Introduction
The following examples are designed to help you calculate the application response time (ART) for an application.
Example: CPU with Ethernet I/O Scanner Service in a Main Ring
In this example of an RIO main ring, the local rack contains a CPU with Ethernet I/O scanner service. The ART is calculated from the perspective of either of the BM•CRA312•0 X80 EIO adapter modules that are associated with the MAST task in the RIO main ring:
This ART value is calculated from the perspective of the adapter module in the either of the RIO drops. Consider these application-specific elements when calculating ART:
The maximum potential hop count is 3, which represents the maximum number of switches a packet might pass through from the adapter module to the CPU with Ethernet I/O scanner service in the local rack.
NOTE: The hop count includes all switches located along the route between the source input module and the CPU, including the switches embedded in the BM•CRA312•0 X80 EIO adapter module.Jitter is introduced into the system only from the two main ring drops.
Given these factors, ART computation parameters include:
Parameter |
Maximum value (ms) |
Comments |
|---|---|---|
BM•CRA312•0 process time (CRA_Drop_Process) |
4.4 |
The sum of BM•CRA312•0 input scan time and queue delay. |
BM•CRA312•0 input RPI (RPI) |
– |
User defined. Default = 0.5 * CPU period. |
network input time (Network_In_Time) |
(0.078 * 3) = 0.234 |
Hop-count is 3 from the BM•CRA312•0 X80 EIO adapter module in the RIO drop (3) to the CPU with Ethernet I/O scanner service in the local rack (1), which includes the switches in both the BM•CRA312•0 X80 EIO adapter module and the CPU with Ethernet I/O scanner service. |
network input jitter (Network_In_Jitter) |
(0.078 * 2) = 0.156 |
For delay occasioned by devices (2) and (3). |
CPU Ethernet I/O scanner service input jitter (CPU_In_Jitter) |
(1 + (0.07 * 2)) = 1.14 |
To read packet |
CPU scan time (CPU_Scan) |
– |
User defined, based on application. |
CPU Ethernet I/O scanner service output jitter (CPU_Out_Jitter) |
1.21 |
CPU Ethernet I/O scanner service internal queue delay |
network output time (Network_Out_Time) |
(0.078 * 3) = 0.234 |
Hop-count is 3 from the BM•CRA312•0 X80 EIO adapter module in the RIO drop (4) to the CPU with Ethernet I/O scanner service in local rack (2), which includes the switches in both the BM•CRA312•0 X80 EIO adapter module and the CPU with Ethernet I/O scanner service. |
network output jitter (Network_Out_Jitter) |
0 |
Does not apply. No distributed equipment is connected to the RIO network. |
BM•CRA312•0 process time (CRA_Drop_Process) |
4.4 |
The sum of the BM•CRA312•0 X80 EIO adapter module output scan time and queue delay |
For an explanation of each parameter, refer to the ART Computation Parameters topic. |
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Recall that the ART formula is:
ART= (2*CRA_Drop_Process) + (RPI) + (Network_In_Time) + (Network_In_Jitter) + (CPU_In_Jitter) + (2*CPU_Scan) + (CPU_Out_Jitter) + (Network_Out_Time) + Network_Out_Jitter)
Thus, for a CPU scan time of 50 ms and RPI of 25 ms, maximum ART is:
max ART = (2*4.4) + 25 + 0.234 + 0.156 + 1.14 + (2*50) + 1.21 + 0.234 = 136.774 ms