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Appendix A – Connecting Peripheral Instruments
A-15
Circuit Grounding Issues
By far the most important issue in electrical connections is proper
circuit grounding. In addition to all the other electrical gremlins that
poor grounding gives rise to, instrument signals are easily scrambled
if they are badly grounded, resulting in erratic and unreliable displays.
Installations with both AC and DC current have separate ground
circuits for each. In such cases, the use checklist that follows to
ensure that the AC and DC ground circuits are kept fully isolated:
þ Use isolating transformers or a separate power-inverter to
run PC, monitors, and other sensitive electronic instruments
or devices.
þ Use an isolating transformer with weather FAX audio cables.
þ Use an RS-232/NMEA converter with optical isolation on
the signal lines.
þ Use PCs and other devices that are directly DC powered.
Proper Cable Shielding
For all but the shortest length cable runs, data cables must be shielded
to prevent them from picking up electrical noise from other devices
and to keep them from radiating potential interference. If you
encounter a data corruption problem, a good place to begin trouble-
shooting is with the cable’s shielding, especially on longer runs.
Make sure the cable’s shielding hasn’t been scrapped off by being
squeezed through a tight area.
Preventing Interference and Crosstalk
You should always run data cables as isolated as possible, and as far
away from high current carrying AC and DC power lines and antennas
as possible. While it may be convenient, bundling power and data
cables together causes crosstalk between them, resulting in scrambled
and unusable data. If you must cross power cables when running data
cables, try to make the crossing as close to 90° as possible; never run
power and data cables parallel within the same bundle. If you have
long runs planned for your data cables, and are using NMEA equip-
ment, use shielded twisted-pair cable to eliminate the danger of
interference and crosstalk.