"KEY BUS TROUBLE" (identified as SBUS expander trouble in the manual) indicates that the control panel has lost communication with or cannot recognize a device on the SBUS, such as a remote annunciator or expander module (Page 9-1) [1].
Verify that the module's DIP switches are set to the correct ID as assigned in the system programming (Page 10-2) [1].
Ensure the device has been added to the system using the JumpStart feature or manual programming (Page 10-2) [1].
Check the wiring between the panel and the module for correct polarity and secure connections (Page 10-2) [1].
Measure the voltage at the module's (+) and (-) terminals; it should be in the range of 27.2V to 27.4V when AC power is present (Page 10-2) [1].
Observe the module's green LED; a flash pattern of 0.5 seconds on and 0.5 seconds off indicates the device is physically connected but not programmed (Page 10-2) [1].
