Long time reader - first time poster.

I have a Siemens BACnet MBC running firmware MBCE V3.0 BACnet 4.2g. This MBC is an open processor only, gathering data via FLN1 from a Trane Summit BMS interface. This MBC controls our chillers. The MBC is installed in an enclosure with a second MBC and point termination modules. The MBC communicates with Insight via Ethernet.

I frequently (100+ times per day) lose communications with this MBC. All or most of the points will go "FAILED". The panel can stay FAILED for 30 seconds or 30 minutes depending on it's mood.

I am monitoring the MBC diagnostic points and I get many NO.RESPONSES and my NAK.COUNT climbs continuously.

We have replaced the Siemens Trunk Interface II. We have re-terminated the FLN connection (and made sure only one end of the shield has been landed.) We have verified the FLN is set for 9600 baud. We have also cold-started the processor several times. Siemens techs so far unable to provide much assistance.

Can anybody provide any more ideas of different things we could try?