Let's say you know that a certain point on a data bus has a defective
device that is always trying to drive the bus low; Was wondering if anyone
has any good tricks in finding the defective device, other than
cutting/lifting pins?

JW wrote:
Let's say you know that a certain point on a data bus has a defective
device that is always trying to drive the bus low; Was wondering if anyone
has any good tricks in finding the defective device, other than
cutting/lifting pins?

Drive the bus high, e.g. with a 1k resistor to Vdd, and use
millivoltmeter. You can see the voltage drops in the traces--where the
voltage drop is changing, current is flowing.

Cheers

Phil Hobbs

On Wed, 13 May 2009 07:51:38 -0400 Phil Hobbs
wrote in Message id:
:

JW wrote:
Let's say you know that a certain point on a data bus has a defective
device that is always trying to drive the bus low; Was wondering if anyone
has any good tricks in finding the defective device, other than
cutting/lifting pins?

Drive the bus high, e.g. with a 1k resistor to Vdd, and use
millivoltmeter. You can see the voltage drops in the traces--where the
voltage drop is changing, current is flowing.

Hi Phil, That sound like a good possibility, thanks. I have a Philips
PM2534 DMM that has a 300mV range with 100nV resolution. The only thing
might be that if the bus is active (changing states) the readings might be
jumping around too much. Often I see a tied node on a TTL bus that has
transitions from .5 to 2 volts or so. Any ideas for a situation like that?
Maybe try to crash the processor?

JW wrote:
On Wed, 13 May 2009 07:51:38 -0400 Phil Hobbs
wrote in Message id:
:

JW wrote:
Let's say you know that a certain point on a data bus has a defective
device that is always trying to drive the bus low; Was wondering if anyone
has any good tricks in finding the defective device, other than
cutting/lifting pins?
Drive the bus high, e.g. with a 1k resistor to Vdd, and use
millivoltmeter. You can see the voltage drops in the traces--where the
voltage drop is changing, current is flowing.

Hi Phil, That sound like a good possibility, thanks. I have a Philips
PM2534 DMM that has a 300mV range with 100nV resolution. The only thing
might be that if the bus is active (changing states) the readings might be
jumping around too much. Often I see a tied node on a TTL bus that has
transitions from .5 to 2 volts or so. Any ideas for a situation like that?
Maybe try to crash the processor?

Yep. Hit the halt line. Alternatively, disable the clock. As a last
resort, you could put an RC filter on the meter.

Cheers

Phil Hobbs

On May 13, 4:50*pm, JW wrote:
Let's say you know that a certain point on a data bus has a defective
device that is always trying to drive the bus low; Was wondering if anyone
has any good tricks in finding the defective device, other than
cutting/lifting pins?

Does the bus point show low ohms to ground in cold state?(powered
down). If so, leave a sensitive ohmmeter connected to it, hover over
each device feeding the bus with a hot air blower. When you see a
sudden pronounced change in ohms to ground you're hovering over the
culprit.
Jango

On Wed, 13 May 2009 07:11:07 -0700 (PDT) jango2
wrote in Message id:
:

On May 13, 4:50*pm, JW wrote:
Let's say you know that a certain point on a data bus has a defective
device that is always trying to drive the bus low; Was wondering if anyone
has any good tricks in finding the defective device, other than
cutting/lifting pins?

Does the bus point show low ohms to ground in cold state?(powered
down). If so, leave a sensitive ohmmeter connected to it, hover over
each device feeding the bus with a hot air blower. When you see a
sudden pronounced change in ohms to ground you're hovering over the
culprit.
Jango

I always check for that - if I see a low resistance to GND when power is
not applied, that's an easy one. I have a Toneohm with a 200m Ohm range.
Also useful for finding etch/solder shorts. Thanks, though.

JW wrote:
Let's say you know that a certain point on a data bus has a defective
device that is always trying to drive the bus low; Was wondering if anyone
has any good tricks in finding the defective device, other than
cutting/lifting pins?

Hang a CRO off each line in turn, & look for one that's stuck low. Once
you've found it, pulling chips out of their sockets is probably the
easiest way to find the culprit.

--
W
. | ,. w , "Some people are alive only because
\|/ \|/ it is illegal to kill them." Perna condita delenda est
---^----^---------------------------------------------------------------

JW wrote:
On Wed, 13 May 2009 07:51:38 -0400 Phil Hobbs
wrote in Message id:
:

JW wrote:
Let's say you know that a certain point on a data bus has a defective
device that is always trying to drive the bus low; Was wondering if anyone
has any good tricks in finding the defective device, other than
cutting/lifting pins?
Drive the bus high, e.g. with a 1k resistor to Vdd, and use
millivoltmeter. You can see the voltage drops in the traces--where the
voltage drop is changing, current is flowing.

