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 1 provides the guidelines of the Bus Performance Index.
|
Index Value |
Description |
|---|---|
|
1.0 - 1.9 |
Best performance, no performance problems |
|
2.0 - 2.9 |
Acceptable performance |
|
3.0 - 5.9 |
Possible slower performance, bus may be overloaded |
|
6.0 or Greater |
Poor performance, devices going offline, other problems |
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.