Ivy Biomedical Systems 101NR, Service Manual

Page: 38 / 68

THEORY OF OPERATION
+12 V. Other output pulses (+5 TTL) may be generated by removing U12 and changing C56 and R111. See
component matrix on schematic.
Rate Computation
The EVENT signal goes to PLD U36 in the digital section where the 200 ms blanking removes all but one pulse for
each heartbeat. The ouptut of the flip-flop goes to the interrupt input of the microprocessor U48. The
microprocessor measures the time between interrupt pulses and computes the heart rate.
Alarm Generation
The microprocessor compares the current heart rate to the limits set by the user. If the limits are exceeded, the
appropriate alarm signal is generated.
CRT Signals
Refer to the Key Signals portion of the Testing and Troubleshooting section of this manual for diagrams of each of
the crt signals.
Vertical Deflection
The display generation is broken up into two timing periods, one for the raster display (digital information and
alarms) and the other for the stroke display of the ECG waveform. Each time period is 9.2 ms.
The raster display also contains two time periods. The first is for the left display and the second for the right
display. The left display contains the heart rate display, the setup information, and the ALARMS OFF indicator.
The right display contains the alarm indicators.
During the stroke display period, the vertical deflection is the ECG waveform signal. This signal is selected
through FET Q15 to the vertical deflection amplifiers U6 and U9 and transistors Q1, Q2, Q5 & Q6. During the
raster display period, the vertical deflection is the vertical ramp generated by U8B, with its timing set by the
V SYNC signal. The vertical ramp is passed through Q15 to the vertical deflection amplifiers.
Horizontal Deflection
The horizontal ramps are generated by U16 and U17 with timing controlled by the H SYNC signal. Four ramps are
generated in each timing cycle. The two small ramps are for raster display, and the large ramps are for stroke
display. An offset for the right side raster display is added to the circuit through U11D.
Horizontal Correction
Because the face of the crt is flat and the beam originates at a single point, the speed of the beam as it strikes the
face is greater near the edges than at the center. Therefore, correction circuitry is required to produce a waveform
that is linear over the entire screen. The linearity correction is provided by U17 and associated components.
The correction circuitry produces two signals that are added to the horizontal ramp to correct the linearity. First,
U16A integrates the horizontal ramp to produce a parabolic waveform. This waveform is the mathematical
equivalent of the speed of the beam across the crt face. This signal is inverted by U16B. The original signal and its
inverse are fed into the SYM potentiometer RV6, which selects the portion of the positive or negative waveform
required to produce a symmetrical trace between the left and right sides of the crt.
The second signal is an S-shaped waveform generated by U17A, which integrates the parabolic signal from U17B.
This S-shaped signal, when added to the horizontal ramp has the effect of slowing the beam near the edges of the
32 Model 101R/NR Service Manual