Owner`s manual

Page A-4 Appendix A
CABLE CONSTRUCTION GUIDELINES
If you want to construct your own cables, there are a few things you should keep in mind
before soldering the cable connectors:
Cable Length
We strongly recommend that RS-232C cables be not more than fifty feet long. As you
increase the cable length beyond fifty feet, the reliability of the data signal decreases.
The cable’s susceptibility to noise and cross-talk is proportional to its length and
bandwidth, so RS-232C restricts both. This is based on untwisted paired cable. The
following restrictions apply:
1.Slew rate—The slew rate is the speed with which the signal changes from high to
low. The higher the slew rate, the more likely you are to create noise interference
on other devices. To control interference on neighboring circuits, the slew rate of
drivers is limited to 30V/us.
2.Bandwidth—The bandwidth is limited on receivers to reduce cross-talk (the effect
of one line on another).
These are some things you can do if the cables absolutely must be longer than fifty feet:
1.Alternatives to the RS-232 standard (RS-422 and RS-423) do allow
communication over cables longer than fifty feet. You will need some special
equipment to implement these standards, however.
2.Signal conditioning equipment (for example, a "short haul modem") can improve
signal quality.
Cable Type
We recommend you use a twisted paired overall shielded and jacketed cable— which is
rated at least CMP Level 3. You can use level 5 CMP cable for lines longer than 100
feet. The CMP stands for Communication cable Plenum rating. Most cities now require
that any low voltage communication cable have a Plenum rating when the cable is run in
air return spaces. This type of cable should comply with your local fire codes for
installation in your facility. Using a high quality twisted paired, overall shielded cable,
helps minimize electromagnetic interference. Reducing this interference not only
protects your system from signal noise, but protects other devices around your Alpha
Micro computer system (such as a TV or radio) from interference radiated by an
improperly shielded system.
Eagle Series Computer Owner’s Manual, Rev. 03