Epson PX-8 Technical Manual page 51

REV.-A
*
The detailed voltage waveform at the anode of D6, shown below, illustrates an oscillation
which occurs during charge/discharge from/to the inductance L3.
----
-------
~
Damping oscillation
I I
I
I
I
IE
~I
Discharge from L3 (Oscillation)
Fig. 2-22 Details of L3 Discharge and Charge Cycle
2.2.5.3 RS-232C Regulator
This regulator is also a DC-DC converter, which is enabled only when the RS-232C or the serial
interface is used. The circuit includes a control feature which prevents its output voltage
from being used for data transmission during a certain period of the rising time until the voltage is
sufficiently stable to be used for the RS-232C levels. IC 4C performs this control function .
• Circuit Operation
IC 4C initially outputs a high signal at pin 26 (SWRS) and a low signal at pin 27 (INHRS).
The SWRS signal is inverted by IC 12D and fed to the base of the transistor Q8, turning it on.
This causes the battery voltage (VB, +5V) to be output at the collector of the transistor. The
INHRS signal is inverted high by 12D and then input to the base transistor Q18, cutting the
transistor off. Q17 is also cut off, leaving the transmission line (TXD) floating.
A pulse signal of approximately 35 kHz, generated by a CR oscillator circuit, is supplied to pin 9
of IC 14D through R45, C29, and R46. The inverted output is fed to the base of transis-
tor Q31, switching it on and off. This causes transistor
Q
17 to also start switching, thus re-
peating a discharge/charge from or to the inductance L2. This discharge and charge voltage is
half-wave rectified by the diode D2, and the positive output voltage is filtered by capacitor
C13 to produce a DC voltage of +8V. -8V is generated at the negative pole of capacitor
C14 by the negative component charge to capacitor C15 and a negative voltage swing at
the anode of diode D3 due to the charge.
Fig. 2-23 shows the timing relationship among the voltages discussed above.
2-27