Hill-Rom VersaCare P3200, Service Manual

Page: 118 / 379

Electrical System
Chapter 3: Theory of Operation
Electrical System
Logic and Motor Control
Power is supplied to the Logic Control by the P3200 linear power supply. The
power supply provides an unregulated 19 - 35 V DC reference to signal GND
at 2.5 A max, 1.75 A typical (see Power Supply Node HDD for input supply
capabilities). The GROUND is common among all nodes in the bed.
The switch mode power supply consists of an input capacitor for input ripple
current control, an integrated Switch Mode Control Integrated Circuit, the
feedback choke, the output capacitor, and a high current, high speed Schottky
diode. This buck regulator provides DC at 8.2 V and provides 3.5 A of current.
The under voltage detection circuit consists of a series resistor and zener diode
connected to the input voltage source. The zener voltage, along with the base
resistor, provides base current for the NPN bipolar transistor.
When the zener stops conduction due to an under voltage condition, the zener
voltage is off, and turns off the transistor. The input voltage through the series
resistance turns on the NPN transistor switch to the regulator. This will turn off
the regulator.
A diode/bulk capacitor is added to the input of the +5 V regulator to keep the
logic supply up longer than the +8.2 V supply. This time depends greatly on
load current and functions active.
The Logic Controller consists of an embedded controller, a derivative of the
8051 family. The Input/Output structure is configured as an active low for both
input and output. This allows pull ups to be used without the microprocessor
running and not cause any operation to occur. The micro processor can be
disconnected and not have any inputs or outputs enabled. The clock can also be
disabled and not cause any operation to occur. All signals to and from the
microprocessor are buffered. All signals that exhibit a slow rise are Schmidt
triggered to prevent oscillation from occurring. Debounced inputs are provided
for all switch inputs to the microprocessor.
The embedded processor is a Atmel 89C51CC01. This is an 8051 variant with
internal 64K flash program and 256 + 1.2K EEPROM data memory. External
to the microprocessor are the CAN transceiver, watch dog reset circuit, 2K
EEPROM and the buffering circuitry for the H bridge motor controllers, and
the sensors for motor position feedback.
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