Operation - Data General DASHER D100 User Reference

Interface Logic
The interface logic provides a communications interface.
This asynchronous serial interface allows communication
in full-duplex mode between the microprocessor and a
computer via a 20mA current loop or an EIA RS232-C
communications line. When the split baud/printer option
is present, an additional asynchronous serial interface
(printer interface) allows communication between the
microprocessor and a local printer via an EJA RS232-C
communications line. Both interfaces use a universal
asynchronous receiver/transmitter (UAR/T).
The line characteristics (e.g. baud rate, parity) for these
interfaces are selected by on-board switches. When the
terminal is powered-up or switched on-line, the
microprocessor senses the setting of these switches and
programs the interface(s) to function accordingly.
Power Supply
The power supply provides the five d.c. supply voltages
necessary to drive the various circuits within the terminal.
The primary power input is either 85-132 V a.c. or 187-264
V a.c., 47-63 Hz, as selected by hardwired jumpers on the
The power supply uses a switching regulator to provide
the output voltages. The incoming a.c. voltage is converted
to a d.c. voltage and fed through a high frequency oscillator
and into a transformer. The outputs of the transformer are
rectified and filtered to provide the various output voltages.
Constant power output is maintained by varying the duty
cycle of the oscillator (pulse width modulation). Protection
circuits prevent damage in the event of an over-current or
over-voltage condition.
The switching of the supply is synchronized to occur when
the display screen is retraced, i.e., when the electron beam
in the CRT is shut off and the CRT is preparing for the
next line scan. This prevents switching pulses from
appearing on the screen.
The CRT (cathode ray tube) is a vacuum tube that projects
a beam of electrons towards a fluorescent screen. When
this beam hits the screen it forms a luminous dot. Signals
generated by the CRT driver circuits control the position
and intensity of this dot.
on the screen. Eighty column positions are defined within
OPERATION
The terminal operates in two different modes: on-line and
off-line. On-line the terminal functions as two different
I/O devices — a keyboard input device and a display
output device. Communication between these two devices
must be provided by the computer's software. This means
that all displayable information entered at the keyboard
must be "echoed" to the display by the executing program.
Off-line the terminal functions as-one device. Figure 5.4
shows how the terminal functions in both of these modes.