Raspberry Pi Sense/Net Bridge
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-Need 5V level conversion on Raspi I/O pins
-Use multiplexer/demultiplexer to select one of three RS-485 transceivers
  -The receive output of the transceiver seems to be tri-state.
   
   
Component Selections
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-Demultiplexer: NXP 74HCT238PW,112
-Multiplexer: NXP 74HCT151PW,112
-Level shitfer mosfet: Diodes DMN63D8LDW-7

Rev. 1 Issues
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-No PTCs on I/O lines
-Should draw power from external source, to avoid overloading the 5V rail
-MCP23009 is open drain; replace with MCP23008
-R/W not connected to gnd
-Need pull up on E line

Rev. 3 Additions
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-Get rid of 6 wire. Instead, automatically select RS-485 or 1-wire for each bus.
  -Use analog switch (per channel) for 1-wire data line
  -Use mosfet (per channel) for extra ground.