Page 62 - The Mechatronics Handbook

FIGURE 4.6 Example of multilayer communication.
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 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,
2
RS-485, I 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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