Serial Communications

Each protocol builds on the older technology.

RS232 - point to point - one computer and one device per port. Defines a standard set of data rates but no software protocol.

NMEA 0183 - this is a simple software protocol that uses RS232 for its hardware layer. NMEA 0183 is used for exchanging information between a master station (usually a computer or a "chart plotter") and a controller or sensor. Originally NMEA 0183 was intended for use on boats but also widely used in GPS equipment, most GPS receivers can be progreammed to output a NMEA 0183 data stream.

RS422 - Like RS232 but with differential signaling. Still point to point.

RS485 - Differential signaling. Adds bus topology that can be half or full duplex. In half duplex mode there can be one master and many slave nodes. Master polls slaves to avoid contention; only one slave talks at a time. No software protocol defined.

CAN (Control Area Network) bus - devised by Intel and Bosch for automobiles. Uses collision detection, so any node can talk anytime it needs to.

USB Universal Serial Bus - now found on all new desktop computers and many peripherals; has pretty much replaced RS232 on computers. Like RS485, it uses half duplex and differential signaling. Like CAN, it has a software protocol defined. It also has connectors and cabling included in the specifications.

NMEA 2000 - complete specification for the physical and software communications; designed to be used on boats. NMEA 2000 is based on CAN, but like USB, the allowed data rate, connectors and cabling are included in the spec.