When viewing the Device Properties of an MNB device in the Workplace Commissioning Tool (WPCT) the controller status indicates Operational, Read-Only?

Issue

When viewing the "Device Properties" of an MNB device in the Workplace Commissioning Tool (WPCT) the controller status indicates "Operational, Read-Only"?

Product Line

TAC I/A Series

Environment

BACnet MNB controllers, WorkPlace Commissioning Tool all versions

Cause

The MNB-xxxx series controllers contain circuitry that is designed to protect the integrity of the embedded flash memory under low-voltage or questionable input voltage conditions.  In the event of a controller-perceived low-voltage condition, the controller will set a read-only flag and lock out all writes to memory, as well as turn off controller outputs.

Resolution

The Read-Only status can help serve as an indicator that the input voltage to the controller may be questionable.

Attention should also be paid to the wire distance between the central transformer and the secondary loads, especially in the case of half-wave input devices like the MNB-70, MNB-V series and MNB-300 controllers and the MNB-1000-15 remote I/O module. With half-wave type input devices, significant AC input current spikes can occur during the positive half-cycle of the AC input. Large resistances due to the wire lengths can cause significant voltage drops as seen from the controller AC input. In extreme cases, the controller may enter the read-only mode at apparent AC voltages exceeding 20 Vac due to the asymmetrical nature of the AC input voltage waveforms. In these cases, reducing the load on the transformer, reducing the wire length between the controller and the transformer, and using higher current rated wire will correct the problem.  For complete wiring guidelines consult the Micronet BACnet, Wiring, Networking, and Best Practices Guide.

Once the power issue is resolved cycling power to the controller will clear the read-only portion of the status.