GE Industrial Solutions POWER LEADER ModBus Monitor - Manual (Page 8)

POWER LEADER Modbus Monitor
Chapter 1 – Introduction
2
Theory of Operation
As mentioned previously, the POWER LEADER
Modbus Monitor has two modes of operation. It can
operate as a stand-alone master to one or two Modbus
segments of compatible devices or it may operate in
conjunction with the GE Power Management Control
System (PMCS) software. In this second mode, the
Monitor collects the data it needs to display and also
watches for the PMCS to go off-line. If for some reason
the Monitor does not sense the PMCS being online as
Modbus master, the Monitor will immediately and
transparently assume the role of master on the Modbus
segment(s).
We’ll explain briefly how PMCS operates, and then the
relationship of the Monitor to the PMCS, and finally the
operation of the Monitor independent of PMCS.
PMCS operation
The GE Power Management Control System consists of
four basic parts: (1) power management devices, (2) an
RS-485 network (“Modbus segment”) connecting these
devices to (3) a host computer running GE’s Power
Management Control System software and a Dynamic
Data Exchange (DDE) server, which monitors the
devices for data requested by (4) client software
programs, and feeds this data back to the clients
requesting it.
The power management devices vary in complexity and
intended use, but in general their function is similar: they
collect data on specific electrical metered, status and
event parameters (volts, amps, power consumption,
peaks, trips, etc.). The various information they monitor
is recorded in specific memory locations called registers.
Registers are organized by address. The complete list of
a devices’ registers, their addresses and their contents is
referred to as a register map.
PMCS has been preprogrammed with the register maps
of a variety of GE and third party devices. When the user
configures PMCS with the details of what devices are
connected to his network, the software automatically
associates the appropriate register map with the given
device type. This enables the software to locate
information in a device based on its type.
PMCS operates using a client-server architecture called
DDE - Dynamic Data Exchange. DDE is a means of
exchanging information between software programs
operating on a PC. Client programs request information
from a Server program; the Server program fills these
requests. In the case of the PMCS DDE Server, these
requests are for power management information being
gathered by physical power management devices on
Modbus segments connected to the PC. An example of
PMCS DDE communications is shown in Figure 2.
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Figure 2. PMCS DDE communications cycle.
Client software such as the PMCS Event Logger or
Waveform Capture is used to analyze particular data
from devices. Each client program needs particular
pieces of data from particular devices. The client sends a
DDE request to the Server, the Server queries the
devices for the requested information, and when it has
received the information from the devices, it replies to
the client’s request with the desired information.
An important aspect of DDE is its ‘dynamic’ nature;
once a client has requested information from the Server,
a ‘DDE link’ to that data is established. The Server will
continue to monitor that information and, if any changes
occur, will forward the new data to the client.