Service manual
3-1
Theory of Operation
This chapter contains an overview of the cardiograph operation, and circuit descriptions of
the major subassemblies. Except as noted, the information in this chapter applies to the
M1770A PageWriter 200i/300pi, the M1771A PageWriter 200, and the M1772A
PageWriter 100.
The last section in this chapter describes the Option #A05 printed circuit assembly.
2Operational Overview
This overview covers two areas of cardiograph operation: the path that ECG data follows
from collection to display, and the cardiograph’s power-on/power-off sequence.
ECG Data Path
Analog ECG data are collected through the patient cable to the ECG Front End on the
CPU Assembly Board in the cardiograph. The ECG Front End performs analog-to-digital
conversion before sending the data to the System Gate Array. The System Gate Array
receives the ECG data and processes it. The data may be interpreted (200i/300pi only),
displayed (200/200i/300pi only), printed, or stored in system RAM. PageWriter
200/200i/300pi cardiographs equipped with Option #A05 can also store up to thirty ECGs,
and transmit ECGs to TraceMaster ECG Management Systems, other PageWriter
cardiographs, facsimile machines, and other HP equipment.
Power-on and Power-off Sequences
When the cardiograph is turned on or off, it follows a sequence of events in applying
power to or removing power from its circuits. The System Gate Array controls this
sequence because the System Gate Array remains powered up even when the cardiograph
is in Standby.
Power-on
The system gate array is powered from a supply which is always turned on (Unswitched
5V). When it detects that the key has been pressed, it sends the ENBL5V
signal to the power supply to turn on the Switched 5V supply and reset the other circuitry.
When the CPU is taken out of reset mode, the processor begins execution of the software
located in ROM. The software sets up the system, decides what type of keyboard is
attached, calibrates the ECG Front End, and initializes the display, if present.
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