Mallinckrodt NELLCOR NPB-4000, Service Manual

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Section 13: Microprocessor Computer and Control –Theory of Operation
13-5
of them is buffered by a logic gate that is connected to 5 volts. The inputs to
these buffers are TTL-compatible and will switch when the input goes below
0.8 volts or above 2.4 volts.
13.3.5 Analog Power On/Off - Port 1 Bit 1
This signal turns on the power to the NIBP circuits via a regulator run from the
battery. To turn on the NIBP circuits, port 1 bit 1 must be set by software to a
high state.
13.3.6 3-Way Valve On/Off - Port 1 Bit 2
This bit controls the three-way pressure valve which is a back safety valve for
the cuff pressure. A high signal allows a blood pressure measurement to occur,
while a low signal dumps the air and the cuff is prevented from pressurizing.
13.3.7 Pvenable - Port 3 Bit 7
This bit is generated by the NIBP circuits for software monitoring of the
over-pressure condition. Normal operation occurs when this signal is at a high
level, indicating the cuff pressure is normal, while a low signal indicates an
over-pressure condition.
13.3.8 Valve PWM Signal - FPGA
The FPGA control circuit produces a pulse-width modulated signal that is routed
to the control valve for relieving the cuff pressure in an orderly manner for blood
pressure measurement. The software programs an eight-bit register in the FPGA,
which outputs a high pulse level for a programmable time, then a low level for a
period of time. The high level is programmed by the software and the FPGA
circuit automatically generates the low portion of the signal.
When this signal is low, the valve is fully closed . As the duty cycle of the PWM
is varied from 0 to 100 percent, the valve changes smoothly from fully closed to
fully open. The PWM repetition rate must be a minimum of 9 kHz. The circuit
uses an eight-bit counter clocked at 5 MHz. This gives a 19.5 kHz
programmable signal period, well above the 9 kHz minimum.
13.3.9 Pump Motor PWM Control Signal - FPGA
The software programs an eight-bit register in the FPGA circuit to generate a
PWM control signal to the pump motor. When the signal is at a low level, the
pump is off. A PWM signal is generated to turn on the pump in a smooth
fashion by varying the duty cycle from 0 to 100 percent. There is an eight-bit
counter that is clocked at 78 kHz. This gives a 304 Hz signal frequency for
pump operation. Minimum pump PWM frequency for proper operation is
300 Hz.
13.4 RECORDER OPERATION
See Figure 13-3. The recorder is a thermal recorder option that is supplied by
General Scanning Corp. This recorder has a self-contained microprocessor and
control circuits for properly operating the recorder from commands sent to it via
UART serial connection. The detailed operation of the recorder is specified in
the operational manual supplied by the vendor.