This is Nagios plugin for Modbus TCP and RTU.
The first version of this plugin was taken from
http://exchange.nagios.org/directory/Plugins/Network-and-Systems-Management/Check_modbus_tcp/details

Added many useful features:
- support of IPv6 / IPv4 
- support of DNS names
- serial port connections (RS232 and RS485) with Modbus RTU (when compiled with libmodbus >= 3.0.0)
- support of many data formats  (unsigned/signed int16_t, int32_t, float, double)
- support of several data orders (LSB, MSB and some others)
- retries can be sent to get reliable answer
- supports performance data
- supports creation dump of the registers (binary, hexadecimal, decimal)
- support binary dump as an input source for data (useful for off-line data)


In general, plugin can be build as usual

./configure 
make
make install


When you are trying to build plugin under MinGW, then maybe you will need to define path for libmodbus explicitly.

libmodbus is installed by default in /usr/local/lib. In MinGW you will need to run 

./configure LDFLAGS=-L/usr/local/lib CPPFLAGS=-I/usr/local/include

instead of just ./configure


For usage details see manual page or run ./check_modbus --help.