Specifications

In some cases, there are statistical counters that count when recovery actions have been taken. The following
sections define four resources monitored by the BACnet software and describe what you may notice if the resource
throughput is reduced.
The Object Engine Input Queue
The Object Engine Input Queue handles all incoming messages intended for object level operations. The Object
Engine processes these messages in order by priority, one at a time. If this queue is backing up, operation is slow
and applications sending messages may encounter errors.
If the Object Engine Queue is overloaded, you may notice the following:
The BACnet® protocol messages (starting with the lowest priority) received by the queue are aborted (discarded if
in the Alarm or Critical state). The Protocol Engine counter, Rcv Messages Discarded (under BACnet Protocol
Eng, Diagnostic tab), counts and displays the number of discarded messages.
If the Object Engine Queue is critical, all messages sent to the queue (including internal messages) receive an error
and BACnet protocol messages received from the IP port are discarded. Discarded Frames (under Eth IP Datalink,
Diagnostic tab) counts and displays the number of discarded messages.
Available Free Memory
Free memory is required to buffer all incoming and outgoing messages, as well as to save information needed for
internal processing. If available free memory becomes low, some communications fail and devices may drop offline.
If available free memory becomes completely depleted, all processing fails.
If the free memory available is low, received BACnet® protocol messages (starting with the lowest priority) are
aborted (discarded if in the Alarm or Critical state). The Protocol Engine counter, Rcv Messages Discarded (under
BACnet Protocol Eng, Diagnostic tab), displays the number of discarded messages. Additionally, BACnet protocol
messages waiting to be transmitted are also discarded. If an external node is waiting for a reply, an abort is sent
unless the condition is alarm or critical. The number of aborted messages are counted and displayed in Abort Tx
(under BACnet Protocol Eng, Diagnostic tab). The Protocol Engine counter, Tx Messages Discarded (under BACnet
Protocol Eng, Diagnostic tab), displays the number of discarded messages. If this state exists for more than a few
seconds, external BACnet protocol devices may be flagged offline.
If the free memory available is at the alarm level or worse, checking for the online/offline status of external BACnet
protocol devices stops. This may cause external BACnet protocol devices to be flagged offline if they are not frequently
sending data to their supervisory device.
If the free memory is critical, messages to be routed from one network to another through the controller are discarded.
Additionally, information for routing to previously unknown network numbers are not saved. BACnet protocol messages
received from the IP port are discarded. The IP port counter, Discarded Frames (under Eth IP Datalink, Diagnostic
tab), displays the number of messages received and discarded by the IP port.
Protocol Engine Input Queue
The BACnet® Protocol Engine receives messages from external BACnet devices and holds them until the Protocol
Engine can process them. If this queue becomes too large, the Protocol Engine is not capable of holding all of the
messages in progress, causing some communications to fail.
If the Protocol Engine input queue is critical, BACnet protocol messages received from the IP port are discarded.
The IP port counter, Discarded Frames (under the IP Datalink, Diagnostic tab), displays the number of discarded
messages.
58MS/TP Communications Bus Technical Bulletin: Appendix: Maximizing and Troubleshooting the MS/TP Bus