12-27-2007, 09:15 PM
I'm confused. My original 25C came with a splitter cable and one 60 ohm terminating resistor. I am now planning to network my 522 in and the sample system shows (2) 120 ohm resistors. I know that two 120 ohm resistors in parallel equates out to 60 ohms but the 120 ohm resistors are at each end of the "backbone". Will my single 60 ohm reisitor work with the added tee's and cables or do I need to get some 120 ohm resistors?
12-27-2007, 10:29 PM
From everything I understand about the NMEA network you are setting up (I have a similar setup, but not yet completed) you need two 120 ohm terminators to make this work. Another thing to check is if the 522 and the 25 are both "blue" connectors. If not you will need a "blue" to "red" adaptor.
Ditto on the advice - I added in the fuel flow sensor, and tried for the last half of the season to make it work with just the one resistor. Lots of problems with gps antenna not being recognized, fuel sensor not being seen, etc. All very intermittant. Would go for days thinking all was great, then see problems, then it would be great again. Redid it this last spring as the latest instructions advise - build a backbone with Tees, each end of the backbone terminating in a resistor, and plug your stuff into each Tee. Dump the Y cable, and use individual Tees and single lines for every device. Resolved all problems!
A bit more -
I was advised by a respected depthfinder repair shop that I could get away with one device plugged into the end of the backbone (ie - two devices into one tee) and then use one 120ohm terminator at the opposite end of the backbone. The reasoning being that the devices "were all 120 ohm", hence the system would be balanced. No go - just as intermittant as before. The "Y" cable also isn't as clean, electrically as a Tee is, hence Lowrance's advice to not use it when expanding the network. (Works fine for just the Depthfinder and GPS antenna, but adds problems when you patch in additional items). This technology harkens back to the early days of computer networking using coax cable, and is real touchy as to impedences, proper termination, stray signal rejection, etc. Built as described before, all is stable, but variations aren't well tolerated. It is irritating to have to buy additional (pricey) tee's and terminators to get it all to work, when there are two open ends that "should" be able to be used.....
Lowrance told me, BTW, that the only reason for the change from blue connectors to red was to match new standards from NMEA as to salt water resistance at the connectors - blue was good, but red is better. Hence the need for adaptors for us with slightly older equipment, if your local sources are out of the color you need.