Specifications

Protocol Engine Output Pending Queue
The BACnet® Protocol Engine has an output pending queue used to hold messages when slower communications
links, such as MS/TP, are not able to keep up with the rate at which outgoing messages are generated. If this queue
becomes too large, the time required to send a message becomes excessive, resulting in slow operation. When this
slow operation occurs, applications begin to encounter errors and some devices may drop offline. Messages that
are waiting are also counting down their APDU Timeout.
If the Protocol Engine output pending queue is in the trouble state or worse, reading the initial values at startup is
delayed.
If the Protocol Engine output pending queue is critical, originated outgoing messages are discarded and the sending
applications receive an error.
Building Efficiency
507 E. Michigan Street, Milwaukee, WI 53202
Metasys® and Johnson Controls® are registered trademarks of Johnson Controls, Inc.
All other marks herein are the marks of their respective owners.© 2015 Johnson Controls, Inc.
www.johnsoncontrols.comPublished in U.S.A.
59MS/TP Communications Bus Technical Bulletin: Appendix: Maximizing and Troubleshooting the MS/TP Bus