Device-level Connection Through Ethernet Daisy-chain for Data Acquisition Systems
Sometimes abbreviated
as DAQ or DAS, data acquisition is the process
by which events in the real world are translated
into machine-readable signals. This typically
involves the acquisition of signals and waveforms
and then processing the signals. The acquired
data is analyzed and stored on a computer using
vendor supplied software to obtain the desired
information.
The Various Types of Device Level Communication Topology
Industrial Ethernet requires infrastructure
equipment such as hubs and switches to form
a network. Think of the network as a kind of
field bus that is separate from but still connected
to the main IT network. The topologies that
can be used to connect field devices are (1)
Star, (2) Star-bus, (3) Daisy-chain, and (4)
Ring, which are shown in greater detail in the
following figures:
Fig. 1: Star
Topology
Fig. 2: Star-bus Topology
Fig. 3: Daisy-chain Topology
Fig. 4: Ring Topology
Why Use Ethernet for Data Acquisition Infrastructures?
Ethernet infrastructures are now widely used
in industrial environments for automation, control,
process, and monitoring because of the versatility
it brings. An Ethernet infrastructure can support
various communication protocols, allowing systems
from different manufacturers to be interconnected.
But industrial Ethernet provides more than just
interoperability. It also enhances the overall
performance of networks, allows for distance
wiring, and increases bandwidth.
It is important to note that industrial Ethernet components must be specifically designed to work in harsh environments subjected to extreme temperatures, high humidity, and vibrations that exceed the normal range for IT equipment.
Traditional Data Acquisition:
Data collection in the form of waveforms, on/off
signals, or temperature readings has been greatly
improved by Ethernet technology. With traditional
data acquisition infrastructures a controller
must be used (generally a PC or PLC) to connect
to sensors and the SCADA system. This kind of
setup is more centralized.
Data Acquisition with Ethernet:
Ethernet allows systems to be more distributed,
and remote monitoring is also possible. In addition,
bandwidth can be increased since the performance
is shared by several controllers, and there
is greater flexibility since sensors do not
need to be connected to a specific controller.
Fig. 5: From a centralized and localized system to a distributed and remotely monitored system
Ethernet daisy-chain is now one of the most
widely used topologies for industrial automation
networks. In this case, cable segments are used
to interconnect multiple devices and unite all
the devices together.
The Benefits of Ethernet Daisy-chain for Data Acquisition at the Device Level
The best approach to deploying a distributed
Ethernet-based data acquisition system is to
create a device link using a daisy-chain topology.
With Ethernet daisy-chain topology, no external
hubs or switches are required. All that is needed
are dual port Ethernet switches at the device
level. The two Ethernet ports are used for connecting
each device to the ports of its two neighboring
devices. Ther are two major benefits:
1. The distance between devices
is extended and the total cost of ownership
is reduced:
A common Star topology requires networks to
be located in a centralized location. In this
case, field connections to the devices and sensors
and subsequent switches are limited. In addition
to diagnostic problems, this type of topology
requires a large number of homerun cables, is
more labor intensive, and is altogether more
costly.
Fig. 6: When using external
switches and hubs, the connection distance is
limited and more homerun cables are reruired.
With an Ethernet
daisy-chain topology, the devices have a built-in
Ethernet switch. This way cabling is much easier
and less cabling is required, so cable and labor
costs are significantly reduced and there is
no need to purchase external switches. In addition,
failure points are eliminated by the absence
of external switches.
Studies indicate that with $300K projects, the
total cost of ownership can be cut to 15% when
using daisy-chain enabled devices. A maximum
cable length of 100 meters is more than adequate
for Ethernet daisy-chain topologies.
Fig. 7: With embedded Ethernet
devices wiring is easier, less homerun cabling
is required, and distance is extended.
2. Space saving and fewer failure
points:
Moxa's built-in
Ethernet switch ports can be cascaded to other
Ethernet enabled devices, which is a big advantage
since remote sites tend to have numerous sensors,
devices, and Internet connections.
Take roadside cabinets, for example. In this
case, remote monitoring applications require
using an additional switch or hub to connect
Ethernet I/O devices to an IP camera. A better
method would be to connect an Ethernet device,
such as a PLC controller or an IPC, in the same
cabinet.
This solution not only saves valuable cabinet
space but also reduces the number of failure
points associated with using a switch or hub.
In addition, wiring for power signals and the
network can be significantly reduced.
Fig.
8: Embedded Ethernet devices save valuable cabinet
space, which is perfect for remote cabinets
and wiring panels.
The
Limitations of Ethernet Bypass and Switch Connections
Ethernet bypass uses high-speed relays to ensure
that the uplink/downlink ports of an Ethernet
switch can still transmit data if the power
fails. This means that when an Ethernet daisy-chained
device loses power, the uplink/downlink ports
will still be connected by bypassing the normal
switch link.
In theory, with Ethernet bypass, engineers can
replace failed devices without affecting data
transmission; however, this is not possible
unless the bypass mechanism is detachable, which
is one of the limitations of Ethernet bypass.
Another more serious limitation is cable length.
Since cable length is limited to 100 meters,
if the uplink and downlink cables are both 100
meters in length, you will need 200 meters of
cable to implement the bypass function. But
this is not possible since the maximum cable
length is 100 meters.
Fig. 9: Ethernet bypass limitation
According to some industry
reports, the average latency for Ethernet switch
systems is 20 to 40 microseconds, and most SIs
agree that the maximum switch link should be
no more than 32 nodes if real-time process control
is required. In reality, the majority of Ethernet
data acquisition systems are not time-critical
(at the level of microseconds). Consequently,
there are no limitations to the number of daisy-chain
nodes for your Ethernet data acquisition network.
Redundant
Ring for Primary Control and Daisy-chain at
the Device Level—a Perfect Balance
The optimal Ethernet data
acquisition system consists of different network
topologies. Moxa's solution involves two parts.
(1) A primary infrastructure, which includes
the SCADA PC and controllers, is connected through
a ring topology using managed switches, and
is designed to provide reliable communications.
(2) Field devices are connected through a daisy-chain
topology, which overcomes wiring problems and
saves a significant amount on implementation
costs.
Fig. 10: Optimal wiring infrastructure:
Redundant ring for the primary infrastructure
and daisy-chain connection at the device level
Summary
Daisy-chain enabled devices, such as data acquisition
equipment, and daisy-chain topologies are an
up and coming trend. Because of an increase
in demand, it will be up to the manufacturers
to embed an Ethernet switch inside their devices
to deliver the benefits of easy cabling, simple
maintenance, and cost-effectiveness. Moxa has
taken the lead in this regard by providing data
acquisition products, at a competitive price,
that can be daisy-chained together.
Learn more about Moxa's daisy-chain solution for data acquisition.
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