Dear Sir,
Iam planning to design an 8 Lines Telephone Exchange. Now the thing to note over here is that my objective is not to design a PBX or private automatic branch exchange which you may have seen in offices where a usually a operator attends , transfer, puts on hold a call etc..and if any subscriber or user wanna talk to other user in the same office they can use the same PBX as internal intercom , these systems are avaiable in market and most famous are Panasonic onces .. www.kx-td.com.
But My only Objective is to make simple intercommunication system for 8 lines, my project does not involve any activities related to external telephone service lines or speaking technically i wnat to design my own SLIC "( Subcriber Line Interface Circuit) but TLIC ( Telephone LIne Interface Circuit).
Now for the design of this central exchange to work with normal pulse dailing telephone sets, the designer should have knowledge about working of telephone exchange , that when the telephone set in on-hook there is 60 volts Dc, when the ring comes about 60 Volts AC comes and finally when the set goes off-hook due to drop in impedance voltage drops and also converts from AC to 12 volts Dc, so teh conversation take place on Dc voltage, also issues like cross-talking, Tone-Generators etc should be kept in mind.
I don't want to go in the complicated method of PCM i.e. Pulse Code Modulation, i want to use simple and economical analog switching matrix using MT8816 or something related. Now as far as my work on this uptill now is as follows:-

Hardware is complete using basic telephony and electronics knowledge:

Details are as follows:-

1) Most important SLIC design ( not using market ICs) , know for those who don't have much knowledge about SLIC:-
SLIC or Subcriber Line Interface Circuit , is like a linking circuit between user telephone sets, it is avaiable in form of IC from Market but can be designed on your own by using following components, the SLIC i designed is one SLIC for 4 Users :-
1) Four, 12 Volts Relay,
2) For Each Relay , One Current Source Transistor C828 or C945 , One Switching Transistor 1383 or A673, back to back zeniors 3.3 volts or 3 volts.
3) one LM339 or LM3900N ( op-amps, 4 comparator) when ever a telephone set goes off-hook "Low Signal" goes to this opam and low goes at out-put. Usually this is used for Off-Hook Detection.

2) Then comes Tone, When ever a telephone Set goes off-Hook the user must hear a Sound which is similar to what we listen when we pick-up the normal telephone set connected to external service line , that sound is called "TONE", now i think that can be generated using 8051 by playing with frequencys but a good sound is produced using 555 Timer which generates a Square Wave , using a de-fily-flop IC to make digital pulses into sionsiudal pulse using adder like LM334.

3) Since we will be connecting Pulse Dailing Telephone Sets with our Exchange so no need to go for DTMF Receiver circuit. I think the Pulse work can be carried out using 8051.

4) Then there is MT8816 IC, for anaglow switching matrix.

5) Power Supply is connected parallel to all components, and it supplies 12 Volts, needed for MT8816, 5 Volts , supply is designed usually the tradition method of using 7805, 7905 , bridge 1M4007 or diodes etc..

6) Since the designer is going for 8 Lines Telephone Exchange so using two SLIC and each SLIC has a Shift Resistor liek HCF ICs,

7) Then there are other minor components like Ringer IC KA2410 etc..etc..

Now after atking so much time of the reader, firstly i like to thank you for giving so much time for this post and if you can help me on this please provide any suggestion which you understand..

Now my Objective is to make a software for this using 8051 and Assembly.

At this moment i don't understand how to start writing my software, like what is the alogaritum ?

Please Suggest if iam missing something or if i should up-grade on some. I working with asm files using X51 assmebler, then L51 and AVS51 simulator...as iam a 18 years hobbist so i don't have much money to buy window based softwares, if any one knows any free-downloadable simulator for windows or any other up-graded thing that i should install please tell me.

The Final Out Come of this project should be useful in following case:-

Let Suppose you are living in an 8 storey high building containing 100 apartments. Now at the main enterance of this apartment centre, security guards are provided with 3 normal Pulse Dailing Telephone Sets and 3 digit "7-Segment Display", also each apartment resident also has a pulse dailing telephone set in their flat. Now all these Telephone Sets are connected with a Central Box using a Single Pair Cable. When ever a visitor comes at the Main Gate the security guard ask him which apartment the visitor want to go, the secuirty guard dails the apartment number using a three digit dialing. In the same way each apartment resident can call at Main Gate by just dailing "9" and when ever the resident call at Main Gate their apartment Intercom Number appears on the 7-segment display, showing the security guards from where they are receiving the call. Similarly each apartment resident can call other apartment resident by just dailing their specific number. So its like a Mini.Telephone Exchange only for that apartment centre.
I think i have made of mountain out of mole.

Usman Perviaz
talentshow@engineer.com