Tektronix 2445, Instruction Manual

Page: 70 / 351

Theory of O peration-2445 Service
character address register U2960 (though they have not
been shifted into their correct positions fo r addressing
the RAM).
A t this point, the last character b it remains to be shifted
into the registers, but the operating mode must be set up
firs t to ensure correct operation upon shifting the final bit.
The eight bits of mode data are shifted into the mode
control register U2865 by the ROS2 strobe. Bit Q4
(WRITE),, along w ith the ROS2 and the R/W Dlyd signal
are applied to the RAM enabling circuitry and determine
when new character information w ill be written into the
Character RAM. With U2865 loaded w ith the mode data, a
final ROS1 strobe clocks the eighth b it of character data
from U2960 to U2860 on the negative edge, and the
positive edge o f the strobe clocks the eighth character
address b it (an unused bit) into U2960.
With control b it Q4 from U2865 LO, the outputs
of U2860 are enabled and the eight bits of character data
(CDO through CD7) are written in parallel into the Char­
acter 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).
8 HORIZONTAL DOT PO SITIO NS
W IT H IN EACH HORIZONTAL
DISPLAY AREA-DETERMINED BY
B IT S 0 - 2 OF CHARACTER ROM
SMALL CHARACTERS
ARE DISPLAYED IN
TOP HALF OF A
*
n o t e
:
r e f e r e n c e l e v e l i s
WITH D D 3 , D D 4 , DD5 , AND
DD6 ALL LO
AREAS-DETERMI NED BY
CHARACTER COUNTER
3831-02
Figure 3-7. Developing the readout display.