Technical data

3-16 Theory of Operation
3.23 LED Board
The LED board for Model 400s systems houses the LEDs that are visible on the front
panel of the system unit. These LEDs indicate the progress of the boot ROM through
its initialization, report status codes during normal Domain/OS operation, and display
a FRU code in the event of an error. They also indicate power-on and Service mode
status. Chapter 2 describes the LED Normal operation codes for Domain/OS; Chapter
4 describes the LED boot ROM error codes for systems that use Domain Compatible
and HP-UX Compatible modes.
The LED board for Model 400t and 400dl systems houses nine LEDs instead of the 10
LEDs found on the Model 400s LED board. The power-on LED for Model 400t and
400dl systems is located on the power-on switch plate instead of on the LED board.
The Model 400t and 400dl LED board also contains the system Reset switch and
Service/Normal switch. The Service/Normal switch is a push-button switch that only
retains its state (Service or Normal mode) when the system is on; the system defaults to
Normal mode when you turn it off. The LED board also routes the power supply dc V
power-on signal between the power-on switch and the power supply.
3.2.4 Apollo Token Ring Network Controller-AT
The Apollo Token Ring (ATR) Network Controller-AT manages the system’s interface
to the ring network. The network controller incorporates
• A modem for modulation and demodulation of data from the ring
• The input-filtering and amplification hardware that converts data from the ring
to a digital bit stream
• The receive logic responsible for converting digital data from serial format to
parallel format
• The transmit logic responsible for converting digital data from parallel format
to serial format
• A dual-ported RAM buffer
• The control logic that regulates the operation of the other functional units
• The relays that remove the system from the ring during power-off or offline
operation