Hi @bengfeld ,
Looks like it's Industrial-Shields/arduino-Modbus this one..
documentation is in the git, linked above..
mac short for "media access control"..
a unique identifier assigned to a network interface..
normally programmed by the manufacturer..
i would stay away from the commonly used DEAD BEEF FEED, it's cute but not so unique..
the exclamation point NOTs (reverses) the return value of readInputRegisters to demonstrate how to catch errors..
slave is the EthernetClient that has been connected to the remote machine..
I do believe the rest has been explained..
I would approach this scientifically, experimentation..
if (master.readInputRegisters(slave, 1, 844, 2)){
//success
}
be my first try..
good luck.. ~q