Hill-Rom Advanta P1600, Service Manual

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Theory of Operation
Chapter 3: Theory of Operation
Page 3 - 34 Advanta™ Bed Service Manual (man168ra)
ZoneAire® Sleep Surface Air Interface System
Switch Matrix
The ZoneAire® Sleep Surface user interface requires 10 switches and 12
indicators. When a switch is pressed or released, there is a minimum 50 -
millisecond debounce. A network output message is sent via the LON network
to the ZoneAire® Sleep Surface providing the state of all 10 of the system
switches. When a network input message is received from the system, the state
of the 12 ZoneAire® Sleep Surface indicators is updated to one of four states:
off, blink slow, blink fast, on.
Membrane Key Panel Switch Multiplex Circuit
The switches in the membrane key panel are arranged in 4 rows of 8 columns.
Address 0 through 7 of the decoder/selector circuit select columns 0 through 7
individually. Switch states are read on the switch input nibble from the A
outputs of the 74HC244 tri-state buffer, U5. The A output is selected when any
of the addresses 0 through 7 are selected on the decoder/selector circuit. The
1N4148 diodes provide protection of the selected column when multiple
switches in the same row are activated. Half of the 10K resistor RN2 are used
as pull-up resistors for the switch input nibble. The 74HC14 Schmitt Triggered
Inverter, U6, buffers the input lines to U5.
LEDs
The switch LEDs and the service required LED are strobed by the LED driver
chip (U3). The neuron instructs the driver to turn the LEDs on or off via I/O_8
and I/O_9 (Neurowire).
Membrane Key Panel LED Driver
The Motorola MC14489 LED display driver, U11, provides drive for 22 of the
26 LED’s located on the membrane key panel. LEDs D1 through D11 and
LEDs D17 through D2 are driven by the MC14489 in a multiplexed mode. The
refresh rate on bank 1 through bank 5 is between 700 and 1900 Hz. The
Neuron controls this display driver through the Neurowire Object.
Configuration of the MC14489 is set as an eight bit configuration register. The
LEDs are arranged into 5 banks in a common cathode configuration. LED
states are controlled by a 24-bit display register. The 3.3K brightness resistor
R3 sets the LED current to each LED to approximately 10 mA peak.