Radio Shack Quick Printer II Reference Handbook

generated back in the address decoding discus-
sion? We said
VID* will select the Video RAMs.
Notice that pin 1 of the 3 multiplexers is tied to
VID*. When the CPU wants control over the
Video RAM, the address decoder recognizes the
Video RAM address and causes VID* to go low.
When VID*
is low, the multiplexer switches the
"0" inputs over to the multiplexer outputs. The
counter chain addresses are switched out of the
circuit, and the CPU has control over Video
RAM. When VI D* goes back high, the CPU is
switched out and the counter chain takes over.
Most
of the time, the counter chain is in control
of Video RAM. The CPU only takes charge
when it needs to. You can see on the display
screen when the CPU robs the counter chain of
Video RAM control. Ever notice black streaks
allover the screen while graphics are being
drawn? These streaks are the result of the count-
er chain losing control over Video RAM.
Aside
from chain and CPU address, there are in-
puts to the multiplexer we have not yet men-
tioned. The first of these inputs is the resistor at
pins 13 and 6 of Z49. These two inputs, which
are not needed in the counter chain's control
over Video RAM, are pulled up to 5 volts by
R49. Outputs pins 12 and 7 correspond to the
inputs at pins 13 and 6. When the chain has con-
trol over Video RAM, pins 12 and 7 output a
steady state high. Pin 12 goes to the RIW (Read!
Write) control of all the RAMs. Since the count-
er chain never stores data in RAM at the address
it specifies, pin 12 should be high when the
chain is in control. Pin 7 of Z49 goes to the
Video RAM data buffer. When the chain is in
control, the RAM data bus should be disabled. A
high
on VRD* (Video Read) guarantees this bus
will be off.
We also find WR* and RD* tied to pins 14 and
5 of Z49. When the CPU takes charge of the
Video RAMs, multiplexer output at pin 12 be-
comes VWR* (Video Write). The CPU can store
data into the video RAMs by causing VWR* to
go low. If the CPU wants to read data from
video RAMs, RD* can pass through Z49 and
activate VRD*. A low here will open data
buffers Z60 and Z44. Addressed Video RAM
data is then placed on the data bus. The CPU
can process this data like any other data.
Alphanumeric Format
The CRT (Cathode Ray Tube) in the display
will
be scanned twice per second. The electronic
beam in the CRT travels from top to bottom of
the screen and left to right. Each screen or frame
consists of 264 scan lines. 192 scan Ii nes are
used
in the "picture". 72 lines are used during
vertical interval and as upper and lower bound-
aries. Nothing is ever "written" or visible within
these 72 lines. There are 1024 character loca-
tions per screen (or 512, depending on status of
MODESE L). Each character-line consists of 64
characters (or 32, depending on the status of
MODESEL). There are 16 character lines. Each
character-line consists of 12 scan lines. An
alphanumeric character uses seven scan lines
while there are five blank scan lines between
character lines. We'll worry about graphics for-
matting later.
Part of Z65 and Z50 specify the column address.
Z32 specifies the row (or character-line). Z12
specifies the scan line in any character line. The
outputs from Z 12 are labeled L1, L2, L4 and
L8. These four lines are not used in Video RAM
addressing
because we already stated a row and
column address will specify anyone of the
1024 rectangles in our rectangle array. Z12's
outputs are used in the Video Processing. L1,
L2
and L4 will enable the character generator
to output correct data for any character since it
knows where the CRT's electron beam is scan-
ning. L8
is used by the Video Processor to blank
(turn off) the five lines between character lines.
Notice NO R gate Z30, pins 8 and 9. These pins
enable Z30's output to produce a signal called
BLAN K*. BLAN K*
is used by the Video Proces-
sor to give the 72 scan line blanking for the
upper and lower boundaries. It also defines the
blank boundaries on the left and right of the
screen.
33

Radio Shack Quick Printer II Specifications

General

Brand	Radio Shack
Model	Quick Printer II
Category	Desktop
Language	English