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SECTION FOUR
THEORY OF OPERATION
The ADM-2 theory of operation can be divided
into four main logical subsections:
• Logic Board
• Monitor
&
Display
• Keyboard
• Power Supply
All four of these subsections are detailed in the
following description.
4.1 MAIN LOGIC BOARD
The main logic board in the ADM-2 is the center of
activity for all terminal operations. The board is
a microcontrolled device which is responsible for:
• Clock and timing signals
• Generating the displayable dot patterns
• Generating the cursor
• Keeping track of cursor position
• Coordinating all data transfers
• TX and RX data from the computer, modem,
or printer
• Storing data for display and TX
• RX
control information from keyboard and
modem
• Interpreting all control commands and con-
trolling terminal activities accordingly
• Data status control (unprotected/protected)
for display and transmission.
The basic functional organization of the ADM-2
main logic board is illustrated in Figure 4-1.
Figure 4-2 is a more detailed block diagram of the
main logic board. Each block in this diagram is dis-
cussed in the following subsections.
(WHILE READING THE FOLLOWING DATA IN
THIS SECTION (4.1), REFER TO THE SCHE-
MATIC DIAGRAMS IN APPENDIX A.)
4-1
4.1.1 Clock (Schematic P. 2)
The ADM-2 uses a crystal controlled oscillator as
its master clock.
The oscillator frequency is
12.4925 MHz for 60 Hz operation; 10.4104 MHz
for 50 cycle operation.
The oscillator output
(erR)
is applied to counters and gates throughout
the logic board to provide basic timing signals.
ADM-2
basic timing signals are shown in Figure 4-3.
4.1.2 Counters (Display Generation)
(Schematic P. 2)
Display generation in the ADM-2 is provided by a
series of counters. Dot Position, Character Posi-
tion, Character Height, and Character Line counters
are used to produce, on the CRT, a 5" x 7" char-
acter and a 7" x 9" cursor. Counter operation is
started by a clocking pulse from the oscillator.
The dot position counter is a divide by
7
type,
4-bit counter.
Its outputs (DPCx) are input to
the video line register, and the indicator logic
(shift register) for transmission to the video logic
and the monitor drive logic, respectively. A DPC
output signal also clocks the character position
counter.
The character position counter is a divide by 104
type, 8-bit counter.
It
is used to keep track of
the cursor position in the video timing logic.
MAC is used to monitor the cursor position on
the CRT. Character position counter output signals
(CPCx) are sent to the data memory address logic,
video line register logic, video logic, MACC control
logic, indicator timing logic, and monitor drive
logic. Output signals CPC4, 16, and 80 clock the
character height counter.
The character height counter is a divide by 11 type,
4-bit counter.
It
is used to count the number of
rasters in a line. The output signals (CHCx) are
input to many functions of the video logic, includ-
ing the character generator multiplexer, indicator
video logic, and cursor enable F/F. The (CHC9)
output signal clocks the character line counter.
The character line counter is a divide by 26 type,
4-bit counter.
It
is used mainly to keep track of
the row position of the cursor. The character line
counter output signals (CLCx) are sent as inputs
to the video driver, indicator logic, and the cursor
tracking logic of the MACR.
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