There are five general types of serial protocols, each dependent on the number of hosts and receivers. The most common is RS-232, while RS-422 and RS-485 support additional systems. Microwire and SPI are master and slave types of serial protocols developed by National Semiconductor and Motorola, respectively.
Serial communication interfaces can vary in many different ways depending on the purpose. Consequently, general methods, or types of serial protocols, can 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 under 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 each other in what’s called full-duplex, meaning communications can go in both directions at the same time. They can optionally synchronize their communications via a clock, although more commonly peers communicate asynchronously. Limited in speed and distance, it is commonly used to connect serial peripherals to computer systems.
Another of the recommended standards, number 422, which arrived a bit later, details a different type of protocol that can support additional systems. With RS-422 based serial protocols, additional receivers can be supported from a single transmitter. This is referred to as a multi-drop protocol. The speed is significantly increased over what is available with RS-232, including the allowable cable length. The cost, however, is that it only operates in 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 it is another recommended standard that describes a multipoint option for serial protocols. Known as RS-485, the multipoint technique allows multiple peers to send and receive each other over the bus. Like RS-422, RS-485 serial protocols are very fast and capable of great distances. Even like RS-422, however, additional cabling is required to go beyond half-duplex communication between peers, which can be limiting and costly.
In addition to the recommended set of 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 more slave devices connected to the serial bus. One such type is known as Microwire created by National Semiconductor®. It is of 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 protocol, developed by Motorola®, further added the ability for multiple masters to communicate with multiple slaves in full-duplex.
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