Today, almost every engineer depends on the PC to help perform design, simulation, test-program generation, or results-evaluation tasks. Now, PCs also are the essential control and data-processing backbones for many instrumentation, data acquisition, and ATE applications. For any one of these functions, the PC’s flawless performance—even in the event of a power outage—is becoming increasingly important.
Fortunately, uninterruptible power sources (UPSs) are available to provide clean power in spite of brownouts or short-term power interruptions. The UPS uses built-in or auxiliary batteries to furnish power during power outages for periods ranging from 10 to 30 minutes, depending on battery capacity and load. Since it usually is not known how long an outage will last, it is common to curtail PC operations, save work in progress, close open programs, and shut down the system long before the battery power is used up.
To affect an orderly and timely shutdown, two requirements must be met: The UPS must provide data on which a shutdown decision can be based, and human or automatic resources must exist to suspend computer operation.
UPS as a Vital Data Source
A visual or audible alarm to indicate loss of input power is the minimum feature offered by most UPSs. LEDs also may show input and output voltage, overloads, and battery conditions.
During a power outage, it often is essential to obtain information about how long the battery can support all equipment connected to the UPS. To provide this data, many UPSs include measurement facilities to determine the power consumed by the load. Some UPSs also feature remotely controllable switchable outlets that selectively disconnect the least essential equipment fed by the UPSs.
Input-voltage variations and power consumption may be selectively or continuously monitored to generate long-term operational records. Other information from UPSs also may include environmental data, such as ambient temperature and humidity, or equipment temperature rise.
A variety of communications options forwards data to an adjacent or remote- control station. Information may be transferred via switch closures or low-level analog signals or it may be digitized and structured in conformance with a proprietary or standard protocol. Almost all full-featured UPSs provide digital data via an RS-232 interface. A few offer direct LAN connections or space for a range of plug-in interface boards.
Safe Shutdown, Surveillance, Control
For noncritical installations consisting of a PC adjacent to a dedicated UPS, a power-loss warning light and built-in LED status indicators may be sufficient. However, an operator must be present to be aware of the warning and then take immediate action to achieve an orderly shutdown.
But neither presence of nor prompt action by an operator can be guaranteed. As a result, it is becoming common to interconnect the UPS with one or more PCs via a communications link and provide software that monitors and controls actions automatically.
In its simplest form, the software provides only shutdown management. When an activating signal is received from the UPS, the software notifies operators that shutdown is imminent. After an interval set by the user, it saves all work in progress, closes open applications, and shuts down the computer—all without operator intervention.
A more comprehensive set of management software also may automatically reboot the system upon return of normal power, restart applications, and restore all windows to the same state they were in before the power outage occurred. Many more real-time surveillance and control features made possible by UPS-resident sensors, the communications link, and software are available.
For example, many of the current UPSs and their accompanying software provide alarms indicating potential problems. Whenever the line voltage, the applied load, the battery charge level, or other user-defined attribute is outside pre-established limits, a warning appears on the PC monitor. Then, preventive measures can be taken to alleviate the situation or minimize potential consequences.
Some power-management software allows you to configure and customize how the UPS will react to various power events, such as under what conditions it should revert from line power to battery power and vice versa. You also may choose specific types of power disturbances the UPS should monitor and the data to be logged by the PC.
Software-controlled or regularly scheduled UPS diagnostic self-checks or battery tests may be performed unattended and out-of tolerance performance reported. When warranted, battery-replacement warnings may be posted automatically. Then, the system may send an e-mail message to the information systems manager or initiate a page to summon a maintenance technician to rectify potential problems.
Network Environment Requirements
Most PCs used for scientific, engineering, or business applications are operated in a networked environment. For adequate data protection in case of a power outage, all networked equipment including PCs, workstations, servers, hubs, and routers must be fed from UPSs. The UPSs should include measurement and control features, and the power-management software must monitor and control all interconnected power-providing and power-consuming equipment.
Facilities also must exist so a network manager can configure automatic shutdown sequences for all devices, specifically multiple servers and data-forwarding and storage equipment. For instance, shutdown of the most critical equipment, such as data bases or file servers, should occur last, after all work in progress at the PCs and workstations has been saved. Similarly, shutdown of hubs, switches, routers, and communications servers must take place in a systematic sequence appropriate for the particular network topology.
Unattended automatic shutdown of all computer systems in a network is a must. But not all UPS software programs provide the capabilities or flexibility to deal with the variety of equipment and operating systems involved. Some simple UPS software packages only shut down the servers, but not necessarily the PCs logged onto the servers. In this case, any open files on the PCs or workstations may be lost.
Since networks may be composed of a variety of computer types and operating systems, the UPS software should work equally well on all platforms. Ideally, it provides the same features regardless of whether it is running on Windows NT, Novell, or UNIX. Preferably, the software is not only compatible with the supplier’s own UPSs, but also with those from other manufacturers.
Integration capability with server management programs, such as IBM’s Netfinity, Compaq’s Insight Manager, or Microsoft’s System Management Server, is another desirable asset. But to provide the most versatile and complete remote monitor and control capabilities, compatibility with the Simple Network Management Protocol (SNMP) is offered by many UPS suppliers.
Although originally designed to work with the internet-inspired Transmission Control Protocol (TCP), SNMP now manages virtually any network type, including non-TCP architectures such as Ethernet bridges or OSI transports. All major network-related equipment suppliers presently support SNMP.
UPSs that support SNMP broadcast warnings regarding power events and UPS status to all users or any network management station, regardless of location. UPS-SNMP compatibility permits network managers to monitor and control all UPSs in the network and to manage the power for clients, servers, and internet equipment from one management console.
Most importantly, shutdown scheduling of all equipment as well as load-shedding decisions can be made centrally by the network manager, providing more efficiency and security. For instance, after assessing power availability, you can run a command file that transfers work in progress from a PC that is about to be shut down to another unaffected PC to avoid work stoppage.
The hardware required to provide SNMP compatibility may be built-in to the UPS or, more often, offered as an optional adapter unit. UPS-SNMP software may be generic or targeted to suit specific topologies and operating systems.
Since the extent of automatic monitoring, warning, and control capabilities is very much a function of the software’s capabilities, it is prudent to evaluate its features as thoroughly as those of the basic UPS hardware. The key performance characteristics of typical UPSs and some power conditioning equipment are presented in chart that accompanies this article. Note that all the companies in the chart equip some or all their UPSs with RS-232 communication ports and offer SNMP support.
Legend for Power Conditioning Chart
Inter = Line Interactive UPS
IT = Isolation Transformer
On-L = On-Line UPS
Reg = Regulator
Sup = Surge Suppressor
Surge Suppression (2)
ANSI = Conforms with ANSI/IEEE Surge
Output Waveform (3)
˜ = Sine Wave
App = Approximate Sine Wave
P = Power Consumption
O = Overload
B = Battery Status
Copyright 1997 Nelson Publishing Inc.