Application Response Time for M580 Hot Standby CPUs

Introducing Application Response Time

Each Ethernet RIO input signal packet travels from an RIO drop to the CPU, and the CPU sends an output signal back to the RIO drop. The time it takes for the CPU to receive the input signal and effect a change in the output module based on the input is called application response time (ART).

In an M580 system, ART is deterministic, which means you can calculate the maximum time the CPU uses to resolve an RIO logic scan.

Calculating ART for M580 Hot Standby CPUs

The Modicon M580 Standalone System Planning Guide for Frequently Used Architectures describes both a simplified method and a more complex method of calculating ART for a standalone CPU.

To calculate a maximum ART for an M580 Hot Standby CPU, it is necessary to add to the standalone CPU ART calculation estimates of the maximum time for:

A switchover event, during which the standby PAC takes over the role of primary PAC, after the original primary PAC ceases to be operational or loses communication.
A swap event, during which the standby PAC takes over the role of primary PAC in response to a user or application command.

ART: General Calculation for an M580 Safety Hot Standby PAC in a Multi-task Configuration

A: missed input scan	8: operation of application logic (1 scan)
B: missed output scan	9: additional jitter due to the PAC multitasking
1: input turns ON	10: CPU output jitter
2: input filtering and sampling (Safe modules)	11: network delay
3: CRA drop processing time	12: network jitter
4: CRA input request packet interval (RPI) rate	13: CRA drop processing time
5: network delay	14: output check and actuating (Safe modules)
6: network jitter	15: output applied
7: CPU input jitter	–

The more complex method of ART calculation, in the Modicon M580 Standalone System Planning Guide for Frequently Used Architectures remains valid. Refer to that topic for an evaluation of the TCOM_IN and TCOM_OUT components.

NOTE: The following evaluations assume that the PAC is not overloaded, and that total bandwidth of all tasks is less than 80%.

Hot Standby effect: An additional time (Transfer Standby) is necessary to exchange the user data from the primary PAC to the standby PAC. This time is included in the execution time displayed in the PLC → Animation screen if the primary PAC is connected to the standby.
Safety effect:
- An additional time (Safety Comparison) is necessary to exchange and compare the safe data between the PAC and the safety coprocessor. This time is always included in the execution time displayed in the PLC → Animation screen.
- All the safety I/O modules have an internal cycle used for filtering and diagnostics. They introduce a delay on the sampling and the activation of the external signals.
Multi-task effect: This introduces a jitter before the emission of the output message.

The following elements are necessary for the ART evaluation:

Name	Description		Value
TINPUT	Time used by the safety input modules for the filtering and the sampling of the external signal.		6 ms
TCOM_IN	Sum of all the time used by the communication with input modules.		See1
Lost_scan	Lost scan time because the input message arrived too late, after the begin of the scan.		TPER2
Eff_scan	Effective scan which computes the outputs depending on the latest sampled input value.		TPER2
Multitask_jitter	Jitter introduced by the multi-task system on the output message emission. The real value is based on the task execution time, for simplification they are capped by the task period.	Mast task	TSAFE3 + TFAST3
		Safe task	TFAST3
		Fast task	0
TCOM_OUT	Sum of all the time used by the communication with output modules.		See1
TOUTPUT	Time used by the safety output modules for the diagnostics and for actuating the external signal.		6 ms
1. For a description of these elements, refer to the more complex ART calculation method in the Modicon M580 Standalone System Planning Guide for Frequently Used Architectures. 2. Refer to the Processor Load topic for a discussion of these elements. 3. The configured cycle period for the respective task (SAFE, FAST).

