Epson MX-80 Technical Manual page 23

In this circuit, the following voltages are produced.
+
24V DC ... Power supply for driving print head solenoid and stepper motors
+
5V DC .... Power supply for TTL logic
+
12V DC ... Power supply for buzzer, LED's, etc.
+
14V DC ... For stepper motor holding current
+
12V AC .. , Power supply for serial interface (option)
Should the supply voltage become abnormal, this circuit detects such condition and
causes a reset signal to be generated in the control circuit so as to return the control cir-
cuit to its original state, and at the same time, inhibits voltage from being applied to the
print head solenoid driving transistors, thus assuring proper energization of the print
head solenoid.
In this circuit, the voltage level at the zener diode ZD1 connected to the rectified output
of the
+
24V line is normally 12V and the voltage level at the zener diode ZD2 connected
to the base of transistor 03 is normally 7V. When a total of these two voltages exceeds
19V, a current is supplied to the base of transistor 03, causing transistor 04 to turn on to
permit the supply of
+
12V to the head driver circuit. Should the voltage of the 24V line
reduce to below 19V for some reason or other, transistor 04 is caused to turn off, thus
prohibiting the supply of
+
12V to the base of the head driver transistor. The print head
solenoid is thus inactivated to prevent the dot wires of the print head from being
damaged.
+
24V is produced from the 23V supplied from the power transformer into a current of
1.5A on the average at 24V by a series regulator consisting mainly of zener diode ZD4
and transistor 04, after being processed through a bridge rectifier. The
+
5V supplied to
the logic circuit is obtained by producing a current of approx. 1.5A with three-terminal
regulator SR2.
(3) 8155
Data and signal exchange with an external computer (e.g., standard parallel computer)
is performed by LSI "8155" which incorporates MCS85 series I/O's and a 256-byte RAM.
The 8155 is operated in the ALT3 mode according to the command from the main
microprocessor of LSI "8049". In this case, port PC2 is used as the input port for the
strobe pulse (5TB), and ports PAO- PA7 as the input ports for all data.
Accordingly, when one of the data is taken in by the STROBE signal. Ready (RDY) signal
is immediately output to port PC1. The strobe pulse of the data is latched by the 8155 at
the trailing edge of the pulse. The ports PBO- PB7 and PC4 of the 8155 are used to out-
put the respective drive signals for the 9 print head· solenoids.
The 256-byte memory (RAM) in the 8155 is used as the storage area for data.
Data exchange with the main microprocessor of the 8049 is performed through address
data bus lines ADO-AD7. Fig. 2.6 shows the flow of data and signals through the 8155
and Fig. 2.7, the 8155 and its peripheral circuits. The ports PAO-PA7 of the 8155 are
respectively connected to the optional interface connector for system expansion, to per-
mit communication between the MX-BO and any optional device when attached to the
Printer.
2-11
REV. A