Epson PX-8 - Page 77

REV.-A
Circuit operations
< While power is on>
While power is on, port 70 of the 4-bit sub-CPU is held low. This low signal is fed to the base of
transistor Q25 after being inverted twice by ICs 3E and 120, maintaining the transistor in con-
duction. This causes Vcc to be supplied to IC 3E, enabling it to oscillate.
VB
To 3E, pin 14 (Vee), 2
7508 (2E)
P70
Fig. 2-53 CR2 Oscillation Control
< While power is off>
While power is off, the backup voltage VB is supplied to the collector of the transistor Q26, as
well as to the base through the resistor R126. A zener diode, Z03, is inserted across the base of
Q26 and ground. Z03 has a zener breakdown voltage of 4V and breaks down when the base
potential rises towards the VB (+5V) voltage beyond 4V. The zener breakdown is removed
when the base potential falls below 4V. An infinite repetition of this alternation causes Q26 to
continue switching on and off, outputting a voltage of approximately 4V at the emitter. This out-
put is connected to the same line as the collector of transistor Q25 and causes Vcc to be sup-
plied
to IC 3E, ensuring the same oscillation as when power is on.
Q 26
VB
To 3E, pin 14 (Vee), 2
Fig. 2-54
2-53