ART: M580 Safety Hot Standby PAC in a Multi-task Configuration During a Switchover

A switchover occurs in a Hot Standby system when the primary PAC ceases to be operational or loses communication. The standby PAC, after a detection time, will restart the MAST task in its role as the new primary PAC. Thereafter, the SAFE and FAST tasks will be able to start in the new primary PAC. The worst-case switchover scenario from an ART standpoint, i.e., the one that takes the longest to complete, is shown in the following diagram:

The following elements are necessary for the ART evaluation in the case of a switchover:

Name	Description		Value
TINPUT	Time used by the safety input modules for the filtering and the sampling of the external signal.		6 ms
TCOM_IN	Sum of all the time used by the communication with input modules.		See1
Lost_scan	Lost scan time because the input message arrived too late, after the begin of the scan.		TPER2
TDETECT	Time used by the standby PAC to detect and confirm the primary PAC has become non-operational.		15 ms
Eff_scan	Effective scan which computes the outputs depending on the latest sampled input value.		TPER2
Multitask_jitter	Jitter introduced by the multi-task system on the output message emission. The real value is based on the task execution time, for simplification they are capped by the task period.	Mast task	TSAFE3 + TFAST3
		Safe task	TFAST3
		Fast task	0
Additional_jitter	Jitter introduced by the multi-task system to restart the task on the new PAC.	Mast task	0
		Safe task	TSAFE3
		Fast task	TFAST3
TCOM_OUT	Sum of all the time used by the communication with output modules.		See1
TOUTPUT	Time used by the safety output modules for the diagnostics and for actuating the external signal.		6 ms
1. For a description of these elements, refer to the more complex ART calculation method in the Modicon M580 Standalone System Planning Guide for Frequently Used Architectures. 2. Refer to the Processor Load topic for a discussion of these elements. 3. The configured cycle period for the respective task (SAFE, FAST).

ART: M580 Safety Hot Standby PAC in a Multi-task Configuration During a Swap

A swap occurs in a Hot Standby system when the user requests it, either by program logic or through a communication request (for example, from the Hot Standby screen, an animation table, the HMI, and so forth).

On request, the primary PAC checks that all the conditions necessary to authorize a swap are met, then confirms that all the tasks have updated the standby PAC with the latest data. The primary PAC then goes into wait mode. The remote PAC switches to the primary mode, starting the MAST task first and then the other tasks SAFE and FAST). In the meantime, the other PAC (i.e., the original primary PAC) goes to standby mode.

The worst-case swap scenario from an ART standpoint, i.e., the one that takes the longest to complete, is shown in the following diagram:

The following elements are necessary for the ART evaluation in the case of a switchover:

Name	Description		Value
TINPUT	Time used by the safety input modules for the filtering and the sampling of the external signal.		6 ms
TCOM_IN	Sum of all the time used by the communication with input modules.		See1
Lost_scan	Lost scan time because the input message arrived too late, after the begin of the scan.		TPER2
TTRANSFER	During the diagnostics of the MAST task, the PAC accepts the Swap command and begins to perform the transfer of all the latest data for each task.		Refer to the formula, below.
Eff_scan	Effective scan which computes the outputs depending on the latest sampled input value.		TPER2
Multitask_jitter	Jitter introduced by the multi-task system on the output message emission. The real value is based on the task execution time, for simplification they are capped by the task period.	Mast task	TSAFE3 + TFAST3
		Safe task	TFAST3
		Fast task	0
Additional_jitter	Jitter introduced by the multi-task system to restart the task on the new PAC.	Mast task	0
		Safe task	TSAFE3
		Fast task	Min(TFAST, 5 ms)3
TCOM_OUT	Sum of all the time used by the communication with output modules.		See1
TOUTPUT	Time used by the safety output modules for the diagnostics and for actuating the external signal.		6 ms
1. For a description of these elements, refer to the more complex ART calculation method in the Modicon M580 Standalone System Planning Guide for Frequently Used Architectures. 2. Refer to the Processor Load topic for a discussion of these elements. 3. The configured cycle period for the respective task (SAFE, FAST).

TTRANSFER can be calculated as follows:

max((K3 x (MASTKB + 2 x SAFEKB + FASTKB) + K4 x (MASTDFB + 2 x SAFEDFB + FASTDFB)) / 1000, TSAFE)

Where:

TTRANSFER is measured in milliseconds.
TASKKB = Size of the data (in Kbytes) exchanged for the TASK between the primary PAC and standby PAC.
MASTDFB = The number of DFBs declared in the TASK.
K3 and K4 are constants with values determined by the specific CPU module used in the application, as follows:

Coefficient	BMEH582040S	BMEH584040S or BMEH586040S
K3	46.4	14.8
K4	34.5	11.0