Pase66
@
The I/O
(input/output)
circuit
board handles all
data
transfers
between the
Terminal and all external
de-
vices,
such
as a reader/punch
or a digital computer.
See
Pictorial
+-t0
(Illustration
Booklet, Page
11).
These circuits contain separate
8-bit
parallel
input
and output
ports
and a
4-bit
parallel
handshake.
It
also contains a serial
input, serial output,
and a UART
{universal
asynchronous
receiver/transmitter)
circuit
that
interfaces the 8-bit
parallel
system
bus with the
serial
I/O
ports,
A
special
character detector,
a baud
rate
generator,
a data
priority
latch, and a
reader
start-stop control are
also located on the
I/O circuit
board.
The
parallel
input
connects to the master
ASCII bus
through IC620,
an octal
tri-state buffer
that is
normally turned
off. An external
device can signal
to
the Terminal
that there is
parallel
data
available at
the
parallel
input
by setting the DAV,
input to zero. This
causes a logic 1 to
appear at the
data
input
of IC616,
the data
priority
latch. At
the start
of the TPU cycle,
the logic
1 is
clocked through the latch
and into a
priority
encoder, IC's
611C, 619A, 6198. The
priority
encoder
permits
only one data available
signal
[DAV)
to be transmitted
to the TPU
at a
given
time.
The
UART
data available
signal
(DAV,)
has the
highest
priority,
the
parallel
data
available
IDAV')
is
next,
and the keyboard
data available
signal
(DAVk)
has
the
lowest priority.
The TPU
reacts
to the
parallel
DAV
signal by
turning
on the outputs
of the tri-state
buffers,IC620.
This puts
the
parallel
input
data on the
master
bus, which
allows it to be written into
the
RAM at
the cursor location.
When
the
Terminal
is finished
writing the
parallel
input
data
into
the RAM, it
tells the
external device
that it
is finished
by transmitting
a negative
going
pulse
on the
RD,ffi
output.
If
the Terminal has
data to transmit
to an external
device, it
will first load
that data from the bus into
parallel
output latches
IC621
and IC622. It
then
transmits
a
DAV' signal to the
external device by
setting
flip-flop IC618B.
When the
external device
has
finished reading
the
data, a negative-going
pulse
is
transmitted
to
the
RTEV6
input.
I/O
(Input/Output)
The serial input and
output can be
jumper-wired
for
standard
TTL.
EIA,
or
TTY
(20
mA
current
loop) input
and output signals. The two logic
states of a
serial
input
or output are called a
"mark"
and
a "space."
A
mark is
defined as a TTL logic
1, an EIA minus
voltage,
or 20 milliamperes
flowing
in
the current
loop. A space is
defined as a TTL zero, and EIA
positive
voltage,
or zero current flowing in the 20 mA
current loop.
The
idle
state of
both
the input and
output
is
defined as a
mark.
Pictorial 4-17
flllustration
Booklet, Page 72) shows
EIA input connections. In its idle state, the EIA
input
is
a negative voltage and it keeps transistor
Q603
biased
off. Since
the collector
voltage
ofthe
transistor
is
5
volts, it
places
a logic 1 at the input
(pin
20)
of
the
UART. When
the EIA input
voltage
goes
high,
Q603
turns
on and
its
collector
voltage
goes
low.
This
places
a space
(logic
0) at the
input
of
the UART.
The
serial output
from the
UART
(pin
25) drives
IC609A,
which, in turn, drives
IC609B. The output of
IC609B
drives opto-isolator
IC604. When the output
is a mark,
the
photo
transistor in
IC604 turns
off
and
Q602,
a
constant
current source, turns
Q601
on.
The
current
flows from the
*12-volt
supply through
R631 and
Q601
to the
-12-volt
supply.
This makes the EIA
output approximately
-
10 to
*72
volts. As the output
of
IC609B
goes
low, the LED
(light
emitting diode)
in
the
opto-isolator
turns on the
photo
transistor,
which
turns
Q601
off.
This
causes
the EIA output
to
go
to
approximately
+ 12 volts.
Refer
to Pictorial
4-18
(Illustration
Booklet,
Page 12)
for
the
20 mA
current
loop connections.
When
a mark
is
applied, loop current
passes
through diode
D601
and opto-isolator
IC605, which turns
Q603
off.
A
logic
1 then appears at the serial
input to the UART.
When the loop current changes
to a space
(or
zero
current),
the
photo
transistor turns off
and
Q603
turns
on.
A logic 0 then appears
at the serial
input to the
UART. The reader start/stop
circuitry
works
in
exactly
the same manner.
It is
composed
of opto-
isolator IC606
and
transistor
Q604.
As long as there
is
a
mark
current
flowing in the loop, the collector
of
Q60a
is high.
This tells the TPU that the external
device
wishes to see another character.
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