Ohmeda 3000 - Page 10

1/Functional Description
Port 3 is used to perform remaining tasks required
by the control system. Connections 3.0 and 3.1, (re-
ceive and transmit respectively), are used in con-
junction with the serial interface chips so that com-
munication to an external microcomputer is possi-
ble. Connection INTO/P3.2 is a line frequency inter-
rupt line that is used to aid in timing subroutines
found within the system software. Connection T0/
P3.4 sends serial data to the display driver while
connection T1/P3.5 provides clocking to the driver.
Line Frequency
The line frequency circuit converts a 60 or 50 Hz
sinusoidal signal into a square wave signal. The
output of the circuit is used to clock the 4020B
counter and to provide a low frequency clock source
for the system software. The 1N4001 diode haif-
wave rectifies the 8 Vac (nominal) signal for use
with the Schmitt trigger NAND gate. With one line
tied “High”, the output of the trigger will be inver-
ted. Since the gate will not respond until the input
exceeds 1.9 volts minimally, the duty cycle of the
output will be slightly more than 50%.
Heater Status
The Heater Status function signals the microcontrol-
ler and the safety. circuitry as to whether or not the
heater is “On" or “Off”. The input to the Schmitt
trigger is “High” if the heater is “Off” and “Low” if
the heater is “On”. Small glitches appear when the
heater is “On". Consult the Functional Description
of the power supply board for further explanation.
The output of the NAND gate is inverted because
one input is tied “High”.
Hardware Triac Test
The 4020B 14 bit binary counter, U9, counts at a
rate equal to the line frequency and responds to the
negative edge of the clock pulse. The clock signal is
received from a Schmitt trigger NAND gate, pin 6 of
U8. The counter resets when the 7418123 retrigger-
able one shot flip flop outputs a “High” level pulse
on the O output Ime. With CLR tied “High” and A
tied “Low”, the counter will reset when B of the
7418123, US, is “High” at a time equal to (04)
+(013) +(014) or after 12296 counts (04 = 8, 013
= 4096, 014 = 8192). Approximately 6.19 usec.
later the output of the one shot will return to its in-
itial “Low” state.
Q13 and Q14 of the 4020B are tied to a 2 input
“And” gate which will go “High” after 12288
counts. On odd numbered counts Q1 of the counter
goes “High”. Q1 is tied to the CLR pin of D flip flop
Ui, which when “High” allows the output O to
equal the input D on the next positive edge of the
clock pulse. Therefore the output at pin 5 will up-
date on even counts. After 12288 counts (3.4133
minutes on 60Hz units, or 4.096 minutes for 50Hz
units) the signal at the D input of the flip flop goes
“High”. This signal is also input to the microcontrol-
ler through the 1/0 expander U4. The software will
then switch “Off” the heat. Two counts later the
“High" input on D is clocked to the output Q. The
heater status (“Off"-“Low", “On"-“High") sent from
the Schmitt trigger NAND gate pin 11 of U8 is al-
ways present at the input of U2 pin 5. If the heater
is still “On” after the 2 counts, the output of the
“And” gate pin 6 of U2 will clock the second D flip
flop. The outputs of the flip flops switch ~O goes
“High” and “Not Q” goes “Low”. A “Low" on “Not
O" sets off the audio alarm and drops out the non-
1-4
resettable safety relay causing the heater to switch
“off”.
Heater Status LED
A heater status LED is located on the control board
for troubleshooting. The LED can be seen through
the rear of the controller assembly cover. When the
status line from the Schmitt trigger is “High”, (hea-
ter “On”) the transistor O2 switches “On” causing
the LED to emit light. If heat is “Off”, the LED is
“Off”.
Watchdog Timer
A watch dog timer is used to “check” that the
microcontroller is working properly. After every
cycle through the system software the microcontrol-
ler sends a “Low” pulse to the A input of U3, a
74L$123. The RC network connected to the RxCx
and Cx pins create a time constant, t = 0.45xRxC
= 0.263 seconds. If a pulse is not received at the
input before the time constant expires, the output
will go “Low". The high priority alarm will then be
activated due to the microcontroller failure. Note
when the microcontroller detects a high priority
alarm condition, pulses to the watch dog circuit
stop.
Alarm Tone Generator and Control Circuits
The alarm circuit consists of an alarm tone
generator and control circuitry for high or low prior-
ity alarm conditions. Under a no alarm condition the
7556 timers are both inactive, (reset lines low).
Low Priority Alarm
Under normal operating conditions the input to U8
pin 9 is “High”. When the microcontroller detects a
low priority alarm a 1 Hz square wave is output to
U8 pin 9. The timer activates, causing a 2 kHz audio
output. This results in a one second “On”, one sec-
ond “Off" audio alarm. The 2 kHz signal is adjusted
within + 100 Hz by R38. The volume of the audio
alarm is adjusted by R37. This should be adjusted
fully CCW for maximum volume.
High Priority Alarm
The high priority alarm is activated if the microcon-
troller quits sending pulses to the watchdog timer.
This occurs when a high priority alarm condition is
detected or if the microcontroller fails. The high
priority alarm is also activated if the hardware triac
test circuitry detects a failed triac. Both timers be-
come active with one timer feeding a 1 Hz signal to
the control line of the second. The 1 Meg resistor
changes the output frequency of the second timer
to produce a warbling effect (two tone alternating
alarm). If high and low priority alarms are both
“On", the output of the “And” gate overrides the
low priority signal, keeping both timers active.