Specifications
4.6.4 Speaker Interface
ASIC2 provides the logic for a
PC
compatible
speaker
port.
The
speaker
logic signal is
amplified by a Darlington transistor circuit which provides approximately
.1
watt
of
audio
power
to
an
external 8
ohm
speaker.
As
in
the
standard PC, the speaker circuit's output
frequency
is
based on one
of
two sources: The
V4O's
CTC Channel
2;
and the programming
of
two bits, 0 and
I,
in
I/O
port 61h.
The
system's 7.16
MHz
master clock
is
divided down
to
1.193
MHz
by a divide by
six
counter
in ASIC 1, which is used as the input time base for the V4O'sCTC channel 2. Bit
0
of
I/O
port 61h
is
used
to
enable CTC Channel
2,
so that a logic 1 on
this
bit causes CTC channel 2's
programmed frequency to appear on
the
V4O's
output pin, and
at
the
input
of
ASIC2. A logic
o causes CTC channel 2's output pin to stay at a logic
1.
In
addition, Bit 1 of
I/O
port 61h
is
one term
of
a 2-input
AND
gate in ASIC2, the other term
being the output from CTC Channel
2 Thus, setting bit 1 to a logic 1 enables the output
of
CTC Channel 2 to the speaker, and a logic 0 disables it.
If
the V 40's CTC Channel 2
is
disabled (by setting bit 0
of
port 61h to a 0), then bit 1
of
port
61h can be used to
directly
control the speaker.
4.6.5
PC.
Configuration
Switches
The configuration switches normally used in a PC system to inform the BIOS and operating
system
of
hardware setup information
is
supported by a set
of
jumpers connected to ASIC2.
The use
of
these switches is discussed in Chapter
2,
under the jumper configuration sections
of
the appropriate board devices.
4.6.6
PC
Serial
Port
An
8250 serial controller device provides a full duplex asynchronous communications channel
having compatibility with the
COMI
or COM2 serial port
on
a standard PC. As in the PC,
the device contains an internal oscillator and baud rate generator, supporting asynchronous
data transfer rates of up to 19.2K baud, based
on
a 1.8432
MHz
crystal (limited by the RS232
drivers).
As
in the PC, this serial port offers a choice of two
I/O
port address blocks (2F8-2FFh and
3F8-3FFh) and two
system interrupts (IRQ3 and IRQ4), programmable via jumpers
on
the
board.
These
selections determine whether the port is the primary
or
secondary serial device.
The serial controller's TTL level serial
I/O
signals are converted
to
and from RS232C signal
levels by unique +5-volt-only RS232C interface IC's which internally generate the +
/-9
volts
needed for the RS232C output signal levels.
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