The most commonly used serial devices in PCs are modems. The word modem is actually an acronym for Modulate-Demodulate.
Modems allow PCs to send data over a phone line to another computer. Digital signals from the processor are modulated into analog tones that can be carried over a phone line.
On the receiving end, the analog tones are demodulated back into digital signals that can be processed by the receiving PC.
The invention of the PC modem can be directly attributed to a gentleman by the name of Denny Hayes.
Before the introduction of the Hayes SmartModem, modems worked on a synchronous basis. Each modem has an internal clock. The clock in each modem was used to synchronize data transfer. This type of modem is known as a synchronous modem.
The SmartModem created a relatively low-cost modem by eliminating the clock. Hayes was able to eliminate the clock by specifying when a data transmission begins and ends by using start and stop bits.
The one challenge Hayes faced was how to control the modem. To accomplish this task he created the AT command set.
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