Serial communication interfaces can vary in a number of different ways depending on purpose. As a result, the general methods, or types of serial protocols, are able to be narrowed down into some general categorizations. There are, therefore, five general types of serial protocol, each somewhat dependent on the number of hosts and receivers. From there, each serial bus, depending on the communication protocol used, will have additional features that will offer varying functionality.
The most common is the recommended standard 232 (RS-232) protocol. This falls into the peer, or point-to-point group of serial protocols, and a number of serial interfaces have been developed based on its concepts. Here, two devices, or peers, communicate directly with one another in what's called full-duplex, meaning that communications can go both ways simultaneously. They can optionally synchronize their communications via a clock, though most commonly, the peers communicate asynchronously. Limited in speed and distance, it is commonly used for attaching serial peripheral devices to computer systems.
Another of the recommended standards, number 422, that came a bit later details a different protocol type that can support additional systems. With RS-422-based serial protocols, additional receivers can be supported by a single transmitter. This is referred to as a multi-drop protocol. The speed is increased dramatically over what's available with RS-232, including the allowed length of the wire. The cost, however, is that it operates at only half-duplex, therefore only allowing communication over the wire one way at a time unless additional wires are established between the receivers and the transmitter.
Similar to RS-422 is another recommended standard describing a multi-point option for serial protocols. Known as RS-485, the multi-point technique allows for multiple peers to send and receive to one another over the bus. Like RS-422, RS-485 serial protocols are very fast and capable of great distances. Also like RS-422, however, additional wires are required to go beyond a half-duplex communication between the peers, which can be limiting and costly.
Beyond the array of recommended standards, several companies have also developed a number of different types of serial protocols for use with particular products, but which have also contributed to other developments. These types involve the use of a master device and one or many slave devices attached to the serial bus. One such type is known as Microwire™ created by National Semiconductor®. It is a master and slave type, where the master device sets a clock for synchronized communication with a slave device. Similar to the master to slaves technique provided by Microwire™, the serial peripheral interface (SPI) type of serial protocols, developed by Motorola® further added the ability for multiple masters that can communicate with multiple slaves in full-duplex.