Image Processing Reference
In-Depth Information
Max(ms) = 8.0559, mean (ms) = 7.995, min (ms) = 7.9381, std Dev. (ms) = 0.017907
Round trip time interval cyclic update interval of 8 ms
8.3
Table 1. PLC1 packet time interval
Max(ms) = 16.366, mean (ms) = 8.5406, min (ms) = 7.8568,
Std Dev. (ms) = 1.8972
Table 1. PLC1 packet time interval results
No.
of
switches
No.
of
PLC
Std
Dev.
(ms)
Mean
(ms)
18
PLC1
Number of switches
Data size (bytes)
8.2
Packet
1
3
5
8
216
440
Baseline
1
2
8.068
0.05
0.32
0.05
0.04
0.05
0.11
16
Switches
20% load
90% load
2 connections
3 connections
5
2
8.068
8.071
8.043
8.081
8.072
Avg.
St. Dev.
8.000
8.000
8.000
8.000
8.000
8.000
5
2
0.018
0.017
0.018
0.018
0.005 0.006
8.1
5
2
14
1
3
1
4
8
12
Table 2. Roundtrip time interval results
10
Table 2. Round trip time interval
7.9
Round
trip
Number of switches
No.
of
PLC
Std
Dev.
(ms)
Data size (bytes)
No.
of
Switches
Mean
(ms)
8
1
3
5
8
216
44
Avg.
8.000
8.000
8.000
8.000
8.000
0.005
8.000
0.00
Baseline
Switches
20% traffic
90% traffic
2 connections
3 connections
1
5
5
5
1
1
2
2
2
2
3
4
8.541
8.643
8.686
8.686
8.635
8.669
1.89
2.18
2.14
2.18
2.04
2.11
7.8
St. Dev.
0.018
0.017
0.018
0.018
6
500
1000
1500
2000 2500
Packet number
3000
3500
4000
4500
500
1000 1500 2000 2500
3000 3500 4000 4500
5000
Packet number
FIGURE .
Sample plots of round-trip timing measurements for PROFINET (a) and EtherNet/IP (b). Here,
the round-trip times for packets between two PLCs are plotted for a large number of packets to obtain a time distri-
bution. he embedded tables represent the consolidation of a number of these graphs, where the testing environment
is modified in terms of switches between sender and receiver, data size transmitted, and network loading (the plots
shown represent the “Baseline” case).
capabilities, and network management capabilities
∗
[]. As part of this effort, parallel multilayer
switched Ethernet testbeds were developed utilizing each of these technologies, where the network
layout was representative of the structure being utilized at a leading automotive manufacturer.
The results indicate that both protocols and protocol devices are fairly similar and are adequate to
the task of providing industrial networking capabilities at the PLC level and higher []. However,
distinct differences were observed, such as those illustrated in Figure ., that indicate additional
improvements in device performance may be needed if the solution is to be deployed down to the
I/O level.
23.3.5 Impact of Ethernet Application Layer Protocols: OPC
OPC is an open communication standard that is often used in industry to connect SCADA systems
and human-machine interfaces (HMIs) to control systems and fieldbus networks [,,]
†
.Itis
based on the Microsot DCOM standard [] and is the dominant factory-floor application layer pro-
tocol utilized for diagnostics and is beginning to be used for sequential control []. he main benefit
ofOPCisthatitallowsanyproductsthatsupportthestandardtosharedata.AlthoughOPCactu-
ally consists of many different communication specifications, its most commonly used form is called
Data Access, which supports both client-server and publisher-subscriber communication models.
The server maintains a table of data values, and the client can read or write updates. The overhead
associated with OPC (and DCOM in general) is significant, as shown in Figure .. Most of this
delay is due to the software implementation of the OPC protocol; OPC was never intended for a
real-time environment. However, it is very useful to push data up from the low-level controls to the
higher-level supervisors or diagnostic systems. It can also be used to send commands down from
the HMIs to the control systems. Its high level of interoperability enables the connection of mul-
tiple control systems from different vendors in a unified manner. However, when OPC is used to
send control data, the additional delay caused by the higher-level application layer protocol must be
considered.
∗
Many industrial Ethernet protocols are available, with Modbus/TCP the most widely utilized [,], however in this
instance the manufacturer was interested in comparing these two industrial Ethernet protocols.
†
While OPC originally stood for “OLE for Process Control,” the official stance of the OPC Foundation is that OPC is no
longer an acronym and the technology is simply known as OPC.
