Table of Contents
Advertisement
DIDS402-2AM13
8TH CHARACTER
10 COUNT
11 COUNT
------------________________
~r_l~
__ __
(HORIZONTAL DRIVE)
L J
DIDS 68-510
Figure 4-21.
Character Counter Outputs
4-18.7 Line Counter
The purpose of the line counter is to count the number of displayable lines
on the CRT screen.
The line counter is a divide- by-14 counter (13 displayable
and 1 retrace) which produces two decoded outputs.
These outputs are described
as follows:
a.
Vertical Drive - This decode is similar to the horizontal drive decode
(discus sed previously).
During the CRT scan period, 13 horizontal
lines are displayed before the CRT scan reache s the bottom-right
corner of the screen.
This downward movement is produced by the
vertical drive signal (see paragraph 4-19.1.2).
During this entire
period, the line counter is counting from line 1 to line 13.
When the
line counter toggles to line 14, the vertical drive decode is enabled to
produce a vertical retrace signal, 80 characters wide.
The absence
of drive enable s the CRT scan to return to the top of the screen to
begin another vertical scan.
b.
Line 1 - This output is produced when the line counter is toggled by
the next 80th character clock pulse.
From the previous discussion,
eight more characters (horizontal retrace) are counted before display
can once again occur.
The line· 1 decode is enabled during these
eight characters and remains enabled until the CTS of the last display-
able character of line 1.
During this period, an output from the 7th
character decode is combined with the line
1
decode output to produce
a F(.A) pulse.
This pulse coincides with character count 87 and has a
CR T reference point of CTS of the first displayable character of line 1.
4-18.8 1200-Baud Circuitry
The purpose of the l200-baud circuitry is to count down the CTS bit-counter
output to develop l200-baud pulses for clocking data to and from the interface
circuitry.
Unlike all other timing circuitry, the 1200-baud counters can be
switched on or off by the display terminal logic circuitry on Communications
Control Board A12.
A level, present when the Display Terminal is about to
receive or transmit data, enables CTS pulses to enter the first of two divide-by-
eight binary counters.
4-47
Advertisement
Chapters
Table of Contents
