Revise MKR 485 communication tutorial

[sebromero]

Updated tutorial on sending data between two MKR 485 Shields, including new sections on RS485 standard, differential signaling, and circuit connections.

What This PR Changes

• Added missing:

  • An explanation of what is termination in RS485 and how does it need to be configured. -> To avoid signal reflection. Needed at both ends unless communication is unidirectional (then only at the receiver end)
  • What is a differential signal and how is it calculated in RS485. Basically Voltage A - Voltage B > Threshold = 1, else 0.
  • What is half duplex / full duplex mode
  • An explanation that in half duplex mode only Y Z are used

• Fixed incorrect:

  • The dip switch should be at half duplex (as this is being demoed here. Full duplex would be a different setup)
  • Termination should be enabled on Y Z on both boards
  • They should be connected Y <-> Y and Z <-> Z. No crossover necessary in RS485 because it uses a differential signal with which the polarity is always clear.
  • ISO GND should be connected to ISO GND, not the board’s GND

Contribution Guidelines

• I confirm that I have read the contribution guidelines and comply with them.