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Appendix B Serial Port Information
PCMCIA Serial for Windows 98/95 B-2 © National Instruments Corporation
RS-485
RS-485, as specified in the EIA-485 Standard, Standard for Electrical
Characteristics of Generators and Receivers for Use in Balanced Digital
Multipoint Systems
, expands on the RS-422 standard by increasing the
number of devices you can use from 10 to 32 and by working with
half-duplex bus architectures. Unlike the RS-422 standard, RS-485
addresses the issue of using multiple transmitters on the same line. RS-485
defines the electrical characteristics necessary to ensure adequate signal
voltages under maximum load, short-circuit protection, and the ability to
withstand multiple drivers driving conflicting signals at the same time.
Table B-1 lists the features of the RS-232, RS-422, and RS-485 standards.
Table B-1. RS-232, RS-422, and RS-485 Features
Feature RS-232 RS-422 RS-485
Type of
transmission lines Unbalanced Differential Differential
Maximum number
of drivers 1 1 32
Maximum number
of receivers 1 10 32
Maximum cable length 50 ft. 4,000 ft. 4,000 ft.
Maximum data rate 20 kb/s 10 Mb/s 10 Mb/s
Maximum CMV ±25 V ±7 V +12 to –7 V
Driver output 5 to 25 V 2 to 6 V 1.5 to 6 V
Driver load >3 k
Ω
100
Ω
60
Ω