Application Response Time Example

Introduction

This sample Modicon M580 application is designed to help you calculate the ART, or application response time.

Example of a BM•CRA312•0 Module in an RIO Sub-ring

This example calculates the maximum ART, representing the longest packet path from a BM•CRA312•0 X80 EIO adapter module in an RIO sub-ring (8, in the following graphic) to the CPU with Ethernet I/O scanner service in the local rack (1). The calculation is performed from the perspective of the BM•CRA312•0 X80 EIO adapter modules in this M580 network design:

The primary rack includes an M580 CPU.

The secondary rack includes an M580 CPU.

The Hot Standby ports on the primary and standby CPUs are linked.

A DRS on the main ring supports an RIO sub-ring.

A DRS on the main ring supports a DIO sub-ring and a DIO cloud.

6, 7

RIO drops on the main ring include BM•CRA312•0 X80 EIO adapter modules.

8, 9, 10

An RIO drop on an RIO sub-ring includes a BM•CRA312•0 X80 EIO adapter module.

A DIO cloud is connected to a BMENOS0300 network option switch module in an RIO drop.

An HMI device is connected to a BM•CRA312•0 X80 EIO adapter module in an RIO drop.

Calculation

In this example, assume there are breaks in two cables:

main ring: There is a cable break between the CPU with Ethernet I/O scanner service in the local rack (1) and the DRS (4).
RIO sub-ring: There is a cable break between the DRS (4) and the BM•CRA312•0 X80 EIO adapter module in an RIO drop (8).

In this example, ART is calculated from the perspective of the adapter module in the RIO drop. Consider these application-specific elements when you calculate the ART:

In this example, the maximum potential hop count is 10. That is, 10 is the maximum number of switches a packet can potentially pass through between RIO adapter module 8 and the CPU with Ethernet I/O scanner service in the local rack (1). This is the case when a packet follows this path from RIO adapter module 8 and the local rack (1): 8, 9, 10, 4 (x2), 6, 5 (x2), 7, 2, 1.

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. Count each DRS as two devices when you calculate the hop count.
Jitter (packet queue delay) is introduced into the system by these design elements:
- DIO sub-ring
- RIO sub-ring, on which the BM•CRA312•0 X80 EIO adapter module is located
- RIO drop
- HMI
- DIO cloud

Given these factors, ART computation parameters include:

Parameter	Maximum value (ms)	Comments
CRA drop process time (CRA_Drop_Process)	4.4	The sum of CRA input scan time and queue delay.
CRA input RPI (RPI)	—	Default = 0.5 * CPU period.
network input time (Network_In_Time)	(0.078 * 10) = 0.780	The hop count is 10 from the CPU with Ethernet I/O scanner service in the local rack (1) to the BM•CRA312•0 X80 EIO adapter module in the RIO drop (8). hop sequence: 8, 9, 10, 4 (x2), 6, 5 (x2), 7, 2, 1 (Count each DRS [4, 5] as two devices when you calculate the capacity of your main-ring.)
network input jitter (Network_In_Jitter)	((0.078 * 5) +	RIO: The value 5 represents the number of BM•CRA312•0 modules plus the number of CPUs based on a packet size of 800 bytes.
	(0.128 * 2))	DIO: The value 2 represents the number of packets from distributed equipment based on a packet size of 1500 bytes.
	= 0.646
CPU input jitter (CPU_In_Jitter)	(1 + (0.07 * 5)) = 1.35	Read packets from the distributed devices attached to DRS 5 and the BM•CRA312•0 modules (6, 7, 9, 10).
CPU scan time (CPU_Scan)	2 * CPU_Scan	User defined, based on application.
CPU output jitter (CPU_Out_Jitter)	(1 + (0.07 * 5)) = 1.35	CPU Ethernet I/O service internal queue delay (owing to BM•CRA312•0 modules)
network output time (Network_Out_Time)	(0.078 * 10) = 0.780	See comment above for Network_In_Time.
network output jitter (Network_Out_Jitter)	(0.128 * 1) = .128	Owing to distributed devices.
BM•CRA312•0 drop process time (CRA_Drop_Process)	4.4	The sum of the BM•CRA312•0 X80 EIO adapter module (6) output scan time and queue delay.
For an explanation of each parameter, refer to the topic ART Computation Parameters.

The maximum ART value is equal to the sum of values in the Maximum value column. Therefore, the ART calculation for a CPU scan time (CPU_Scan) of 50 ms and an RPI value of 25 ms looks like this:

4.4 + 25 + 0.780 + 0.646 + 1.35 + (2*50) + 1.35 + 0.780 + 0.128 + 4.4 = 138.834 ms ART

NOTE: If a cable break exists on the network, add an additional time period, equal to 50 ms + RPI, to the above ART calculation. The added time allows for RSTP recovery from the cable break.