12Miniature Modems

Miniature short range modems offer a simple, low cost and highly reliable method to connect digital devices in a local, campus or metropolitan environment. They also offer very easy installation since in many cases they do not require AC power, but derive their power from the interface signals.

Miniature short range modems offer the same communications functions as AC- powered modems but with fewer features, such as limited diagnostics capability and no BER tests. Usually they operate over shorter distances at lower rates (normally up to

19.2kbps). However, certain modems can operate at data rates up to 115.2 kbps in async transmission and up to 128 kbps in sync transmission. Like AC-powered modems, miniature modems are designed to overcome distance limitations of standard data communications interfaces such as V.24/RS-232. They also overcome noisy environments and provide ground isolation.

Miniature short range modems connect computers, terminals, bridges/routers and similar data communications devices inside buildings, campuses or within city boundaries. They support simple point-to- point communications as well as complex multipoint campus-wide systems. They also support flow control when connected to a printer or a multiplexer.

Typical communications problems, such as distance limitations and noise, are overcome by using special signal modulation and line equalization techniques that allow for error-free transmission over longer distances. RAD miniature modems have transformers to isolate the equipment from the line and ensure safety. The transformers are rated at over 1,500 VRMS and are approved by carriers worldwide for connection to their lines.

What is the difference between

interface-powered and AC-powered

modems?

Interface-powered and AC-powered modems perform the same basic communications functions. Interface- powered modems incorporate innovative VLSI and hybrid circuitry that enable them to consume very little power. These modems draw power from the interface signals without loading the adjacent equipment or interfering with normal operation. For computers that do not support control signals, there are special models that derive their power solely from the data signals.

How do you select the right modem for

your application?

Several factors should be considered when selecting a modem:

•Computer/DTE interface: V.24/RS-232, V.35, etc.

•Communication method: sync, async or both

•Data rate

•Distance

•Media: twisted pair, coax, fiber

•Point-to-point or multipoint application

•Control signals (does the application require transmission of data only, or also transmission of one or more control signals?)

•Environment (is the environment noisy?)

A Quick Reference Guide on pages 158-159 will help you select the most suitable modem for your application.

146 RAD Data Communications 2006 Catalog