SA Bus traffic reduction - Metasys - LIT-12011034 - General System Information - Metasys System - Release 11.0

MS/TP Communications Bus Technical Bulletin

Product name
Metasys System
Document type
Technical Bulletin
Document number
Release 11.0
Revision date

If the COV rate exceeds 500 for inputs or the command rate exceeds 500 for outputs, use the following strategies to reduce the traffic on your SA Bus:

  • If possible, connect fast process inputs and outputs directly to the Metasys Equipment controller or engine . Fast processes include airflow, static or differential pressure control, central plant temperature control, and mixed air control. Move slower process inputs and outputs on the Metasys Equipment controller/engine to devices on the SA Bus, and move the inputs and outputs for the fast processes to the Metasys Equipment controller or engine . The Metasys Equipment controller or engine internal update process does not contribute to SA Bus communication issues.

  • Limit the number of processes controlled across the SA Bus to use 8 AOs or fewer.
  • If an IOM/XPM is used strictly for monitoring, place the device on the Field Controller (FC) Bus instead of the SA Bus. Alternatively, relocate AIs used for monitoring to a separate IOM/XPMs on the FC Bus.
  • If the AIs must remain on the SA Bus, configure the Filter Setup attribute to monitor only. If the AI is part of a process, reduce the resulting update interval by modifying the time constant attribute of the analog input object with the following steps:
    1. Set the Filter Setup attribute to the value Process Data.
    2. Set the Time Constant attribute to 0.3 times the desired update interval value. This configuration allows the low pass filter to continue to operate properly.
      Note: Update interval has units of hundredths of a second. A time constant of 30 results in a 1 second update interval (displayed as 100 hundredths of a second).
  • If you must leave inputs on the SA bus, slow down the process unless it will affect Pattern Recognition Adaptive Control (PRAC).
    Important: Use caution. Changing the PID interval or the analog input filtering may affect the PID controller’s ability to provide proper control. This change also may affect the adaptive tuner’s ability to tune the control process. Make sure that the AI has an update interval less than or equal to the PID interval. We recommend an AI update interval that is half the PID interval or less.

    Change the inputs of the PID Pre Processor logic block in the CCT to match the current outputs (Table 1). Refer to the PID Pre-Processor section of the Controller Tool Help (LIT-12011147) for details on this block.

    Table 1. Inputs and matching outputs

    PID pre-processor inputs

    PID pre-processor block outputs


    Process range

    Eff Process Range


    Time constant

    Eff Time Constant


    Process dead time

    Eff Process Dead Time



    Eff Period

    (Release 5.1 and later)

    For the Interval value, add several seconds to the Eff Period value. Interval should be in the range of Time Constant/10 to Time Constant/5.

    Proportional band

    Eff Proportional Band

    (Release 5.1 and later)


    Integral time

    Eff Integral Time (Release 5.1 or later)


    Saturation time

    Eff Saturation Time (Release 5.1 or later)


    Standard tuning