Tektronix 2445A - Page 59

by U2965A to produce a positive transition that shifts the
data bit present at U2960 pin 9 (OsHl into U2860. After
15
ROS1 strobes have occurred, seven bits of character
data
are latched into U2860, and the eighth character bit
and seven of the character address bits are latched into
character address register U2960 (though they
have not
been shifted into their correct positions for addressing the
RAM).
At this point, the last character bit remains to be shifted
into
the registers, but the operating mode must be set up
first to ensure correct operation upon shifting
in the final
bit. The eight bits of mode data are shifted into the mode
control register U2865
by the ROS2 strobe. Bit 0
4
( WRiTE ), along with the ROS2 and the R/ W DL YD sig-
8 HORIZONTAL DOT POSITIONS
WITHIN EACH HORIZONTAL
DISPLAY AREA-DETERMINED BY
BITS 0-2 OF CHARACTER ROM
Theory of Operation-2445A/2455A Service
nal are applied to the RAM enabling circuitry and deter-
mine when new character information will be written into
the Character
RAM . With U2865 loaded with the mode
data, a final
ROS1 strob strobe clocks the eighth bit of charac-
ter data from U2960 to U2860
on the negative edge, and
the positive edge of the strobe clocks the eighth character
address bit
(unused bit) into U2960.
With control bit
0
4
from U2865 LO, the outputs of
U2860 are enabled
and the eight bits of character data
(CD0 through CD7) are written in parallel into the Charac-
ter RAM at the location selected by the seven-bit address
from U2960. Register U2960 is enabled only
when the
Readout
is not displaying characters (the REST signal at
pin 15 of U2960 is HI).
SMALL CHARACTERS
ARE DISPLAYED IN
TOP HALF OF A
GIVEN DISPLAY AREA
•
•
•
• •
\
SUBFRAME•
8 CHARACTERS
I
LARGE CHARACTERS
REQUIRE 2 ADJACENT
DISPLAY AREAS
,.-1',
16 VERTICAL
DOT POSITIONS
ABOVE LINE
REFERENCE-
DETERMINED BY
BITS 3-6 OF
CHARACTER ROM
*cAS LINE
REFERENCE
•
•
• •
•
•
•
•
-
•
\
\
\
\
1 V
4..IQ.I....LJU:::b..l:~~li_Li...L.l---*cAS HI
\
\
CLINE
REFERENCE!
ONLY SMALL CHARACTERS - ----'':::......,.----<-i
MAY BE DI SPLAYED ON t::::t::=f=e!!::!=t=:l=====:t==========
BOTTOM LINE --'-..J-J......L.L....J.....L...JL- _______________ *cAS LO
*NOTE REFERENCE LEVEL IS
WITH 003, 004, DDS, AND
006 ALL LO
32 HORIZONTAL DISPLAY
AREAS-DETERMINED BY
CHARACTER COUNTER
Figure 3-6. Developing the readout display.
CLINE
REFERENCE!
3831-02
3-27