This module provides a basic Modbus client for reading from and writing to
various ModBus slaves using RTU or ASCII modes.  If you have a serial device
(that uses RS-232 or RS-485), this module will work for you.  It also supports
the notion of a ether-to-serial converter, like the Lantronix or Digi devices.

Note that this module does NOT support TCP/Modbus (see Protocol::Modbus for a
more complex but more complete interface).