From: whit3rd@gmail.com   
      
   On Tuesday, April 9, 2019 at 8:50:13 AM UTC-7, default wrote:   
      
   > >> For instance, I'm running one part of the circuit at 8 volts and   
   > >> another at 12.  It is necessary to input a 12volt output to a 8 volt   
   > >> input.   
      
   > I already know how to return the pull-up   
   > to the 8 volt side - but want to keep the 12V swing for other circuits   
   > using that same signal line.   
      
   Any protection diodes will NOT necessarily respect the '12V' signal,   
   they might clamp it   
      
   > Texas Instruments CD4520B   
   > Dual up counter   
      
   The data sheet says this WILL diode-clamp input to +8V.  Either your +12V   
   signal or   
   your +8V supply will suffer, if the chip doesn't burn up.   
      
   The (relatively) clean solution is two resistors, making a voltage divider on   
   the (0V, 12V) signal   
   to make (0V, 8V) output.   Something like a 10K + 20K ohm will do, depending   
   on what   
   the resistor value of the open collector pullup is.   Accurate values are not   
   needed, for   
   normal logic operation, any output above 6V is 'high' enough for that CMOS   
   with 8V applied.   
      
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