Specifications
Although
the
function
of
DACKO refresh signal is
the
same as
that
in
a PC, its timing is
somewhat different
in
one respect: the V40
CPU
occasionally does a rapid "burst"
of
refresh
cycles, as close as 200
nS
apart. with no other intervening
CPU
cycles. This is
in
contrast to
the
nearly constant
refresh
rate
--
one
every 15 microseconds -- found
in
most PC's.
This
should pose
no
problem with
PC
expansion add-in cards, unless they use the expected refresh
frequency for fUllctionai timing,
or
cannot accept the rapid refresh bursts.
In
the
mM
·XT" computer,
an
input signal,
·CARD
SLCTD," is provided
on
pin B8
of
the
8th
expansion slot. This signal is
intended
to support
an
IBM
bus
expansion subsystem. This
signal is not provided
by
the
Little
Board/PC's
PC
Expansion Bus,
but
should
not
cause
problems, as it is seldom used.
All the other signals
on
the bus conform to what
is
found on a standard PC's expansion bus,
but with slightly less drive current. For details, consult the technical references listed
in
the
beginning
of
this chapter.
The + 12VDC, -12VDC, and -5VDC voltages
of
the
PC
bus are not generated by the Little
Board/PC
for the
PC
Expansion Bus (J9) connector.
If
those voltages are required, they can
be connected via the board's Utility Connector (J4).
4-15