A-17
A2
RS-232
Most popular standard for office machines and
computer systems.
RS-422 and
New high speed standard with noise
RS-485
improvements over RS-232 for longer distances.
Devices employing the same interface standard can usually be connected
together but the user must verify each devices signal requirements
before plugging them together.
When data must be transmitted over long distances, it is typically sent over
the phone company's direct dial network (DDN) as shown in Figure A-8.
Modems are used to convert the serial data bits into tones that will pass
through the telephone system's 300 to 3000 Hz voice band. For low baud
rates, up to 1200 Hz, the modems convert the bits into two tones (frequency
switched keying) that the receiving modem recognizes and converts back
to data bits. These low speed modems are referred to by the telephone
company's designations, i.e.: Type 103 (300 baud) and Type 212 (1200
baud). Higher data rates require more complex modulation techniques and
the modems are referred to by their CCITT specification i.e., V22.
PC or
Computer
Modem
Modem
DDN or Leased Line
2303
Interface
Digital
Device
Digital
Signals
RS-232
RS-232
Chassis
Figure A-8 Long Distance Communication using Modems
