Sigma 6000+ - Page 23

THEORY OF OPERATION (Continued)
On/Off and Memory Circuits
When the ON/OFF key is pressed (assuming the unit is off), U8 pin 1 goes “HT”, turning on Q4 which
turns on Q7, switched +12V. This supplies the 5-volt regulators VR2 (display +V) and VR1 (logic +V).
At the same time the microprocessor turns Q6 via pin 44, a latched output of U19.
When the ON/OFF key is depressed again, U8 pin 1 goes “LOW” which disables Q4. After 3 hours the
microprocessor turns off Q6, which turns off Q7 (switched 12V+). This allows the retention of all
settings in memory for 3 hours retention of programmed settings.
An added feature to the 6000+, which keeps the user fro accidentally turning the unit off while the pump
is infusing, requires that the STOP switch must be pressed before the unit can be turned off. This feature
is accomplished by the use of O11, Q12, U17, U21, associated components and software. Using the
signals from U19 pins 61 and 62, U8 pin 3 is held HIGH while the pump is running. When the pump is
stopped, U8 pin 3 is allowed to go low, when the OFF key is pressed. This allows U8 pin 1 to go low,
turning Q4 off.
Microprocessor System
The heart of this unit is an RCA CDP1802 central processing unit CPU U25. It has a 2.4576Hhz external
clock supplied by crystal X1. Pins 8-15 are an 8-bit data bus and pins 25-32 are address-lines. C37 and
R42 hold the microprocessor in a reset state via U20 pin 4 until the oscillator stabilizes for start-up.
During the main program loop the CPU outputs a stream of regular pulses from pin. 17 to the gate of Q10.
If the CPU locks into a loop outside the program, the pulses stop, turning Q10 on.. This allows the circuit
of U22, R39 and C46 to oscillate, thereby sending a negative reset et pulse via U20 to pin 3 of the CPU and
the AXE circuit (which will be discussed later).
Pins 21, 22, 23 and 24 are external “ON”, “PLUG IN” and “LOW BATT” respectively. They are
controlled by the outputs of U26 a LM3339 comparator. The input levels are adjusted with R24. The
fourth flag pin 24 is used to sense a key change on the door panel membrane. Pins 33 and 34 are TPA &
TPB timing pulses. Pin 7 is memory read, pin 35 is memory write and pin 36 is the interrupted input.
Functions of the LSI Chip (019) Interrupt Controller
The prioritization of interrupts in order highest to lowest are:
1, DOOR OPENED pin 16 — takes precedence over all other interrupts. Controlled by a hall effect
switch mounted on the inside of the front door panel which i is activated by a magnet in the door
handle when opened.
2. LOAD CLIP pin 16 — controlled by a micro switch mounted in the housing which is activated
when the clip is removed. °
A/D OUT pin 17 — tells the microprocessor to read the A/D converter value.
DOOR CLOSED pin16 — controlled by a hall effect switch mounted on the inside of the front
door panel which is activated by a magnet in the door handie when closed.
5. ROTATION pin 15 — this is a single pulse generated once per revolution by a hall effect switch
located on the top of the pump assembly and is compared to the 8 step pulses explained in the
following interrupt. If the 8 to 1 ratio isn’t correcta “HI/LO” alarm occurs.
6. STEP pin 66 — this is a series of 8 pulses per revolution obtained by dividing the motor steps by
25,
7. OFF pin 13 — signals the microprocessor that the ON/OFF switch was pushed.
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Section 1 Page 15