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DIDS-402-2AMI3
4-24.4 Bit Counter
The bit counter (figure 4-56) is a divide-by-eight counter which produces
three outputs: CTS, CTS+1, and CTS+4 . CTS+5. The first
1fi
pulse developed
by the phase counter is inverted and applied to the clock inputs of A20. 12 and
A20.9.
(The third bit counter flip-flop., A19.9, is toggled each time A20. 12
goes low.)
DIDS 68-654
IT COUNTER(+8
12
9
9
AZO
1.20
AI9
8
8
UNBLANKING
~--"""CTS
~12~-------------"CTS+1
~~------------~CTS+4ANDCTS+5
Figure 4-56.
Bit Counter, Logic Diagram
Since the bit counter cannot be preset to any particular count, the first
usable count occurs after from one to eight f/J2 pulses have been .developed by the
phase counter,
Once the phase and bit counters are synchronized, the bit-counter
flip-flops cycle through the eight repetitive counts shown in figure 4-57.
Although
the counter cycles through eight distinct states, only three outputs are decoded.
The se outputs are used for the following pu;rpose s:
a.
CTS - This output is defined as the first bit 'of a character and is in
coincidence with the cursor bit in memory.
Memory characters,
which are eight bits in length, also contain seven data bits which are
in coincidence with the remaining seven counter states.
The CTS
decoded output is used to perform logical operations in coincidence
with the cursor bit and synchronize the remaining counters on the
timing board.
b.
CTS+I - This output occurs in coincidence with the LSB of a 7-bit
data character.
The CTS+l output is ANDed on the timing board
to produce 3(CTS+l), 3(CTS+1}, 1 (CTS+l), and 4(CTS+l) outputs. '
These outputs are principally employed by the buffer register logic
to shift data through the buffer registers.
c.
CTS+4. CTS+5 - This output occurs in the middle of each data
character and is used to accomplish only one function.
When an
Idle character is shifted into the CE register, the decode develops
an output to blank the video amplifier for two additional bit times.
This provides some measure of protection for the often used symbol
at the edge of the monoscope which is used to develop place marker
(short-underline) symbols on the screen.
4-139
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