Datascope Passport 2, Руководство по обслуживанию

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Passport 2®/Passport 2 LT™ Service Manual 0070-10-0441 2 - 19
Theory of Operation Front End Module
2.2.13 Temperature
Patient temperature is monitored by a thermistor probe. Either YSI 400 series or 700 series
probes can be used, with autodetection of probe type. When the 400 series is used, the
temperature channel is accurate to 0.2 °C. Correct calibration is continuously verified, as is
required by European standards.
The YSI 400 series probes contain a single thermistor, wired between the tip and sleeve of a
two-circuit ¼ inch phone plug. The 700 series probes contain two thermistors, and use a
three-circuit plug. The sleeve of the plug is connected to the common of the two thermistors.
The primary thermistor is connected to the tip contact, and a secondary thermistor is
connected to the ring contact. However, only the primary thermistor is used for temperature
measurement. The secondary thermistor is used as the basis for probe type autodetection.
The ring contact of the probe socket is biased through a resistor from the +5 volt supply, and
has its voltage monitored by the 68HC16 internal ADC. When a series 400 probe is
connected, the solid metal sleeve of the plug shorts the socket’s ring contact to ground, so the
ADC reads essentially zero. If no probe is connected, the ADC reads near full scale.
However, if a series 700 probe is connected, a voltage divider is formed by the bias resistor
and the secondary thermistor. Therefore, the ADC reads an intermediate value which
depends on the probe temperature, but is easily distinguished from the readings obtained
when a 400 series probe or no probe is present.
2.2.14 Timers
The only digital input to the processor is Timer Input Compare 1 (Port GP 0), which is used for
pacer detection. This line exhibits a falling edge each time a pacer is detected. Therefore,
this input should be configured to generate an interrupt on falling edges, which invokes the
pacer handling ISR. A timer channel, rather than interrupt input, has been used so that the
exact time of arrival of the pacer is logged; this may prove helpful in generating some of the
timing based on the pacer, although it is not necessary to use this feature. The latency of the
pacer ISR must be minimized, a consideration which must be remembered when assigning
timer interrupt priorities. Timer Output Compare 1 (port GP 3) is the pacer blanking output.
When the monitor is used with patients having conventional implanted pacemakers, this
signal is not used, and is maintained at logic low. In the case of external pacemakers, a
much stronger pacer overload signal results, and the pacer blanking output is used to
activate a sample and hold of the ECG signal for the duration of the pacer overload. In
external pacer mode, TOC 1 is used to generate the pacer blanking pulse. The Pacer ISR sets
the TOC 1 output high, initiating the pulse. It further programs the TOC 1 register to
terminate the pulse after the requisite duration. Therefore, the TOC 1 function is set to drive
the pin low on timer match. Low ISR latency is necessary to ensure that the pulse starts
promptly upon pacer detection.
Timer Output Compare 2 (Port GP4) is used to define the E-trigger pulse sent to the module
bus. The E-trigger pulse is initiated by the R-wave detection software, which also programs
TOC 2 to terminate the pulse after the requisite width.