If you have only 3 devices connected to M1DBH#1 you will connect from J4 on M1DBH#1 to J1 on M1DBH#2 using a crossover cable — see the M1DBH Instructions for the pin out of the crossover cable. Connect your devices to M1DBH#2 and you can install a Jumper on the last “device” connected and not need the RJ45 with resistor built in. You will need to remember later if you install a new device to move the Jumper.