.

4.I have bought 4n25 optocouplers, because it's cheap and I'm using it for another project.
Is it good enough to use them in this serial transmission? (with the speed and reliability consideration).
I have seen a HCPL2606 (if I'm not wrong) datasheet which has 10MHz operation capability, but it's expensive.

Check the page of San Bergmans. His SB-Bus is running on 9600 baud, using opto-couplers.

5. Where I could find in the web, the free material about serial programming with 89C51 and 89C52 ?
Someone (I forgot who, sorry) said that the transmission speed can be increased by optimizing the program by utilizing interrupt line (!INT0 or !INT1).
Anyone know how and have the example of the program ?

Check the page of San Bergmans.

6. Is 89C51 and 89C52 Good enough for this job ? What is the minimum Crystal frequency should I use to achieve that speed ? (9600 baud rate or above)

With 11.059.200 you get the right frequency for 9600 baud and more.

7. How to design and implement and appropiate CRC (Cyclic Redundancy Check)?
How to implement it on the Microcontroller program ?
I saw a CRC aplication notes from Dallas ('Application Notes 27') that describe 8 bit and 16 bit CRC algorithm. Can I use the program mentioned in it straight forward ? Which one should I use, 8 bit or 16 bit CRC ? What is the consideration ?
At the Master side, where should I make it, in the microcontroller program or in the computer program ?

Wagner Lipnharski wrote a CRC-16 program in Basic. You should be able to find his webpage on the internet at www.ustr.net. There is also a translated one that can be found at http://come.to/bascompool
Gives you any idea how things work.

8. In Your opinion or expiriences : Will I need external EEPROM or RAM, with AT89C51 and 89C52, thinking that the microcontroller should handle 100 devices, it's own task and maybe CRC ?

There is a complete board with a 80C535/C515 LN with EPROM and SRAM on my homepage, including PCB-plans, schematics, examples in assembler and basic. There is also some info about VB (a connection between a TCP/IP-machine and the RS232). It is called server535.

Hope you got the right information. I am sure you will get more on this forum.

Bye
Ben Zijlstra