[TangerineSDR] I2C Extender Distance, etc.

[David Witten]

I'm catching up on loose ends related to the magnetometer, and
double checking things I should have verified sooner.

I'm happy to report that when properly configured, the full magnetometer
support setup properly reports three visible I2C devices at the local (DE)
end.  These are typically one MCP9808 temperature sensor on the local board
(default address 0x18), another MCP9808 on the remote board (typically set
to 0x19), and the remote PNI magnetometer unit at 0x20. All seem to work
reliably when separated by at least 50 feet of shielded CAT-6 twisted pair
cable.

One matter that I did state in error previously was the length of the
shielded cable used for testing.  On closer checking I see that I had
purchased two 50 foot lengths of STP from Amazon.  A suitable 100 foot (~30
m) cable was unavailable that day.  So what I can say is that this
arrangement works very well at 50 feet, but 100 feet remains untested.

I plan to contrive some set up that will provide a yes/no answer to the
question 'does this setup work through two 50 foot lengths of cable
connected through a Cat-6 patch panel with appropriately punched-down
female connectors?'.  I will report the result.

Frankie, Troy, and I are also testing Arduino code for the magnetometer.
Although use of this is not anticipated for the PSWS, it is convenient for
reliability testing and for non-PSWS users of the magnetometer.

Dave Witten, KD0EAG
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