GE Industrial Solutions POWER LEADER ModBus Monitor - Manual (Page 9)

POWER LEADER Modbus Monitor
Chapter 1 – Introduction
3
Monitor operation with PMCS
The Monitor’s place in this scenario is as a backup
device to the PMCS software on the host computer, and
also as a remote monitor, enabling viewing of PMCS
device data at locations other than the host PC.
When the PMCS DDE Server is present on a network,
Monitors on the network operate as slaves to the PMCS
system acting as master. Because Modbus networks
operate on a master-slave architecture, which allows
only the master device to initiate communications, a
Monitor cannot directly request information from the
devices it needs to track. A special client application
called the Monitor Proxy provides the means for a
Modbus Monitor to obtain the data it requires from
network devices. The Proxy runs on the host PC
alongside the PMCS DDE Server.
When the Monitor Proxy application is launched, its first
task is to query the PMCS DDE Server, checking for any
Monitor devices on the segment. If Monitor devices are
found, the Proxy records their device names and then
asks the DDE Server for each Monitor’s request list
information. The Monitor’s request list is a listing of all
devices and specific data the Monitor needs to display.
The Monitors send their request lists back to the Server,
which passes this information on to the Monitor Proxy,
completing the DDE transaction.
The Proxy now knows which devices each Monitor
needs to track for data and events. The Proxy uses the
information in the Monitor’s request lists to establish
DDE links with the Server for the devices and
corresponding registers. The Server begins polling these
devices for data and sending the data back to the Proxy.
As a slave device, the Monitor cannot initiate
communications or requests for data. However, there is
nothing to prevent the Monitor from ‘eavesdropping’ on
network traffic. The Monitor watches the segment for
requests to and responses from the devices it is supposed
to track and uses this information to update its displays.
The Proxy itself has no need for the data from the
devices, since its purpose is merely to create the
conversation so that the Monitor can eavesdrop on the
reply. The Proxy discards all device data received from
the DDE Server.
The Proxy’s role in assisting the Monitor in DDE
communications is shown in Figure 3.
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Figure 3. Monitor Proxy and PMCS DDE communications.
The Proxy can also send the PMCS event log to a
Monitor when requested. The Monitor can store the
latest 50 PMCS events if the PMCS Event Logger client
is running on the host PC with the DDE Server.
Monitor stand-alone operation
To a limited degree, the Monitor functions in a manner
very similar to the PMCS DDE Server, acting as master
to a network of power management devices.
The Monitor has been preconfigured with the register
maps of a variety of GE and third-party power
management devices. The user programs the Monitor
with the Modbus addresses and device types of the
devices on the segment(s), and tells the Monitor what
information he wishes to monitor from each device. The
Monitor will query the devices for the desired data and
display it on-screen.
Multiple Modbus Monitors
Up to two Modbus Monitors are permitted on a single
segment. In compliance with the Modbus master-slave
architecture, only one device may act as master at a
given time. However, while one Monitor is acting as
master, the second Monitor is constantly checking for
the master’s presence. Should the master go off-line, the
second Monitor will immediately assume the role of
master device.