Hi Phil, That sound like a good possibility, thanks. I have a Philips
PM2534 DMM that has a 300mV range with 100nV resolution. The only thing
might be that if the bus is active (changing states) the readings might be
jumping around too much.

If it's stuck low, then the signal won't be changing, will it?


--
W
. | ,. w , "Some people are alive only because
\|/ \|/ it is illegal to kill them." Perna condita delenda est
---^----^---------------------------------------------------------------

On Thu, 14 May 2009 00:42:16 +1000 Bob Larter
wrote in Message id: :

JW wrote:
On Wed, 13 May 2009 07:51:38 -0400 Phil Hobbs
wrote in Message id:
:

JW wrote:
Let's say you know that a certain point on a data bus has a defective
device that is always trying to drive the bus low; Was wondering if anyone
has any good tricks in finding the defective device, other than
cutting/lifting pins?
Drive the bus high, e.g. with a 1k resistor to Vdd, and use
millivoltmeter. You can see the voltage drops in the traces--where the
voltage drop is changing, current is flowing.

Hi Phil, That sound like a good possibility, thanks. I have a Philips
PM2534 DMM that has a 300mV range with 100nV resolution. The only thing
might be that if the bus is active (changing states) the readings might be
jumping around too much.

If it's stuck low, then the signal won't be changing, will it?

Yes, it can. It's not stuck/shorted to GND, but fighting with another
latch, driver, or buffer.

On Thu, 14 May 2009 00:41:18 +1000 Bob Larter
wrote in Message id: :

JW wrote:
Let's say you know that a certain point on a data bus has a defective
device that is always trying to drive the bus low; Was wondering if anyone
has any good tricks in finding the defective device, other than
cutting/lifting pins?

Hang a CRO off each line in turn, & look for one that's stuck low.

Hi Bob,

I don't think you understand quite what I'm asking. For sake of argument,
let's say I have five ALS245 transceivers all located on a 8 bit bus. One
of them should be driving the bus at any given moment, but another one has
a defective output that is corrupting data bit 1. I'm looking for any
tricks that would help to narrow the fault to one device. So far, Phil
Hobbs has had the best idea.

Once
you've found it, pulling chips out of their sockets is probably the
easiest way to find the culprit.

Sockets? Heh. If they were in sockets it would be a piece of cake. Have
you taken a look at a modern circuit board with SMD parts lately?

Thanks.

"JW" wrote in message
...
On Thu, 14 May 2009 00:41:18 +1000 Bob Larter
wrote in Message id: :

JW wrote:
Let's say you know that a certain point on a data bus has a defective
device that is always trying to drive the bus low; Was wondering if
anyone
has any good tricks in finding the defective device, other than
cutting/lifting pins?

Hang a CRO off each line in turn, & look for one that's stuck low.

Hi Bob,

I don't think you understand quite what I'm asking. For sake of argument,
let's say I have five ALS245 transceivers all located on a 8 bit bus. One
of them should be driving the bus at any given moment, but another one has
a defective output that is corrupting data bit 1. I'm looking for any
tricks that would help to narrow the fault to one device. So far, Phil
Hobbs has had the best idea.

Once
you've found it, pulling chips out of their sockets is probably the
easiest way to find the culprit.

Sockets? Heh. If they were in sockets it would be a piece of cake. Have
you taken a look at a modern circuit board with SMD parts lately?

Thanks.

Once found a shorted cap by powering the Vcc with a voltage regulated,
current limited source and traced to the hot spot. Once determined that
there was heavy traces, fed a couple of amps and watched the bad cap smoke.
Saved a $5k board that had been passed around everyone else. There were
about 50 or more bypass caps throughout the board and a real pain to have to
replace them all to find the culprit.

JW wrote in
:

On Wed, 13 May 2009 07:11:07 -0700 (PDT) jango2
wrote in Message id:
:

On May 13, 4:50*pm, JW wrote:
Let's say you know that a certain point on a data bus has a defective
device that is always trying to drive the bus low; Was wondering if
anyone has any good tricks in finding the defective device, other than
cutting/lifting pins?

Does the bus point show low ohms to ground in cold state?(powered
down). If so, leave a sensitive ohmmeter connected to it, hover over
each device feeding the bus with a hot air blower. When you see a
sudden pronounced change in ohms to ground you're hovering over the
culprit.
Jango

I always check for that - if I see a low resistance to GND when power is
not applied, that's an easy one. I have a Toneohm with a 200m Ohm range.
Also useful for finding etch/solder shorts. Thanks, though.

