PacComm TINY-2 - Page 139

TROUBLESHOOTING
Loopback
testing
voits
indicates
some
varying
digital
signal.
A
reading
of
over
3.5
volts
or
under
1.5
volts
is
most
likely
an
indication
of
trouble.
A
very
low,
or
very
high
voltage
reading
is
a
sign
of
no
activity,
or
open
and
shorted
circuits.
If
you
cannot
pinpoint
the
trouble
using
this
set
of
guidelines,
then
you
must
either
obtain
an
oscilloscope,
or
send
the
board
back
to
PacComm
for
repair.
It
you
have
an
oscilloscope
available,
the
trouble
shooting
is
much
easier!
The
checks
made
above
still
apply,
but
now
you
can
"see"
trouble
on
the
oscilloscope
waveforms.
If
you
see
a
‘stair-stepped’
wavetorm
on
two
different
pins,
then
it
is
likely
they
are
shorted
together.
Also,
square
waves
of
half
amplitude,
or
lines
with
no
activity
are
dead
giveaways
on
the
scope.
The
scope
also
allows
you
to
trace
the
analog
sections
of
the
device,
including
the
modem.
Be
very
carefu!
about
shorting
pins
on
ICs
when
applying
meter
or
scope
probes
to
the
board.
It
is
a
good
idea
to
attach
a
secure
ground
lead
to
the
meter
or
scope,
one
that
won't
accidentally
short
across
components
on
the
board.
A
good
place
to
pick
up
this
ground
is
on
the
ground
bus
that
completely
encircles
the
perimeter
of
the
board.
Avoid
connecting
in
the
area
where
power
leads
run
to
and
from
the
power
switch
to
avoid
the
possibility
of
a
short.
Loopback
testing
Loopback
testing
consists
of
having
the
transmit
section
of
the
packet
controller
communicate
with
the
receive
section.
If
this
procedure
is
successful,
the
correct
operation
of
both
transmit
and
recieve
sections
can
be
verified.
If
data
is
not
successfully
passed,
clues
to
the
malfunction
may
be
obtained.
There
are
two
types
of
loopback
testing,
analog
and
digital.
For
both
types
of
test,
the
command
FULLDUP
must
be
ON
and
some
type
of
connection
must
be
made
between
the
transmit
output
and
the
receive
input.
Once
the
proper
arrangements
are
made,
testing
consists
of
attempting
to
pass
information
from
the
transmit
section
to
the
receive
section
of
the
packet
controller.
One
method
is
to
send
data
to
and
from
the
PMS
(if
the
PMS
option
is
installed)
by
connecting
to
MYP-
CALL.
You
may
also
connect
to
MYCALL
and
send
and
receive
from
the
terminal,
or
send
UI
frames
and
monitor
them.
Be
sure
to
undo
all
the
loopback
steps
once
the
testing
is
completed.
Analog loopback
Remove
JPR
to
place
the
input
circuit
in
a
high
impedance
state.
Install
a
homemade
jumper
between
pins
1
and
4
of
the
radio
connector
J2.
Send
UI
frames
and
monitor
for
their
reception
or
try
a
connection
as
described
above.
The
output
level
trimpot
R12
may
have
to
be
turned
fo
near
maximum
to
allow
the
modem
to
hear
itself.
Failure
of
the
analog
loopback
test
does
not
necessarily
mean
there
is
a
modem
problem
since
the
transmit
circuits
and
receive
circuits
are
not
op-
timized
for
direct
connection.
18
TINY-2/MIGRO-2
Technical
Ref.,
2
Ed
©
1989,
PacComm,
Inc.