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DIDS-402 -2AM13
4-23.3.9 88-Count Circuit
This circuit consists of the ripple counter and as sociated circuit elements
shown in figure 4-48.
The 88-count circuit is used to accomplish the following
edit or cursor control functions:
Step-down
Step-up
Insert line
Delete line
In the step-down and step-up cursor control functions, the 88-count
ripple counter is initialized and reset by enable-reset flip-flop A5. 9.
In the
insert and delete-line edit functions, the 88-count ripple counter is initialized by
the cros s-connected enable-reset flip-flop formed by A29. 3 and A29. 11.
In
either case, the objective of the circuit is to count 88 clock inputs and then pro-
duce an output (88 Count Complete).
The purpose of the 88 counter in each
of the previously listed functions is de scribed below.
a.
Step-Down - In the step-down function, the purpose of the 88-count
circuit is to count 88 character times beginning at the cursor
position.
This, in turn, informs the step-down circuitry when the
cursor should be reinserted to complete the step-down function.
When the step-down flip-flop is set as a re sult of depres sing (
~'
)
or receiving a VT control code from the CPU, the STEP DOWN
+
VT
input to A6. 6 goes low to produce a high at the input to A5. 9.
When
the cursor is located in the CE register, A5.9 goes high to release
the ripple counter and enable one input to A6. 12.
During each
successive CTS puls'e (except vertical retrace), the ripple counter
advance s in count by one character.
When 88 characters have been counted (signifying the character slot
of the line immediately below the cursor), 88-count decode A36. 8 is
enabled.
This produces an output from AIS. 6 which is inverted and
used to simultaneously furnish an 88-Count Complete level to the
step-down circuitry and set the function-complete flip-flop.
The
function-complete level then terminates the function by setting AS. 8
high and resetting the step-down flip-flop.
b.
Step- Up - The operation of 88-count during the step-up function is
nearly the same as described above for the step-down function.
The
difference is that the 88 count is constantly recycled to count a total
of 1144 CTS pulses (13-88 character lines).
Each time a count of 88
is reached, the circuit is re set but because of a constant low at the
input to A6. 6, the counter is again enabled as soon as the cursor is
once again located.
When the final (13th) count of 88 is detected by
the step-up circuitry, the function is allowed to disable and return
to a reset condition.
Counting 13 lines of 88 characters, in con-
junction with the count of 12
I::J.
pulses by the 12-count circuitry,
enables the cursor to be inserted in the'preceding line immediately'
above where the cursor was previously located.
4-124
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