Appendix C
Serial Port Information
PCI Serial for Windows 95
C-2
©
National Instruments Corporation
(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 C-1 lists the features of the RS-232, RS-422, and RS-485 standards.
Table C-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