29 Mar
2017
29 Mar
'17
6:40 a.m.
Hi, a question about generic analog stuff.
In the process of getting SD cards to work, Dave is seeing square-wave noise
on a line. (1V of square wave, with pulses about 400ns long, running at
375kHz.) The line runs through a flat cable of modest length, along with
other signal-carrying lines. (No, we were not smart, and didn't put ground
lines between each pair of signal lines!)
Could cross-talk cause this kind of noise? We would have thought that you'd
only get spikes, associated with the rising and trailing edges of a signal in
a parallel wire, not a whole square-wave. During the constant-current period
in the middle of the pulse, there shouldn't be any cross-talk? Is there some
mechanism I/we don't understand that could do that?
(My guess is there's a leakage path in the circuitry on one end or the other,
not cross-talk in the cable, but...)
Thanks!
Noel
29 Mar
29 Mar
6:57 a.m.
Is there any load resistance at the end of the line?
Dwight
________________________________
From: cctalk <cctalk-bounces at classiccmp.org> on behalf of Noel Chiappa via cctalk
<cctalk at classiccmp.org>
Sent: Wednesday, March 29, 2017 6:40:22 AM
To: cctalk at classiccmp.org
Cc: jnc at mercury.lcs.mit.edu
Subject: Cross-talk square-wave?
Hi, a question about generic analog stuff.
In the process of getting SD cards to work, Dave is seeing square-wave noise
on a line. (1V of square wave, with pulses about 400ns long, running at
375kHz.) The line runs through a flat cable of modest length, along with
other signal-carrying lines. (No, we were not smart, and didn't put ground
lines between each pair of signal lines!)
Could cross-talk cause this kind of noise? We would have thought that you'd
only get spikes, associated with the rising and trailing edges of a signal in
a parallel wire, not a whole square-wave. During the constant-current period
in the middle of the pulse, there shouldn't be any cross-talk? Is there some
mechanism I/we don't understand that could do that?
(My guess is there's a leakage path in the circuitry on one end or the other,
not cross-talk in the cable, but...)
Thanks!
Noel
8:54 a.m.
On Mar 29, 2017, at 9:40 AM, Noel Chiappa via cctalk <cctalk at classiccmp.org>
wrote:
Hi, a question about generic analog stuff.
In the process of getting SD cards to work, Dave is seeing square-wave noise
on a line. (1V of square wave, with pulses about 400ns long, running at
375kHz.) The line runs through a flat cable of modest length, along with
other signal-carrying lines. (No, we were not smart, and didn't put ground
lines between each pair of signal lines!)
Oops!
Could cross-talk cause this kind of noise? We would have thought that you'd
only get spikes, associated with the rising and trailing edges of a signal in
a parallel wire, not a whole square-wave. During the constant-current period
in the middle of the pulse, there shouldn't be any cross-talk? Is there some
mechanism I/we don't understand that could do that?
Transmission line theory applies. Adjacent lines see the electric and magnetic
fields
nominally seen along transmission lines. Some would say it this way, you get induction
from one line to another based on how those wires are
routed and terminated.
Its only 375khz? No, its pulses with rise and fall times in the Nanosecond region
with bandwidth of hundreds of Mhz.
(My guess is there's a leakage path in the
circuitry on one end or the other,
not cross-talk in the cable, but?)
Nooooo. You have to treat those wires as transmission lines ( like coaxial cable or
parallel pair)
for signals. Its not DC leakage. You send a pulse (or a train of them) down a
transmission line
and if the line is not terminated the pulse energy will be reflected rather than absorbed.
Is there is
a signal line next to it it will see the resulting fields from the currents flowing.
Add to that your ground for the SD card is remote so there will be a current flowing on
that lead as well
from circuit ground and the actual ground pin.
This is why people do not remote SD cards (unless someone is forcing it). Its input looks
like a capacitor
at the end of a transmission line and incorrectly handled you get reflections and ringing.
Just like backplanes
and all sorts of other media.
Allison
Thanks!
Noel
1:06 p.m.
There are few things that come to mind here. The op seemed to indicate the lines are
terminated. If they are not terminated in the characteristic impedance of the source and
the transmission line, it is very unlikely he would be seeing nice square waves at either
end. The reflections would distort the square wave. Given the reported squareness and that
the op indicates terminated line, I do not think impedance mismatch is the issue here.
