title: Writing single bit in modbus
body: <p>I'm interfacing a device over the Modbus protocol.</p>
  
  <p>There's commands for reading and writing registers (register = 2 bytes).</p>
  
  <p>However, often a register contains multiple config flags as bits, and I need to write just one of them or so.</p>
  
  <p>I could read it, modify flag and write it back, but is that the right way?</p>
  
  <p>I've seen some commands for writing coils, but I'm not really sure what a coil is - is it a bit? If so, how do get address of a coil?</p>