Schematic Sheet #4 - Memory Address Generation - Lear Siegler ADM-3 Maintenance Manual

pulsed signal reaches the CURSOR register
through the 7402 (D3) located in zone B4. The
other means by which the cursor can be incre-
mented is during a READ BACK operation
which is normally used for test only. This is
accomplished by D3-10 (7402, zone B4). The
signal on D3-9 is low only during this read oper-
ation and is called READ. XLOAD on D3-8 goes
low when the code under the cursor has been
loaded into the transmit section of the UART
and the cursor can be incremented in prepara-
tion for the transmission of the next character.
The down clock input of the CURSOR register
(CI2-4, zone B3) is simply the decode of the
BACKSPACE control code (USASCII BS,
octal 010).
The gate E6-12 (7410, zone B3) combines the
three ways that the cursor register gets cleared.
The first way is the decoding of a CARRIAGE
RETURN code in the input data buffer. The
function of this code is to return to the Hhome"
position (lower left corner of the screen). This
decode is or'ed in at E6-13. The next way to clear
the. CURSOR register is the signal called
UNDERFLOW entering this circuit at E6-1 and
originating in zone B1. This signal is an indica-
tion that a BACKSPACE operation was at-
tempted from the
~~home"
position. The ADM-3
does not do a wraparound operation when this
is attempted and remains in the
I:~home"
posi-
tion. Therefore, when the UNDERFLOW flop
(7474, B4-9, zone B1) sets, the CURSOR regis-
ter is cleared to maintain the previous location
of the cursor. The last way to clear the CURSOR
register occurs at E6-2. It is cleared when the
START flop sets (zone D4). This accomplishes
two things: (1) The cursor is initialized when a
clear screen operation is initiated and (2) power
on clear also
~~homes"
the cursor.
The gate C4-3 (7400, zone A2) decodes the fact
that the cursor has been incremented beyond
the end of the line and is labeled WC == 80.
This information is latched into flop B4-6 (7474,
zone AI) which is called OVERFLOW. W.hen
OVERFLOW rises, the preset inputs to the
CURS()R register go low thereby loading
information from the parallel inputs into the
register. The information that is loaded depends
upon the position of the AUTO NL switch
located in zone B1. If this switch is closed
(AUTO NL enabled), then the CURSOR regis-
ter will preset to zero and the screen will roll.
Therefore, with the switch in this position,
typing into the last location on a line will
au;tomatically roll the screen and home the cur-
sor. If the AUTO NL switch is open (AUTO NL
6-8
disabled), the register will preset a binary 79
(last position) and the screen will not roll.
Therefore, with the switch in this position,
further typing will overwrite the last position
of the line and the cursor will not move.
6.3.3 BEEPER C'ircuit
The last function on this sheet is the BEEPER
feature. The actual sound is created by putting
one of the system cO'unts (LC2, Fl-12, 74H01,
zone A2) through a speaker (zone A2). The
duration of the
~~beep"
is governed by the one-
shot (D1, 74123, zone A2). Gate D2-6 (7427, zone
A3) triggers the one-shot if a character is being
written into the 71st position of the line which
is used as a line end warning. Gate J3-3 (7400,
zone A3) triggers the one-shot if the USASCII
BEL code has been received into the data input
buffer. Gate D3-1 (7402, zone A3) controls the
clear input to the one-shot. The signal READ
clears the 74123 through input D3-3 and is
present because the initiation of the READ
BACK feature is triggered with the receipt of a
BEL code which normally initiates the beeper.
Therefore, to avoid the noise, the one-shot is
disabled when the READ BACK is started.
The OVERFLOW signal also clears the beeper
one-shot because when operating at a rate
higher than 2400 baud and writing an entire
screen of data, the beeper one-shot never clears
unless this signal is present. This is because
when a character is written into the 71st posi-
tion, the one-shot triggers because of the signal
LINE END. Before the 74123 can time out, the
71st position of the next line is reached and the
one-shot is retriggered. Therefore, a continuous
beep would be heard for the entire duration of
the transmission from the computer. At lower
baud rates, the duration of the sound is normal.
6.4 SCHEMATIC SHEET #4 -
MEMORY ADDRESS GENERATION
This sheet provides all of the memory address
manipulation required by the ADM-3. The two
74157's (F11
&
F12) located in zones A3 and B3
multiplex the addresses corresponding to the
position in the line. The ~~A" inputs of the 2:1
multiplexers come from the CURSOR registers.
This set of inputs is primarily utilized when
there is a character to be loaded into the re-
fresh memory from the data input buffers. The
CURSOR register outputs are also gated to the
memories when the refresh memory is being