HP SureStore Optical Jukebox 160ex Service Manual page 206

NOTE
Theory of Operation
SCSI Interface PCA
The two large components on the PCA are the Field-Programmable Gate
Array and the microprocessor. The microprocessor is an 80C52 that has
flash-programmable memory on board. (There are no boot ROMs on this
PCA).
After the jukebox runs it selftest on wakeup, in instructs the PCA to
come up in whichever mode has been selected by the user. It can come up
either as a repeater or in LUN mode. Communication from the robotic
controller is through the GPIO bus cable. (If the GPIO cable is not
connected at startup, the PCA will come up as a repeater.)
A "heartbeat" LED is located on the bottom edge of the board. This LED
will continuously flash at a slow rate when power is applied. If the LED
is either on steady or off, this indicates a problem with the PCA.
An LED, visible through the top of the interface enclosure, is mounted
between the interface connectors on each side. These LEDs light to show
which interface has been selected. If the wrong interface type is
connected to the interface connector on this PCA, the LED will
continuously and rapidly flash to alert the user to this error. No damage
is caused to the chips on the PCA by having connectors in the wrong
position.
During powerup, the position of the interface selector switch is checked
to see which external bus is active and if the proper bus type is on the
selected interface port.
If the differential bus is active, the DIFFSENSE signal on the SCSI bus
is checked. If this signal is LOW, it means that a single-ended bus has
erroneously been connected to the differential connector. The bus is
immediately made inactive to protect the chips.
In addition to checking the position of the interface select switch on
powerup, the controller is informed of any change to this switch during
normal operation. If the switch position is changed, a BUS RESET signal
is sent to the robotics controller on the internal SCSI bus
It is important to provide proper termination on whichever external
SCSI bus (single-ended or differential) that is in use. If the SCSI bus is
not being daisy-chained to another peripheral (and terminated there)
then termination must be provided at this PCA.
6-14 Chapter 6