An ESR tester can be used to trace down low ohm shorts.



I used to use an 'RCA curve tracer' to troubleshoot bus problems. Sick
gates usually lack a 'sharp knee' on the curve.

binary search is helpful, if you have to cut a trace, try to do it 'in the
middle' (half the circuits on one side of the cut, half on the other). That
way you cut the number of possible bad circuits in half, each time you cut
a trace.

When you narrow it down to a few ICs, you can cut (or unsolder) pins
instead of cutting traces.




--
bz 73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.

remove ch100-5 to avoid spam trap

I've been lurking for a while and this is my first post here. I've been in the
electronics industry for 40 years and have picked up a few tricks along the
way.

Many faulty parts will heat up faster than their counterparts and can be detected
with a liquid crystal sheet. Lay the sheet on a cold unpowered board and apply
the power. Whatever heats up quickest is suspect. I've used this technique to
find faulty ICs, Caps, and even high current traces.

Here is one source http://scientificsonline.com/product.asp_Q_pn_E_3072375

Art


"JW" wrote in message
...
Let's say you know that a certain point on a data bus has a defective
device that is always trying to drive the bus low; Was wondering if anyone
has any good tricks in finding the defective device, other than
cutting/lifting pins?

In message , JW
writes
If it's stuck low, then the signal won't be changing, will it?

Yes, it can. It's not stuck/shorted to GND, but fighting with another
latch, driver, or buffer.
Almost the weirdest fault I ever saw was like that, one PC printed fine
to an old Epson dot matrix, another printed garbage.

The printer had a 74LS244 or 245 (can't remember which, was a *long*
time ago) on the parallel input, measured with a meter on ohms the
offending pin was 2-3 ohms to ground. The old Zenith 8088 drove it fine,
the newer 386 machine failed miserably.

One way to trace is to get some thermochromic labels but if it's just
one input then it may not work and can be fairly random depending on bus
load etc.

Another way that's worked well for me in the past is with a component
tester, the sort that injects a signal into the device and displays the
'curve' of the device under test, Huntron Tracker or Hameg 'scope. You
quickly get used to the shape of the trace produced by various different
chip input/output pins and it's extremely easy to spot a dodgy one.

--
Clint Sharp

JW wrote:
On Thu, 14 May 2009 00:41:18 +1000 Bob Larter
wrote in Message id: :

JW wrote:
Let's say you know that a certain point on a data bus has a defective
device that is always trying to drive the bus low; Was wondering if anyone
has any good tricks in finding the defective device, other than
cutting/lifting pins?
Hang a CRO off each line in turn, & look for one that's stuck low.

Hi Bob,

I don't think you understand quite what I'm asking. For sake of argument,
let's say I have five ALS245 transceivers all located on a 8 bit bus. One
of them should be driving the bus at any given moment, but another one has
a defective output that is corrupting data bit 1. I'm looking for any
tricks that would help to narrow the fault to one device. So far, Phil
Hobbs has had the best idea.

Okay, that's something to work with. Here's how I'd tackle the problem:
Hook your CRO channel 1 to one of the 74x245 enable lines, & use that as
the trigger, while using the 2nd channel to look at the faulty data
line. Probe each of the 74x245 enable lines with channel 1 until you see
a stuck line on channel 2.

Once
you've found it, pulling chips out of their sockets is probably the
easiest way to find the culprit.

Sockets? Heh. If they were in sockets it would be a piece of cake. Have
you taken a look at a modern circuit board with SMD parts lately?

I guess I'm showing my age here. ;^)

--
W
. | ,. w , "Some people are alive only because
\|/ \|/ it is illegal to kill them." Perna condita delenda est
---^----^---------------------------------------------------------------

Artemus wrote:
I've been lurking for a while and this is my first post here. I've been in the
electronics industry for 40 years and have picked up a few tricks along the
way.

Many faulty parts will heat up faster than their counterparts and can be detected
with a liquid crystal sheet. Lay the sheet on a cold unpowered board and apply
the power. Whatever heats up quickest is suspect. I've used this technique to
find faulty ICs, Caps, and even high current traces.

Here is one source http://scientificsonline.com/product.asp_Q_pn_E_3072375

Ooh, nifty. I'd like one of those sheets just for the entertainment value.

--
W
. | ,. w , "Some people are alive only because
\|/ \|/ it is illegal to kill them." Perna condita delenda est
---^----^---------------------------------------------------------------