Chemical Engineering March 2019 - 47

Yokogawa
HIS
ENG
L2SW
L2SW For Vnet/IP
UGS2 dual-redundant
configuration
UGS2
Dual-redundant
control network
HUB
HUB
UGS2
L2SW
For ethernet
communications
can be provided by using two
redundant PCs, which can handle
a variety of interfaces and a
large number of subsystems.
This improves upon the availability
of the system because
many of the controller subsystems
depend on the operation
of a single general-purpose PC.
A high-availability configuration
also prevents data loss and
downtime by contributing to
long-term stable operation.
Therefore, a UGS should be
designed to be a high-availability
connectivity solution (Figure
2). A typical
dual-redundant
Subsystem controllers
FIGURE 2. A dual-redundant network configuration provides
the high availability and reliable connectivity required
for a unified gateway station (UGS)
A UGS is installed between the
DCS and all the external controllers.
It performs continuous bi-directional
communications frame conversion
for process data and alarms to the
DCS regardless of type or vendor
of controller (Figure 1). Using a UGS
provides additional SCADA expandability
to the DCS without reducing
the functionality of the DCS itself.
Because the UGS provides the
main interface to all of the control
systems, its uptime is critical. This
package will consist of two
PCs configured as a logical unit
to achieve a gateway function
for the subsystems. These two
units share an identical Vnet/
IP address (domain and station
numbers), providing other stations
with transparent access to the UGS
tag data without considering which
one of the redundant PCs is active,
and which one is on standby. For redundancy,
both units are connected
using a dual-redundant control network.
Only the active station downloads
the engineering data, but it
synchronizes the standby unit using
the dual-redundant control network
(Figure 3).
If failure of the operating side PC
occurs, it automatically fails over to
the standby PC. Importantly, this
switchover takes less than a second,
which means the connected Vnet/
IP stations and subsystem controllers
are not affected. This is so, even
when the system is performing sequential
control of multiple tasks, as
in a batching system. This minimizes
the probability of data loss and ensures
there is no interruption in control.
Thus, high availability has been
preserved, and safety-critical functions
have not been affected.
As important as high availability is
to maintaining plant safety, having a
unified operating environment is just
as important. As many after-incident
reports have shown, operators moving
from one operator interface to another
tend to make mistakes, which
can have serious consequences.
With the UGS concept, the same
station, for example the DCS operator
interface, can be used to communicate
with and monitor all the
controllers and subsystems in the
control network, regardless of vendor.
The UGS provides an I/O driver
that solves the differences among
protocols of the external controllers
by supporting internal DCS communications
protocols, IEC61850,
OPC-DA, Ethernet/IP, Modbus RTU,
Modbus TCP IP and other interfaces.
When redundant communication is
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Chemical Engineering March 2019

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