Hardware Reference
In-Depth Information
Printers are normally noncritical because whatever is being printed can be reprinted. Don't
connect them to a UPS unless there's a good business need to do so.
Some UPSs and SPSs have sockets that are conditioned but not backed up—that is, they do not
draw power from the battery. In cases such as this, you can safely plug printers and other
peripherals into these sockets.
Real-Time Clock/Nonvolatile RAM (CMOS RAM) Batteries
Most PCs have a special type of chip in them that combines a real-time clock (RTC) with at least 64
bytes (including 14 bytes of clock data) of nonvolatile RAM (NVRAM) memory. This chip is
officially called the
RTC/NVRAM chip
, but it is often referred to as the
CMOS
or
CMOS RAM chip
because the type of chip used is produced using a CMOS Complementary Metal-Oxide
Semiconductor (CMOS) process. CMOS design chips are known for low power consumption. This
special RTC/NVRAM chip is designed to run off a battery for several years.
The original chip of this type was the Motorola MC146818, which was used in the IBM AT dating
from August 1984. Although the chips used today have different manufacturers and part numbers, they
all are designed to be compatible with this original Motorola part. Most modern motherboards have
the RTC/NVRAM integrated in the motherboard chipset South Bridge or I/O Controller Hub (ICH)
component, meaning no separate chip is required.
The clock enables software to read the date and time and preserves the date and time data even when
the system is powered off or unplugged. The NVRAM portion of the chip has another function: It is
designed to store the basic system configuration, including amount of memory installed, types of disk
drives installed, PnP device configuration, power-on passwords, and other information. Although
some chips have been used that store up to 4KB or more of NVRAM, most motherboard chipsets with
integrated RTC/NVRAM incorporate 256 bytes of NVRAM, of which the clock uses 14 bytes. The
system reads this information every time you power it on.
Modern CMOS Batteries
Motherboard NVRAM (CMOS RAM) batteries come in many forms. Most are of a lithium design
because they last 2-5 years or more. I have seen systems with conventional alkaline batteries
mounted in a holder; these are much less desirable because they fail more frequently and do not last
as long. Also, they are prone to leak, and if a battery leaks on the motherboard, the motherboard can
be severely damaged. By far, the most commonly used battery for motherboards today is the coin cell,
mounted in a holder that is part of the motherboard. Two main types of coin cells are used, differing
in their chemistry. Most use a manganese dioxide (Mn02) cathode, designated by a CR prefix in the
part number; others use a carbon monoflouride (CF) cathode, designated by a BR prefix in the part
number. The CR types are more plentiful (and thus easier to get) and offer slightly higher capacity.
The BR types are useful for higher-temperature operation (above 60°C or 140°F).
Because the CR series is cheaper and easier to obtain, it is generally what you will find in a PC. The
other digits in the battery part number indicate the physical size of the battery. For example, the most
common type of lithium coin cell used in PCs is the CR2032, which is 20mm in diameter (about the
size of a quarter) and 3.2mm thick and uses a manganese dioxide cathode. These are readily available
at electronics supply stores, camera shops, and even drugstores.
Figure 18.42
shows a cutaway view
of a CR2032 lithium coin cell battery.
