need help on using a logic probe TIA

[Sierra]

I'm looking for sufficient knowledge before using a logic probe model AT-301
(no instructions for using this probe is available) to troubleshoot on a
monitoring cct board. The board comprises mainly of 40 series logic gates
and buffers driving small transistors based on input condtions. I have all
data sheets showing truth tables and all other data of chips.

The logic probe has a Red led for HI, a green led for LO and a yellow led
for PULSE. A switch for TTL & CMOS. A switch for MEM and PULSE. and its has
a Black and Red lead terminated to banana clips.

Can anyone tell me how to use this probe - like which lead is to be
connected to where and switch positions while attempting to locate faulty
chips.

TIA

 

[CFoley1064]

Subject: need help on using a logic probe TIA
From: "Sierra" zayan@dhivehinet.net.mv
Date: 2/24/2004 6:21 AM Central Standard Time
Message-id: <c1fr0m$1hor2m$1@ID-220738.news.uni-berlin.de

I'm looking for sufficient knowledge before using a logic probe model AT-301
(no instructions for using this probe is available) to troubleshoot on a
monitoring cct board. The board comprises mainly of 40 series logic gates
and buffers driving small transistors based on input condtions. I have all
data sheets showing truth tables and all other data of chips.

The logic probe has a Red led for HI, a green led for LO and a yellow led
for PULSE. A switch for TTL & CMOS. A switch for MEM and PULSE. and its has
a Black and Red lead terminated to banana clips.

Can anyone tell me how to use this probe - like which lead is to be
connected to where and switch positions while attempting to locate faulty
chips.

TIA

Basic logic probes are easy. Connect the red lead to a portion of the circuit
that has the + voltage for the CMOS ICs (don't connect this to a logic output
-- most logic probes require 20 to 40 mA to operate -- be sure the circuit
under test can handle the extra loading), and connect the negative wire to
circuit common or GND. If you've got an axial cap at the output of the logic
power supply voltage regulator, that's just perfect, otherwise, hunt around.
Make sure not to do this bass-ackwards -- you might let the smoke out. Set
your switch for TTL or CMOS, depending on what you're testing. Now you're
ready to use the logic probe.

Place the metal probe on the IC pin to be tested. If it's logic high (usually
more than 70% of Vcc if CMOS, more than 2.4V if TTL), the red HI LED will lite.
If it's logic low (usually less than 30% of Vcc if CMOS, less than 0.8V if
TTL) the logic LO LED will lite. Every time there's a logic transition
(usually from LO to HI) the PULSE LED will lite for half a second or so.

If you don't get any lite, that means the logic output is in an intermediate
state, which almost always means bad news on the board -- either that, or one
of the power supply gator clips came loose 8o).

The only problem I have with most logic probes is that the probe itself is
usually a little clunky -- not so convenient for SMT small pitch ICs. Watch
out for shorting pins to each other with the probe, too. It's safer to probe
with your elbow resting on something to keep things steady.

If you haven't got a scope handy, the logic probe can be the next best thing,
and it fits in your small tool satchel, so you can take it anywhere.

Good luck
Chris

 

[Sierra]

Chris,
Thanks a lot for guiding me. Still there's a bit I don't know. What is this
switch on the probe for MEM and PULSE? Is that switch of any use while
testing?


"CFoley1064" <cfoley1064@aol.com> wrote in message
news:20040224124455.19374.00000267@mb-m16.aol.com...

Subject: need help on using a logic probe TIA
From: "Sierra" zayan@dhivehinet.net.mv
Date: 2/24/2004 6:21 AM Central Standard Time
Message-id: <c1fr0m$1hor2m$1@ID-220738.news.uni-berlin.de

I'm looking for sufficient knowledge before using a logic probe model
AT-301
(no instructions for using this probe is available) to troubleshoot on a
monitoring cct board. The board comprises mainly of 40 series logic gates
and buffers driving small transistors based on input condtions. I have
all
data sheets showing truth tables and all other data of chips.

The logic probe has a Red led for HI, a green led for LO and a yellow led
for PULSE. A switch for TTL & CMOS. A switch for MEM and PULSE. and its
has
a Black and Red lead terminated to banana clips.

Can anyone tell me how to use this probe - like which lead is to be
connected to where and switch positions while attempting to locate faulty
chips.

TIA

Basic logic probes are easy. Connect the red lead to a portion of the
circuit
that has the + voltage for the CMOS ICs (don't connect this to a logic
output
-- most logic probes require 20 to 40 mA to operate -- be sure the circuit
under test can handle the extra loading), and connect the negative wire to
circuit common or GND. If you've got an axial cap at the output of the
logic
power supply voltage regulator, that's just perfect, otherwise, hunt
around.
Make sure not to do this bass-ackwards -- you might let the smoke out.
Set
your switch for TTL or CMOS, depending on what you're testing. Now you're
ready to use the logic probe.

Place the metal probe on the IC pin to be tested. If it's logic high
(usually
more than 70% of Vcc if CMOS, more than 2.4V if TTL), the red HI LED will
lite.
If it's logic low (usually less than 30% of Vcc if CMOS, less than 0.8V
if
TTL) the logic LO LED will lite. Every time there's a logic transition
(usually from LO to HI) the PULSE LED will lite for half a second or so.

If you don't get any lite, that means the logic output is in an
intermediate
state, which almost always means bad news on the board -- either that, or
one
of the power supply gator clips came loose 8o).

The only problem I have with most logic probes is that the probe itself is
usually a little clunky -- not so convenient for SMT small pitch ICs.
Watch
out for shorting pins to each other with the probe, too. It's safer to
probe
with your elbow resting on something to keep things steady.

If you haven't got a scope handy, the logic probe can be the next best
thing,
and it fits in your small tool satchel, so you can take it anywhere.

Good luck
Chris