Specifications
Devices with
an
access
time
of
200 nanoseconds
or
less
can
be
used
in
either
U15
or
U26
without wait states.
When
using wait states, add 140 nanoseconds
of
access time for
each
wait
state programmed.
The
default
on
power-up
is
two wait states.
4.6 ONBOARD
1/0
INTERFACES
The
Little
Board/PC
has seven on-board
I/O
interfaces, which can
be
architecturally divided
into two sections: "internal" and "external" (see
FtgIlfe 4-1).
The
internal section includes four
I/O
interfaces which
are
accessed
by
means
of
the system's
internal address, data, and control buses. They are:
• V 40 serial port
• Parallel printer port
• Keyboard port
• Speaker port
The
external section includes
three
I/O
interfaces
that
are
connected
to
the
board's
PC
Expansion Bus
~
though they were
PC
bus plug-in cards.
They
are
designed
to
mimic entire
individual
peripheral
expansion
cards
normally
present
on
a
PC
bus.
Due
to
advances
in
component technology, each functional element is essentially contained within a single
VISI
integrated circuit.
The
three external
I/O
interfaces are:
• SCSI controller (and
ID
port)
• Floppy disk controller
•
PC
compatible serial controller
The
internal and external peripheral controllers are described
in
the following paragraphs.
4.6.1 V40 Serial Port
As
mentioned
previously,
the
Little
Board/PC
implements
the
V40's
internal
serial
port
controller rather than the 4th
DMA
channel option.
In
this
mode, the V 40's "DACKJ"
and
"DRQ3" pins become serial
data
out
and serial
data
in, respectively.
Because
of
limitations
in
the
V40's
interrupt
logic,
the
V40
serial
port
cannot
be
interrupt
driven unless
the
interrupt
(IRQ1)
normally
used
for keyboard
interrupt
is
not
required.
IRQ1
is
internally
multiplexed within
the
V40,
and
the
ROM-BIOS
initialization
routines
select the external IRQ1 input (driven
by
the keyboard logic)
~
the source
of
IRQ1
rather
than the V 40 internal serial controller logic.
Baud
rate
generation for the V40 serial port is entirely internal to the
V4O,
and is
based
on
the
V40's
internal
counter/timer
channel
CTC!.
Standard
baud
rates
up
to
19.2K
bits/second
are
supported (limited by the RS232C drivers).
The V40 serial port's ITL-Ievel serial
I/O
signals
are
converted to and from RS232C signal
levels
by
unique + 5-volt-only RS232C interface
Ie's
which internally generate
the
+
/-9
volts
needed
to
support the RS232C output signal levels.
4-10