Table of Contents
Advertisement
DIDS-402-2AM13
4-15. 1.2 Character Entry Register
The character entry register (see figure 4-13) is an 8-bit flip-flop register
contained on display logic board A13.
The character entry register perforITls
a two-fold function in terITlS of character generation: keyboard interface, and
refresh-ITleITlory access.
When a character key is depressed, the 7-bit digital code produced by the
ITlatrix is available for character entry.
However, characters cannot be entered
froITl the keyboard until a special bit, called the cursor, is located in the refresh-
ITleITlory loop.
The refresh-ITleITlory loop constantly circulates this bit, and it
appears in the character entry register only once/fraITle.
The cursor bit is asso-
ciated with the visible cursor (J) on the screen and characters can be entered
only at the cursor position.
Thus, characters can be strobed into the character
entry register only every 16.026 ITlS (the frame tiITle).
Since keys cannot be
depressed this fast, it is practical to assume that the character code enters the
register as soon as the character key is depressed.
The character code is
entered into logic by first locating the cursor and cOITlbining the presence of the
cursor with a strobe pulse generated froITl the presence of an INTERLK level.
Once a character enters the character entry register, it is imITlediately
parallel-shifted into the character readout register and serially shifted, bit by bit,
into the refresh-meITlory loop.
4-15.1.3 Character Readout Register
The character readout register (see figure 4-13), is a 7-bit register
located on display logic board A13.
The character readout register is connected
in parallel with the character entry register.
Therefore, when a character code
is present in the entry register, it is siITlultaneously present in the readout
register.
The character code is held in the readout register for one cOITlplete
character tiITle before being replaced with the next code entered from the keyboard
(or from the refresh-ITleITlory loop).
The output froITl the character readout register cOITlprises the original
keyboard output code divided into two segments.
One segITlent consists of the
four ITlost significant bits (MSB
IS)
of the character; the second segment, the
three least significant bits (LSB
IS).
All seven bits are coupled to the monoscope
deflection aITlplifier and applied to X and Y digital-to-analog (D/ A) converters.
4-15.1.4 Monoscope Deflection AITlplifier A3
The purpose of the monoscope deflection aITlplifier is to convert digital
character codes to analog voltages for electrostatically deflel
~ing
the ITlonoscope
scan.
The ITlonoscope scan when positioned to a specific character iITlage on the
monoscope target produces a video output by secondary emission that is peculiar
to the particular character being scanned (refer to figure 4-14).
4-30
Advertisement
Chapters
Table of Contents
