Service manual

Theory
3-7
Video
Video is implemented using Chops 69000 VGA controller with 4 MB of internal RAM that resides on
the PCI bus.
The RGB analog signals are buffered and fed to the CRT display and to two VGA connectors. The
Hsync and Vsync signals also go to the VGA connectors. The composite sync signal goes to the
display.
Ethernet
A Digital 21143 PCI-based Ethernet controller provides the network interface. It is able to support
both a 10-megabit and 100-megabit Ethernet; however, only a 10-megabit Ethernet is used at this
time. This chip directly attaches to the PCI bus and uses a transformer/filter and several passive
components to attach to 10BaseT Ethernet.
ISA Bridge
The ISA bridge connects the PCI bus to an ISA bus and to the components on the ISA bus.
The bridge chip (Intel 82371) also contains several useful support functions:
• Seven DMA channels usable by peripherals on the ISA bus
• Three counter/timers
• Three chip selects
• IDE controller
The bridge and ISA bus components are collectively referred to as the ISA subsystem. This
subsystem is used for less performance-critical peripherals, specifically, audio, wireless LAN,
NVRAM, real-time clock, keyboard, mouse, EPP port, and I/O buffers.
Audio
The audio subsystem is based on a Crystal Multimedia Codec. This chip is capable of supporting
simultaneous stereo in and out in a WAV-type format. It is programmable in terms of analog gain,
data formats, and data sample frequency. An audio amp is used for the internal speaker, and op-
amp buffers are used to drive the line-out connector.
Non-Volatile RAM/Real-Time Clock
The NVRAM and real-time clock reside on the ISA bus. Both functions are implemented with a
Dallas Semiconductor DS1644 Nonvolatile Timekeeping RAM.
The NVRAM chip contains the following functions:
• A 32 K × 8 static RAM.
• A time-of-day clock, which overlays the uppermost 8-RAM addresses.
• An integrated lithium battery with a 10- to 20-year lifetime.
• An integrated clock crystal with ±1 minute/month accuracy.
• Power-fail circuitry to protect the clock and RAM during power down.
The NVRAM is used to hold sysgen values, error logs, and other miscellaneous system data. The
real-time clock keeps track of time while the unit is ON or OFF. An enable bit in the ISA bridge chip
must be set to enable writing to the NVRAM. The device is socket-mounted for easy replacement.