mak kini wrote:
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I want just a basic block diagram.
I want to have communication between two instruments using the telephone line.


A basic block diagram would be quite simple. It depends on how basic you want it to be. There's a keyboard, Display, card-reader, microcontroller/cpu and a modem for communication.

If you want to communicate between two devices usign a telephone line, you will need a modem (for an analog line) or what is called an isdn-modem (in the case of an isdn line). If the line between your devices is dedicated and "galvanic" (ie not loaded and/or amplified, or simply put: a pair of copper wires end-to-end) you can get away with a base-band modem or even a simple line-driver. All depends on the quality of your line.

As for ATM machines: they have no "standardised" way of communicating. It is country dependent and various types of communications are in use. Some have been posted by others (isdn-d-channel). Other ways are: Dial-up on POTS, permanent connection by means of leased-lines (galvanic or other), connection by means of DSL.

As far as I know, communication is _ALWAYS_ encrypted. PIN-codes are NOT sent across the wire. All possible precautions have been made to prevent "eavesdropping" Banks are (fortunately) known for being over-zealous on security issues.



regards
Patrick