Appendix B Serial Port Information
Serial for Windows NT
B-2
©
National Instruments Corporation
the voltage on line B. Differential voltage transmission creates a signal
that is much more immune to noise as well as voltage loss due to
transmission line effects. Thus, you can use RS-422 for much longer
distances (up to 4,000 ft.) and much greater transmission speeds (up to 10
Mb/s) than RS-232.
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
+6 to -.25 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
