A fax machine is best thought of as a dedicated computer that controls an image scanner, a modem and a printer, acting as a middleman to control the inputs and outputs of these devices in order to complete a specific task, in this case the transmission of documents.

To this end, it is best to describe each component and then follow up with the control unit that makes it all possible.

The scanner is the name given to a device which scans text, barcodes, images, handwriting or any other organic or digitally determined image, converting it into a digitally stored piece of information. The scanners that were used in fax machines for the majority of the last 30 years were of the type known as drum scanners. These used a photo multiplier tube, type of vacuum tube which were extremely sensitive when it came to the detection of visible and ultraviolet light. Nowadays, scanners tend to use charge coupled devices, also known as a CCD, like those present within digital cameras.

The principal remains the same however. The scanner has an electrical charge pushed through it, and the imaging device converts the optical signals to electrical ones. This is then captured by the fax hardware.

As the image is scanned and converted into a digital file, the fax hardware engages the modem. The modem will use telephone circuits to connect to another modem and begin transmitting an audible analog signal. By modulating this signal, it is possible to transmit digital information to the modem at the other end of the line, which will then be demodulated and be fed to the receiving fax hardware. This modulation / demodulation process is where we get the name modem.

Finally, the fax machine passes the demodulated digital image file to the printer interface. This effectively reverses the scanning process, by passing a print head over blank paper in the positions that the optical scanner picked up areas of contrasted marking, then writing those marked areas back to the paper.

These systems are controlled by the fax hardware. It provides the logic and the flow control required for the information to pass seamlessly between the three interfaces. While the firmware and specific hardware involved may be propitiatory, fax hardware manufacturers adhere to a standard to enable all the fax machines all over the world to communicate with each other.

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