Unlike the standard serial ports, the ADB interface on older Mac OS machines is synchronous. Hardware handshaking, as employed, for example, between a modem and a computer, is faster but requires additional wires in the cable. Software handshaking, operated by special control codes sent between the devices, is effective, but slows down the interface. To ensure data is sent reliably over a link, some form of handshaking is normally used. Hence the data can be eventually sent over a two-wire circuit, such as a telephone line connected via a modem. These short bursts minimise timing problems, avoiding the need for a separate clock circuit. The Modem and Printer ports are based on the RS-422 standard, with data sent in asynchronous form, consisting of a start bit, followed by eight data bits and a stop bit. All of these convey data in serial form, with each bit of data transported in turn along a common circuit, which makes them rather slow, although they employ convenient low-cost connectors and lightweight cables. Most ‘classic’ Mac OS models have two such ports, usually designated as Modem and Printer ports, as well as Apple Desktop Bus (ADB) connections. These only accommodate one device at a time, although you can use a serial port switch box to choose one of several devices. Older computers often have simple serial ports instead of USB or FireWire connections.
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