Digital Communications
Intercommunication among mechatronics subsystems plays a key role in their engagement of applications,
both of fixed and flexible configuration (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 designers 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 radiotransmission 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 (teleprinter), 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. An example of a multilayer communication concept
is depicted in Figure 4.6.
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 semiduplex
(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 indeterministic access (CSMA/CD
in Ethernet).
An LPT PC port, SCSI for computer peripherals, and GPIB (IEEE-488) for instrumentation serve as
examples of parallel (usually 8-bit) communication available for shorter distances (meters). RS-232C,
RS-485, I
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C, SPI, USB, and Firewire (IEEE-1394) represent serial communication, some of which can
bridge long distance (up to 1 km). Serial communication can be done either asynchronously using start
and stop bits within transfer frame or synchronously using included synchronization bit patterns, if
necessary. Both unipolar and bipolar voltage levels are used to drive either unbalanced lines (LPT, GPIB
vs. RS-232C) or balanced twisted-pair lines (CAN vs. RS-422, RS-485).

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