Appendix A – Connecting Peripheral Instruments
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:
þUse isolating transformers or a separate
þUse an isolating transformer with weather FAX audio cables.
þUse an
þUse 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
