Anyone please HELP ME with this project.
Please tell me by the time you finish reading this, wether this is a Simple, Medium or Complicated project.

I have read the question "Which Protocol To Use", by Todd and its answers.

But I'm a rookie in serial communication and serial programming.

I have a big final project that will use a current loop serial line (with optocoupler line isolation).
I will need a 9600 Baud Rate and above.

I'll use one master, and it would be a PC.
The PC hace one serial communication controller (I should use a AT89C52, with one port connected with the computer's serial port and the other is connected to a current loop serial bus using 2 optocouplers, one receiving and one transmitting data) and a Windows 98 running a program build with VB (Visual Basic) for user interface to monitor all data sent by all slaves and to monitor the line performace it self.

The system would be :
1. Every Devices (Slaves and Master) will use AT89C51 or AT89C52 which are doing a controlling activites
(not specified for this moment, maybe :
A. online temperature monitoring of an engine and controlling fan speed.
B. communication interface to a modem, printer, fax machine or scanner.
etc ... etc ... etc ...)
100 devices or more, connecting in the serial line (current-loop). Any devices can communicate each other.

2. Hot pulg-in and out device ability.

3. A long distance transmission (in range of Kilometers (KM) )


What I want to ask You Guys, the expert ones, are :

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.

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' ?

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 ?

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.

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 ?

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)

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 ?

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 ?


OK, those are my lenghty questions.
Thank's for reading this.
And I hope that anyone willing to help me with this project and want to share the solution with me.



Thank You
Alex