Beehive International B102 Service Manual page 10

Each 74H04 has a 1 kohm feedback resistor
around it. A 10.8864 iVIHz crystal is connected
from the input of the first 74H04 to the output
of the second. The output of the oscillator is
buffered, inverted and fed to the Dot Position
Counter.
Dot Position Counter - This divide-by-seven
counter defines each of the seven dots required
to compose one character. The outputs of this
four-stage counter are labeled DPCl, DPC2. DPC4
and DPC8. The counter actually presets to a
count of 10, counts up through the overflow
point at 15 to a count of zero, and presets back
to a count of 10. The Dot Position Counter
output DPC8 drives the Character Position
Counter.
Character t'\»sctcon Counter - The Character
Position Counter is composed of two binary
type counters that define 96 character posi
tions, each being seven dots wida The output
of the Character Position Counter drives the
Character Height Counter.
Character Height Counter - The Character
Height Counter is a standard counter that de
fines nine scans of 96 characters each, with each
character being seven dots wide. The output
of the Character Height Counter drives the
Character Line Counter.
Character Line Counter - The Character Line
Counter is a binary counter that starts at a
count of zero and counts to a maximum of 29
for a total of 30 character lines. The final out
put of this counter runs at the vertical refresh
rate.
Horizontal and Vertical Drive - The Horizontal
Drive is started when the Character Position Coun
ter leaves the video area of the scan and is active
for the following 40 character positions. The
high active ouiput of this flip-flop is sent to the
monitor on pin 9 of connector J1.
The Vertical Or've is generated during the time
the Character Line Counter is decoding 24.
Cursor Location Counter identifies the location
of the cursor. This is a count made from the Cur
sor Line Counter (called CURL) and the Cursor
Position Counter (called CURP). These two coun
ters, in conjunction with the ROLL counter, are
ised to address the memory to determine the entry
joint of the next character. The cursor location
4-7
counters are compared with the next character.
The cursor location counters are compared with
the Character Position Counter and the Character
Line Counter to generate the signal called CNTR
CURSOR. This signal is used to generate the cursor
displayed on the CRT. Also associated with the
cursor location counters is the appropriate circuitry
to move the cursor up, down, right, left; home, etc
A LINE FEED code causes the Cursor Line Counter
to increment by one. A CARRIAGE RETURN code
clears the Cursor Position Counter.
With the terminal operating in Format Mode, when
the cursor is incremented off the bottom line, the
cursor automatically wraps around to the top of
the display, i.e., the Cursor Line Counter is reset
to zero. However, if the terminal is not in Format
Mode, the display scrolls whenever the cursor incre
ments from a count of 23.
A scroll is initiated by any of three functions
if activated when the cursor is on the last line of
the display and the terminal is not in Format Mode:
a. LINE FEED or CTRL J
b. CURSOR DOWN or ESC B
c. If the cursor is on the last position of the
last line
1. Cursor right
2. Any displayable character
3. Space
Memory is actually a 2048 byte memory. Each
byte consists of 9 bits: 7 for data, one for protect,
and one for blink. Of these 2048 bytes, 1920 are
displayabl& The program does not have the capa
bility of displaying or writing into the remaining
128. In order to write data into the page memory
from the receiver, the memory address is muxed
over to the cursor location registers and the sig
nal WRITE is generated. The UART is then reset
and is capable of receiving the next character. The
page memory output is sent to the character gene
rator input buffer at the proper time to generate
the displayable characters. The program has the
capability of shutting down the screen refresh
for any given operation to increase the program
operating time.
Character Generator is a read-only memory (ROM)
that is addressed by the character (in ASCII). The
scan configuration and the chracter indicate the
pattern desired on that scan. Five-bit dot pat
terns are generated which form a portion of a