Raytheon DIDS-400 Series Technical Manual page 83

DIDS-402-2AM13
The CRT is also a conventional electron gun with the anode operated at
a high positive potential (with respect to the cathode element). The video level
applied to the cathode alternately increases and decreases CRT conduction, and
the changing phosphor excitell'lent produces a visual character on the screen.
The CRT deflection voltages determine where on the screen the character will
appear. Deflection voltages are developed by display terminal till'ling and CRT
deflection circuits which are described in paragraphs 4-18 and 4-19.
4-15. 1. 6 Summary
The preceding text has briefly explained the operation of the keyboard mono-
scope circuits and the manner in which visual characters are developed from the
depression of a character key. Once a character is displayed. however,
it
begins to fade from. view as soon as the monos cope scan is deflected to another
character. The time required for a character to disappear is determined by the
persistence of the CRT screen phosphor material which is type P31 (green) short-
persistence. To prevent this fading, the displayed character is refreshed by
circulating the digital code of the character in the refre sh memory loop.
4-15.2 Refresh-Memory Loop
The purpose of the refresh-memory loop is to provide a constant refresh
of visual characters displayed on the CRT screen. The key to understanding the
refresh memory loop is to remember the following two points: (1) the character
entry register is parallel-connected to the character readout register, and (2) dig-
ital character codes held in the character readout register are used to position
the monoscope scan, and thus develop a corresponding visual character on the
CRT screen.
Digital character code s, whether entered from the keyboard or received
from the CPU, enter the refresh-memory loop at the character entry register.
If the character code is formed locally by the depression of a key on the keyboard.
the code is entered by jamming it into the entry register. If the character code
is received from the CPU, it is serially shifted into the entry register from
buffer register D3. At the end of each character time, the code is parallel-
transferred into the character readout register and a corresponding character is
displayed on the CRT screen. Character codes are constantly returned to the
character entry register for redistribution to the rnonoscope deflection circuitry.
This in turn maintains visual character brightness on the CRT screen.
The refresh-memory loop (shown in figure 4-13) consists of the character
entry register, delay-line electronics A9, and delay line A15. After character
codes initially enter the refresh-memory loop, the code is returned every
16.026 ms, or at a rate of 63 times/ second. This till'le corresponds to the time
required for the CRT scan to move from a character position on the screen,
through a complete scan cycle, and back to the original character position. For
example, assume that the cursor is located at the line one, character one
position on the screen when a character key is depre ssed.· The cursor is constantly
circulating in the loop and eventually appears in the cursor-located flip-flop of
the character entry register. When the cursor is located, the write pulse strobes
the character code into the entry register and a visual character is displayed
on the CRT screen at the line one, character one position.
4-32