Application Response Time
Original instructions
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Introduction
Application response time (ART) is the time a PLC application requires to react to a single input. ART starts when an input signal turns ON and triggers a write command by the PLC, and ends when the remote output turns ON to signify that the data has been received. Each Ethernet remote I/O packet travels from a remote input module that has detected a change in a field sensor to the PLC then back to a remote output module that will effect a field actuator. In a Quantum EIO system, the ART is deterministic, which means you can calculate the maximum time the PLC uses to resolve a remote I/O logic scan.
Overview: ART Computation Parameters
The following diagram displays ART-related events and computation parameters. Refer to the Design Principles of Quantum Ethernet I/O Networks appendix for details.
A: missed input scan
7: packet held in CRP queue (1 scan)
B: missed output scan
8: operation of application logic (1 scan)
1: input turns ON
9: CRP output jitter
2: CRA drop process time
10: network output time
3: CRA input request packet interval (RPI) rate
11: network output jitter
4: network input time
12: CRA drop process time
5: network input jitter
13: output applied
6: CRP input jitter
The ART computation parameters and their maximum values (in milliseconds) are described below:
ID
Parameter
Max (ms)
Description
2
CRA drop process time (CRA_Drop_Process)
4.4
The sum of CRA input scan time and queue delay
3
CRA input RPI (RPI)
–
User defined. Default = 0.5 * CPU period if MAST is in periodic mode. If MAST is in cyclic mode, the default value is watchdog/4).
4
network input time2 (Network_In_Time)
2.496
(0.078 * 32)
The product of (network delay based on I/O packet size) * (the number of hops1 the packet travels). The network delay component can be estimated as follows:
I/O packet size (bytes):
Estimated network delay (μs):
64
20
128
26
256
35
400
46
800
78
1200
110
NOTE: The value 2.496 ms is based upon a packet size of 800 bytes and 32 hops1 (33 hops in Hot Standby).
5
network input jitter (Network_In_Jitter)
6.436
((30 * 0.078) +
(32 * 0.128))
The formula is: ((number of RIO drops) * (network delay)) + ((number of distributed I/O hops1) * (0.128))
NOTE: The value 6.436 is based upon a packet size of 800 bytes.
6
CRP input jitter (CRP_In_Jitter)
6.8
(0.6 + (31 * 0.2)
CRP input queue delay. The formula is either:
without Hot Standby: (0.6 ms + ((number of remote I/O drops) * (0.2 ms))
with Hot Standby: (0.6 ms + ((number of remote I/O drops + 1) * (0.2 ms))
7/8
CPU scan time (CPU_Scan)
–
This is the user defined Control Expert scan time, which can be either fixed or cyclic.
9
CRP output jitter (CRP_Out_Jitter)
1.6
CRP output queue delay.
10
network output time2 (Network_Out_Time)
2.496
Computed in the same manner as network input time, above.
11
network output jitter (Network_Out_Jitter)
3.968
(31 * 0.128)
Distributed I/O device packet output queue delay. The formula is:
(number of hops1) * 0.128
NOTE: This only applies if distributed I/O devices are connected to the remote I/O network.
12
CRA drop process time (CRA_Drop_Process)
4.4
The sum of CRA queue delay and output scan time.
1. A hop is a switch that a packet passes through on the path from a source (transmitting) device, to a destination (receiving) device. The total number of hops is the number of passthrough switches along the path.
2. Regarding the optical fiber impact on network input time / network output time, the total length of the optical fiber that the I/O packet travels * 0.0034 ms/km (0.0054 ms/mi). Example: For 32 drops with 15 km (9.32 mi) of optical fiber (single mode) between drops, the impact is: 32 * 15 * 0.0034 = 1.6 ms.
Estimating ART
Using the parameters described in the preceding table, you can compute the maximum estimated ART—based on the maximum number of distributed I/O and remote I/O devices—for an application, as follows:
ART = (2 * CRA_Drop_Process) + (RPI) + (Network_In_Time) + (Network_In_Jitter) + (CRP_In_Jitter) + (2 * CPU_Scan) + (CRP_Out_Jitter) + (Network_Out_Time) + (Network_Out_Jitter)
NOTE: If a cable break occurs or a cable is reconnected on the network, add an additional time period to the above ART calculation to allow for RSTP recovery. The additional time to be added equals: 50 ms + RPI.