Smiths Graseby 3200, Техническое и сервисное руководство

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Smiths Medical International Ltd.
3200 Service Manual Issue 5 (August 2004)
4 — 2
Processor core
The processor has the ability to address both the paged RAM and the EPROM. The 32K RAM
(IC12) is split into four 8K pages, and the EPROM (IC13) is divided into thirty-two
8 K pages. The paging system is part of the integral memory management system of the
processor.
IC15 is the microprocessor supervisor chip and has four main duties: to control the
microprocessor reset; battery backup switch-over; watchdog timer and CMOS RAM write
protection during power down.
Crystal X1 and its associated components R63, C32 and C33 form a 16 MHz oscillator,
providing clock pulses for the main-processor on pins 67 and 68.
Pins 70 and 71 on the main-processor are used to control its mode of operation. Both pins
are held high to set the processor to address the external EPROM (IC13). Pins 44 and 43
are held at Vcc and ground respectively, as the reference inputs to the processor’s built in
A-to-D converter.
Pin 21 receives pseudo non-maskable interrupts generated by the supervisor circuit in the
event of a system fault being detected.
Pulse Width Modulated (PWM) output PW3, pin 14, controls the offset trim for the pressure
sensing system, and output PW4, pin 15, controls the sounder volume. Pins 12 and 13 are
not configured as PWM outputs, they are used to activate FPSEL1 and FPSEL2 which
define which input/output chips to activate.
IC12, the RAM chip, is used as a buffer for the RS232 interface. IC12 also provides a
duplicate of critical data stored in the on-board RAM so that corrupted data may be detected.
The EPROM chip IC13 stores the system’s software.
IC16 is a real time clock, and crystal X2 provides the reference signal for the clock. When
the pump is switched on, the processor reads the time and date received from IC16, and
when the clock is changed the real time clock’s data is also changed.
The input from the pressure sensing interface circuit, SENSE1, to the A-D converter on the
processor is conditioned by a technique called dithering.
Dithering is achieved by using the signal already present on D0 ( the LSB on the data bus) as
a noise source. C40 and C41 filter this noise source signal which is then applied to the signal
from the pressure sensing interface.
Applying this dithering technique results in a noisy signal being sent to the A-D converter,
this then results in a varying output which is digitally filtered and converted to the pressure
units being displayed. The end result is that the resolution of the ADC is improved.