title: What does the pymodbus "unit" parameter mean?
body: <p>I have some modbus TCP code written under pymodbus 1.2
  the relevent code was</p>
  
  <p><code>result = modbus_client.read_holding_registers(40093, 3)</code></p>
  
  <p>After updating to pymodbus 1.4.0 it wouldn't work until I cargo culted the new unit parameter into the function call (teh examples all had unit=1 in
  them):</p>
  
  <p><code>result = modbus_client.read_holding_registers(40093, 3, unit=1)</code></p>
  
  <p>what does the unit parameter in pymodbus read_ holding_registers() mean?
  I can't seem to find an explanation anywhere.
  The source says ":param unit: The slave unit this request is targeting",
  but I don't understand what this means, nor what selection other than 1 might be used.</p>