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Essentially, for a Q-EIO system of maximum size, the maximum jitter
associated with delays at the switches will be 6,514 ms.
Input Determinism
Unlike traditional fieldbus networks that transfer data from all remote
devices once per network cycle time Q-EIO allows the user to config-
ure the transfer rate for each remote rack individually (ie all racks act
independently). The frequency of input data transfer from the CRA
module in the remote rack to the CRP in the PLC is defined by the RPI
rate assigned to the rack, the RPI value can vary from 5 ms up to 1 500
ms. As a result, input data from a remote rack will be sent across the
Ethernet RIO network every 5 – 1 500 ms (as configured), taking at most
9 ms to arrive at the PLC in our maximum size configuration example.
Output Determinism
Output data from the CRPmodule to the remote racks will be transmitted
immediately after each execution of user logic (ieMAST task). In addition
a user can configure an I/O block to trigger the immediate transmission
of output data from the CPU/CRP to the remote rack. Similarly, output
data from the CPUwill be sent across the Ethernet RIO network once per
MAST task or once per Immediate I/O block and will take at most 9 ms
to reach the remote rack in our maximum size configuration example.
Data faults detection and recovery
Since data is passed between the PLC and the RIO racks based on a
RPI rate, the time taken to recover in the event of a packet being lost
or corrupted (very unlikely on a full duplex switched network, except
by grounding or EMI issues) is one RPI. This reaction is due to the fact
that the next update (sent at the next RPI interval) will include all the
data required for the system to continue operating. If the system has a
defect, for example an RIO rack powered off or a network interruption
(beyond what is recoverable by the ring) then the Ethernet RIO system
will detect this fault within 4 RPI cycles.
Conclusion
Thanks to careful implementation of new techniques in the stabilisa-
tion of the transmission time, jitter management, Input and Output data
transfer, and proper response to data transmission failures; and taking
into account the performance of the system components, the newQ-EIO
system is able to offer a deterministic network for RIO communications
that is resilient even in the presence of other traffic.
Control systems and automation
Supporting calculation
Transmission Time calculation
Transmission time for a packet travelling across any node in a Q-EIO network
is 78 μs.
• Transmission time for an Ethernet RIO packet to traverse the Q-EIO network =
0,078 ms * number of hops
For a Q-EIO system of max size, the number of hops equals the number of
devices, 32
• Max packet transmission time for a Q-EIO network of max size = 2,496 ms
Jitter calculation
The delay for a SCADA/HMI packet that is already being transmitted depends
on the size of the packet and how far through being transmitted it was when the
Ethernet RIO packet arrives at the switch.
• Max transmission delay caused by a max size Ethernet packet (non-RIO) =
0,128 ms
• Max jitter caused by SCADA/HMI or DIO traffic = 0,128 ms * number of hops
For a Q-EIO system of max size, the number of hops equals the number of
devices, 32
• Max jitter caused by SCADA/HMI or DIO traffic in a max size network = 4,096 ms
The delay for another Ethernet RIO packet that is already being transmitted or
waiting to be transmitted is 78 μs per packet.
• Max transmission delay caused by an RIO packet = 0,078 ms
• Max jitter caused by RIO traffic = 0,078 ms * Number of RIO packets possible
in the network at one time
Since there is a max of 31 Ethernet RIO drops in the system and each one will
only transmit one packet per RPI rate there can only be a max of 30 Ethernet RIO
packets in the system at a time.
• Max jitter caused by RIO traffic in a max size network = 2,418 ms
In a worst-case scenario, an RIO packet will suffer delays at each hop due to a
DIO packet that is already being processed and another RIO packet that is next
in queue. This scenario gives us the max system jitter for Q-EIO.
• Max Q-EIO network jitter = Max DIO jitter + Max RIO jitter
• Max Q-EIO network jitter = 4,096 ms + 2,418 ms
• Max Q-EIO network jitter = 6,514 ms
Max Network Transmission Time
The max possible transmission time that can be experienced in a Q-EIO system is,
therefore, the total of the max packet transmission time and the max system jitter.
• Max Q-EIO network transmission time = Max packet transmission tin for a Q-EIO
network of max size + Max Q-EIO network jitter
• Max Q-EIO system jitter = 2,496 ms + 6,514 ms
• Max Q-EIO network transmission time = 9,010 ms
No of
RIO
devices
RIO
Packet
Trans-
mission
Time
(msec)
Max
pos-
sible
jitter
caused
by RIO
packets
(msec)
Max
pos-
sible
jitter
caused
by DIO
packets
(msec)
Total
max
possi-
ble RIO
jitter
(msec)
Max RIO
transmis-
sion time
in a mixed
traffic
network
(RIO & DIO
packets –
msec)
Max RIO
trans-
mission
time
in an
RIO-only
network
(msec)
0
0,000 0,000 0,000 0,000 0,000
0,000
8
0,624 0,000 1,024 1,024 1,648
0,624
16
1,248 0,078 2,048 2,126 3,374
1,326
24
1,872 0,156 3,072 3,228 5,100
2,028
32
2,496 0,234 4,096 4,330 6,826
2,730
A
bout the authors
David Doggett has 18 years’ experience with Schneider Electric in mar-
keting, product development and cyber security. He is currently the cyber
security programme director (industry business). Enquiries: Email david.
doggett@schneider-electric.com.
Manuel J. Palomino joined Schneider Electric in 2006. His current projects
involve the deployment of the next generation of Ethernet architectures,
definition of future network management requirements and the promo-
tion of Ethernet solutions worldwide. Enquiries: Manuel.Palomino@
schneider-electric.com.
Electricity+Control
September ‘12
6