16900A/16902A Cpu Subsystem - Agilent Technologies 16900A Service Manual

8
Theory of Operation

16900A/16902A CPU Subsystem

CPU Board
The CPU motherboard is a standard microATX configuration
with 512 MB of system RAM. The exact CPU and speed
grade will vary over the life of the product. Standard I/Os
built into the ATX board are PS/2, serial port, parallel port,
USB 2.0, 10/100 Mbit LAN, and VGA Video. Additional PCI
slots are available for added I/O such as 1 Gbit LAN or
video.
Disk Drives
The frame contains a standard IDE hard drive and a low
profile CDRW drive. Both are logically controlled directly
from the CPU motherboard.
Power
The frame uses two modular power supplies to provide
enough current for the six DC voltage rails: - 12 V, +12 V, - 5
V, +5 V, - 3.3 V, +3.3 V. These rails use remote sensing to
maintain 0.1% load regulation. The module slots are limited
to 100 W each and the instrument as a whole to 900 W.
This translates to 1300 W maximum draw from the outlet.
Note that the instrument is shipped with a special notched
power cord for the notched input receptical which prevents
the use of lower current rated cords.
The frame has a "soft" power on/off algorithm. Pressing the
power switch notifies the CPU to either wake up or power
down. The CPU then in turn drives the signal line which
actually turns the supplies on or off. A PLD monitors this
process and will force a power down after approximately
two minutes if the CPU does not respond. The small
portion of circuitry involved in this process is powered from
an auxiliary +5 V which is always present as long as the
instrument is plugged in. Additional logic monitors the
voltage levels on the six rails and will also cause an
immediate power down if any one drops below about 90% of
regulation. Power down can also be caused by an over
temperature sense circuit. In this case, the PLD notifies the
CPU, the user receives a warning window, and a soft power
down is started. Again the PLD will force a power down if
the CPU does not soft power down in about two minutes.
16900A, 16902A, and 16903A Service Guide 109