The ELK M1 Gold supervises the data bus and each device connected to the data bus. If a bus device fails to check-in the M1 will Log Expansion Module Trouble and when the device does check in the M1 will Log a Restoral of that bus device. Several reasons why a bus device might fail to check-in could include a voltage drop at the bus device, issues with the data across Data A and Data B, noise on the data bus or perhaps a faulty bus device. The voltage at each bus device should be ~12-14 VDC. There are three methods to wire the M1 Data Bus 1) M1DBH Data Bus Hub which requires a Cat5/6 from the M1DBH all the way to each bus device, 2)M1DBHR Data Bus Retro Fit Hub for existing 4-Conductor Home Runs, 3) Daisy Chaining Bus Devices. There should Never be more than two home runs connected to the M1 Data Bus. With the M1 powered off including any secondary power supplies powering Bus Devices measure the resistance across Data A and Data B at the M1 Main Board — you should measure ~60 – 70Ω Ohms. A reading of 120Ω Ohms means you are missing a terminating jumper on the data bus and a reading of 40Ω or less would indicate too many terminating jumpers are installed.