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138
COMPUTEl
February,
1981
Issue 9
Expanding
KIM-Style
6502
Single
Board
Hal
Chamberlin
Computers
Editor's Note:
Hal
ended
his
first
installment with this·
..
"The
real
question
at
this
point
then
is:
How
many
expansion
boards
can
the
unbuffered
microprocessor
bus
dn've
before
becoming
overloaded?
The
6502
microprocessor
is
rated
to
dn've
slightly
more
than
1
standard
TTL
load
(equivalent
to
five
low
power
shot/Icy
loads)
on
its
address
and
doJa
busses
while
most
of
the
RAM's
and
ROM's
tied
to
the
doJa
bus
can
dn've
two
standard
TTL
loads,
The
6520, 6522, and 6530 I/O
chips
have
the
same
dn've
capability
as
the
microprocesjor,
Thus
in
general
the
answer
is
at
least
four
boards
providd
that
the
expansion
boards
themselves
buffer
the
bus
such
that
only
one
low
power
shot/Icy
load
(.36MA
in
the
zero
state)
is
presented
to
the
bus
by
the
board,
Many
boards
on
the
marlcet
and
par-
ticularly
those
designed
for
an
unbuffered
bus
do
this,
Actually,
any
well
designed
board
would
be
expected
to
buffer
the
bus
in
order
to
provifk
clean
signals
for
the
remaintkr
of
the
board
logic,
The
reason
tho.t
only
four
boards
can
be
driven
instead
of
jive
is
that
some
of
the
address
lines
are
loatkd
by
a
low
power
Shot/Icy
duotkr
Ie
on
the
computer
board
itself
Part 2 of 3
lI"he
Great
Experiment
Of
course
loading
the
microproce~sor
with a full five
loads
puts
the
system
right
at
the limit
of
rated
drive
current.
One
of
the
problems
with
testing digital cir-
cuitry
is
that
there
is
no
obvious indication
of
marginal
operation
that
may
later
develop into a full
fledged failure as
components
age.
In
order
to deter-
mine
the
actual drive
limit,
the
author
took a fully
stuffed
AIM-65
(4K
on-board
RAM,
assembler
ROM
and
BASIC
ROM's)
and
started
adding
Micro
Technology
K-1016
16K
memory
boards,
the
idea
being
to
add
boards
until
failure
due
to
bus
overload
occurred.
These
boards
use low
power
Shot-
tky buffers
onboard
so
each
one
would
be
expected to
add
a
.36MA
load
to the
bus.
Since the
AIM's
40K
of
free addresses would
only
accomodate
two
of
these
boards,
the
most
significant
address
bit was
cut
away
from
the
bus at
each socket position
and
instead
connected
to parallel
output
bits
on
the
AIM's
application
connector.
The
boards
were
then
jumpered
to
respond
to addresses
between
2000
and
5FFFF
(hex). By
programming
only
one
output
bit
to
be
low
at
a
time,
a
rudimen-
tary
bank
switching
setup
was
implemented.
When
the system was reset, all
output
bits
automatically
go
high thus
disabling
all
of
the
boards
and
preventing
interference with the
AIM
monitor
(since A15 was
ignored,
an
enabled
board
would
also
respond
to
AOOO-DFFFF). A
proper
bank
switch
setup
would
have
required
a
two-input
OR
gate
(negative
AND)
ito
be tied to
each
of
the A15 pins.
In
any
case, it
was
adequate
to
run
a
memory
test
program.
The
first trial was to install 4
of
the
16K
boards
which worked fine as expected.
Next,
another
card
file was placed below the first
and
jumper
wires
added
between the two
motherboards.
This
gave a
total
of
9
bus
slots which were filled with
16K
memory
boards.
Again
the
memory
test
program
(which wrote all 144K
of
memory
with
random
data
before
reading
any
of
it back)
indicated
no
problem
and
the
AIM
monitor
and
BASIC
continued
to work
flawlessly. A check with
an
oscilloscope revealed
minimal
signal
degradation.
Finally, a third
card
file was
added
and
bus
jumpers
installed to give a total
of
14 slots.
Three
additional
16K
memory
boards
were
scrounged
(I
had
no
idea
that
more
than
9
or
10
boards
could
be
driven) to give a total
of
192K
of
RAM.
Again
there
were
no
obvious
problems
and
the
bus
was
being
loaded
to three times
rated
capacity! Figure 3 shows
what
the
stack
of
card
files looked like which
is
obviously impractical unless
one
cuts
a hole in the
tabletop to let the two
extra
card
files
hang
below
(I
simply sat
on
a
drafting
stool to use the system).
The
rear
view in figure 4 shows the
interconnected
motherboards
and
individual
Board
Enables
from the
application connector.
Note
the
gridwork
of
copper
braid
between
motherboards
which
makes
the
groundplane
essentially
continuous
between
the
motherboards.
Photographs
of
the
address
and
data
bus signals
were
taken
while
running
the
memory
test
program
and
are
shown
in figure 5.
About
the
only
visible
loading
effect
on
the
address
bus
is
a long tail
on
the
zero-to-one
transition
during
phase
1
of
the
clock.
The
data
bus
appears
to
be
even
cleaner
with
just
a
shade
over
lOONS
required
for the
data
to stabilize
after
the
leading
edge
of
phase 2.
The
microprocessor
was
driving
the
data
bus
for the
data
bus for this
photo
(scope synced to
read/write
line
on
the bus).
The
zero logic levels, which
one
would
think
show
the
effect
of
gross
overloading
most, were still in the
0.3
volt
range
although
the
one
levels were
down
to
only
3 volts from a
normal
lightly
loaded
value
of
nearly 4 volts.
Note
the almost
complete
absence
of
noise.
These
"overloaded"
signals actually look far
better
than
most
S-100 bus signals!
While these results
are
encouraging
and
certain-
ly show
that
a four
board
load
does not
bring
a
system to the
brink
of
failure, it does not
mean
that
loading
rules
can
be
disregarded
altogether.
Some
AIM's,
as well as
SYM's
and
KIM's,
can
be ex-
pected to
have
a weak
component
on-board
that
may
not
be able to drive a
12
board
load
adequately
for
reliable
operation.
Thus
the'
'official"
recommenda-
tion is to stick
with
the spec book
and
limit
unbuf-
fered systems to four
boards.
However,
individual
hobbyists should be able to go
one
or
two boards
over
the
limit with little
probability
of
problems. Ac-
tually,
addressing
limitations
are
more
likely to limit
system size
than
bus drive capability with
today's
dense
boards.
- Bruce Land 1
- Baltimore, MD 1
- Seawell little 2
- 110; 32, 64 3
- 110 levels.) 3
- T:' 5
- Any Others 6
- Owen Sound, Ontario 7
- FALK-BAKER 8
- ASSOCIATES 8
- COMPUTE! 9
- * * * STORAGE 10
- * * * DISPLAY ROUTINE * * * 10
- Expanding 11
- KIM-Style 11
- Computers 11
- KIM, AIM 12
- OnYourSYM 13
- Programs 13
- THE TASK* MASTERS 14
- Theory Of Operation 15
- Conclusion 15
- --.::C~0~M~P~U~T=E! 16
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