DEC VT220 - Page 112

6.2.6.2 Parallel-to-Serial (PIS) Converter -- The piS converter
performs the actual parallel-to-serial conversion of dot matrix
data, and in the process performs dot stretching. It converts
single "on" dots into two adjacent "on" dots (and, due to
construction of the dot stretching circuit, converts the last "on"
dot of a sequence into two dots as well).
Dot stretching (Figure 6-24) occurs only on the horizontal plane,
and does not increase the number of dots in the vertical plane of
a character matrix. It also does not increase the size of the
horizontal plane of a character matrix. Dot stretching simply
compensates for monitor risetime being too slow to reach maximum
brightness for single "on" dots. Figure 6-25 shows that stretching
each "on" bit increases the amount of time allowed for the monitor
to respond, allowing brightness to achieve its maximum.
SCAN CHARACTER ASSTORED IN
NUMBER CHARACTER GENERATOR
ROM
1·
2. • '
3 .. c ••••
4· .•. .• • ••••
S.c •.••. c ....
6 .•••••••...•• c •••
7 •. c •• c ••••••
B.. ••
9 •.
10 • c
20 DOiS FOR 2 CHARACTERS
IN
80 COLUMN MODE
ACTUAL
CHARACTER DISPLAYED
AFTEr, DOT STRETCHING
-
_.c
._ .. -
._ ._ .. _c .. _.
____ • _ _ c
- .-
- _.-
.-
._ ..
Figure 6-24 Dot Matrix Display and Dot Stretching
SIMPLE LOGIC ADDS ONE PULSE TO THE
TRAILING EDGE OF ANY INCOMING PULSEc
~ J..
I I T BRIGHTNESS LOSS
~INPUTL ~
MONITOR
PULSE REsPONSE
THE MONITOR
RESET TiME is TOO SLOW TO REACH MAXIMUM
BRIGHTNESS BEFORE THE SHORT PULSE FALLS.
IL
BY STRETCHiNG ALL PULSES TO A MINIMUM
OF TWO DOT TIMES. FULL BRIGHTNESS IS ATTAINED.
Figure 6-25 Effects of Dot Stretching
6-28
INPUT
•
OUTPUT
JTL