Digital Communications

Digital Communications

Intercommunication among mechatronics subsystems plays a key role in their engagement of applications, both of fixed and flexible configurations (a car, a hi-fi system, a fixed manufacturing line versus a flexible plant, a wireless Pico-net of computer peripheral devices).

 It is clear that digital communication depends on the designer’s demands for the amount of transferred data, the distance between the systems, and the requirements on the degree of data reliability and security.

The signal is represented by alterations of amplitude, frequency, or phase.

This is accomplished by changes in voltage/current in metallic wires or by electromagnetic waves, both in radio transmission and infrared optical transmission (either “wireless” for short distances or optical fibers over fairly long distances).

Data rate or bandwidth varies from 300 b/s (teleprompter), 3.4 kHz (phone), 144 kb/s (ISDN) to tens of Mb/s (ADSL) on a metallic wire (subscriber line), up to 100 Mb/s on a twisted pair (LAN), about 30–100 MHz on a microwave channel, 1 GHz on a coaxial cable (trunk cable network, cable TV), and up to tens of Gb/s on an optical cable (backbone network). Data transmission employs complex methods of digital modulation, data compression, and data protection against loss due to noise interference, signal distortion, and dropouts. Multilayer standard protocols (ISO/OSI 7-layer reference model or Internet 4-layer group of protocols including well-known TCP/IP), “partly hardware, partly software realized,” facilitate an understanding between communication systems.

They not only establish connection on a utilizable speed, check data transfer, format and compress data, but can make communication transparent for an application. For example, no difference can be seen between local and remote data sources.

Depending on the number of users, the communication is done either point-to-point (RS-232C from PC COM port to an instrument), point-to-multipoint (buses, networks), or even as a broadcasting (radio).

Data are transferred using either switched connection (telephone network) or packet switching (computer networks, ATM).

Bidirectional transmission can be full duplex (phone, RS-232C) or semi duplex (most of digital networks). Concerning the link topology, a star connection or a tree connection employs a device (“master”) mastering communication in the main node(s).

A ring connection usually requires Token Passing method and a bus communication is controlled with various methods such as Master-Slave pooling, with or without Token Passing, or by using an in deterministic access (CSMA/CD in Ethernet).