Table C-1. Rs-232, Rs-422, And Rs-485 Features - National Instruments PCI-232/2 Getting Started

Appendix C Serial Port Information
RS-485
PCI Serial for Windows 95
(up to 4,000 ft.) and much greater transmission speeds (up to 10 Mb/s) than
RS-232.
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
Type of
transmission lines
Maximum number
of drivers
Maximum number
of receivers
Maximum cable
length
Maximum data rate
Maximum CMV
Driver output
Driver load
RS-232 RS-422
Unbalanced Differential
1 1
1 10
50 ft. 4,000 ft.
20 kb/s 10 Mb/s
±25 V ±7 V
5 to 25 V 2 to 6 V
100 Ω
> 3 kΩ
C-2
RS-485
Differential
32
32
4,000 ft.
10 Mb/s
+12 to –7 V
1.5 to 6 V
60 Ω
© National Instruments Corporation