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Partitioning example: An external PC Modem.

The figure shows the block diagram of a typical modem. The illustrated device is an external modem, meaning that it sits in a box beside the computer and has an RS-232 serial connection to the computer. It also requires its own power supply.

The device contains a few analog components which behave broadly like a standard telephone, but most of it is digital. A relay is used to connect the device to the line and its contacts mirror the `off-hook' switch which is part of every telephone. It connects a transformer across the line. The relay and transformer provide isolation of the computer ground signal from the line voltages. Similarly the ringing detector often uses a opto-coupler to provide isolation. Clearly, these analog aspects of the design are particular to a modem and are designed by a telephone expert.

Modems from the 1960's implemented everything in analog circuitry since microprocessors and DSP were not available. Today, two microprocessors are often used, but as processing power increases, this can be reduced to one (or sometimes even none, if the main processor of a PC provides the functions needed).

The reason for two processors are interesting and will be discussed in lectures.

Note that the non-volatile RAM requires a special manufacturing processing step and so is not included as a resource on board the single-chip processor. Similarly, the RS-232 drivers need to handle voltages of +/- 12 volts and so these cannot be included on chip without increasing the cost of the rest of the chip by using a fabrication process which can handle these voltages. The NV-RAM is used to store the owner's settings, such as whether to answer an incoming call and what baud rate to attempt a first connection, etc..