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A-1
A
Serial Port Information
This appendix describes the RS-232, RS-422, and RS-485 standards and
explains some of the issues involved with these types of serial
communication.
explains some of the issues involved with these types of serial
communication.
Table A-1 lists the features of the RS-232, RS-422, and RS-485 standards.
RS-232
As specified in the ANSI/EIA-232-D Standard, Interface Between Data
Terminal Equipment and Data Circuit-Terminating Equipment Employing
Serial Binary Data Interchange, RS-232 standardizes serial
communication between computers, and between computer terminals and
modems. Most applications use the RS-232 standard to interface
peripherals to personal computers. RS-232 uses transmission lines in which
the state of each signal is represented by referencing the voltage level of a
single line to ground. RS-232 was designed for serial communication up to
Terminal Equipment and Data Circuit-Terminating Equipment Employing
Serial Binary Data Interchange, RS-232 standardizes serial
communication between computers, and between computer terminals and
modems. Most applications use the RS-232 standard to interface
peripherals to personal computers. RS-232 uses transmission lines in which
the state of each signal is represented by referencing the voltage level of a
single line to ground. RS-232 was designed for serial communication up to
Table A-1. RS-232, RS-422, and RS-485 Features
Feature
RS-232
RS-422
RS-485
Type of
transmission lines
transmission lines
Single ended
Differential
Differential
Maximum number
of drivers
of drivers
1
1
32
Maximum number
of receivers
of receivers
1
10
32
Maximum cable
length
length
50 ft
4,000 ft
4,000 ft
Maximum data rate
20 kbytes/s
10 Mbytes/s
10 Mbytes/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