Specifications

Bus Performance Index
The Bus Performance Index provides a snapshot of the performance of the network by taking the weighted moving
average of the ratio of the actual loop time to the predicted loop time.
The Weighted Bus Performance Constant attribute is an adjustment factor that determines how quickly the index
changes when the actual loop time changes.
Because a master device is allowed to originate messages only when it holds the token, the token loop time is closely
related to network performance. During normal operation, the token loop time should not exceed 2 seconds (2,000
milliseconds). Special Operations (such as Auto Discovery, device downloads, or bus analysis) may cause the loop
time to exceed this value temporarily, but this is no cause for concern; unless, the token loop time continually exceeds
2 seconds, then the bus may be experiencing excessive errors or data overload. The 2-second loop time figure is
the upper limit of an ideal token loop time; however, the Bus Performance Index predicts what the token loop time
should be, based on the number of devices, the baud rate, and the number of messages.
The ideal value for the Bus Performance Index is 1. Table 17 provides the guidelines of the Bus Performance Index.
Table 17: Bus Performance Index
DescriptionIndex Value
Best performance, no performance problems1.0 - 1.9
Acceptable performance2.0 - 2.9
Possible slower performance, bus may be overloaded3.0 - 5.9
Poor performance, devices going offline, other problems6.0 or Greater
High numbers (around 6 and higher) can indicate a physical problem or that the bus communication is overloaded.
If the Bus Health Index is zero, the problem is probably due to an overloaded bus. See Correcting Bus Overload
Problems.
Statistics Attributes
The Statistics attributes of the MS/TP Field Bus Integration object Diagnostics tab (Advanced View) provide useful
information about what is happening on the MS/TP bus. Table 18 describes some of the attributes and values that
indicate there is a problem on the bus. For more details on the following attributes, refer to the MS/TP Field Bus
Integration Attributes section of the Metasys system Help (LIT-1201793).
Send the Latch Statistics command to the MS/TP field bus integration before comparing your values to Table 18.
If the statistics have already been latched and represent old data, send the Clear Statistics command. Wait a minute
or longer for new data to accumulate and send the Latch Statistics command.
For more information on statistics, refer to the Metasys system Help (LIT-1201793).
Table 18: Statistics Attributes
Error Condition IndicatedAttribute
Possible network disruptionUnexpected Frames Rxed
Noise, bad wiring, or another device did not respond.Packets Timed Out
Noise, bad wiring, or mismatched baud ratesFraming Errors
This node is too busy.Overrun Errors
Noise or bad wiringHeader CRC Errors
Noise or bad wiringData CRC Errors
Another device is slow to respond.Reply Too Slow Datalink Errors
The node is too busy to build the reply at this time.Reply Too Slow Application Errors
Low memory or unexpected conditionsInternal Errors
49MS/TP Communications Bus Technical Bulletin: Appendix: Maximizing and Troubleshooting the MS/TP Bus