I also agree that an induced current in an adjacent line would not be square. So I agree
with the op's thoughts that this signal is getting on this line in some other fashion,
I don't believe this is an issue of cross talk. However, some pictures of some
waveforms would be interesting to see
Eugene W2HX
-----Original Message-----
From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Parent Allison via
cctalk
Sent: Wednesday, March 29, 2017 11:54 AM
To: Noel Chiappa; General Discussion: On-Topic and Off-Topic Posts
Subject: Re: Cross-talk square-wave?
On Mar 29, 2017, at 9:40 AM, Noel Chiappa via cctalk <cctalk at classiccmp.org>
wrote:
Hi, a question about generic analog stuff.
In the process of getting SD cards to work, Dave is seeing square-wave
noise on a line. (1V of square wave, with pulses about 400ns long,
running at
375kHz.) The line runs through a flat cable of modest length, along
with other signal-carrying lines. (No, we were not smart, and didn't
put ground lines between each pair of signal lines!)
Oops!
Could cross-talk cause this kind of noise? We would have thought that
you'd only get spikes, associated with the rising and trailing edges
of a signal in a parallel wire, not a whole square-wave. During the
constant-current period in the middle of the pulse, there shouldn't be
any cross-talk? Is there some mechanism I/we don't understand that could do that?
Transmission line theory applies. Adjacent lines see the electric and magnetic
fields nominally seen along transmission lines. Some would say it this way, you get
induction from one line to another based on how those wires are routed and terminated.
Its only 375khz... No, its pulses with rise and fall times in the Nanosecond region with
bandwidth of hundreds of Mhz.
(My guess is there's a leakage path in the
circuitry on one end or the
other, not cross-talk in the cable, but...)
Nooooo. You have to treat those wires as transmission lines ( like coaxial cable or
parallel pair) for signals. Its not DC leakage. You send a pulse (or a train of them)
down a transmission line and if the line is not terminated the pulse energy will be
reflected rather than absorbed. Is there is a signal line next to it it will see the
resulting fields from the currents flowing.
Add to that your ground for the SD card is remote so there will be a current flowing on
that lead as well from circuit ground and the actual ground pin.
This is why people do not remote SD cards (unless someone is forcing it). Its input looks
like a capacitor at the end of a transmission line and incorrectly handled you get
reflections and ringing. Just like backplanes and all sorts of other media.
Allison
Thanks!
Noel
1:13 p.m.
IT was not terminated, 270K is way to high a value (likely 1000*). Its low enough to
assert a weak pull-up or down to a CMOS input but not terminate a line.
Typical transmission line for that is in the 120-250ohm range and the input
to a SD/MMC is a roughly 10pf cap to ground and DC open (equivalent).
Also SD have two modes, one being SPI similar and the other a faster 4bit varient.
I know that as I?ve used SD as ?disk? for CP/M systems with 8085 and Z80.
FYI this is the same problem designers hit with DRAMS back 40 years ago.
Allison
On Mar 29, 2017, at 4:06 PM, W2HX <w2hx at w2hx.com> wrote:
There are few things that come to mind here. The op
seemed to indicate the lines are terminated. If they are not terminated in the
characteristic impedance of the source and the transmission line, it is very unlikely he
would be seeing nice square waves at either end. The reflections would distort the square
wave. Given the reported squareness and that the op indicates terminated line, I do not
think impedance mismatch is the issue here.
I also agree that an induced current in an adjacent line would not be square. So I agree
with the op's thoughts that this signal is getting on this line in some other fashion,
I don't believe this is an issue of cross talk. However, some pictures of some
waveforms would be interesting to see
Eugene W2HX
-----Original Message-----
From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Parent Allison via
cctalk
Sent: Wednesday, March 29, 2017 11:54 AM
To: Noel Chiappa; General Discussion: On-Topic and Off-Topic Posts
Subject: Re: Cross-talk square-wave?
On Mar 29, 2017, at 9:40 AM, Noel Chiappa via cctalk <cctalk at classiccmp.org>
wrote:
Hi, a question about generic analog stuff.
In the process of getting SD cards to work, Dave is seeing square-wave
noise on a line. (1V of square wave, with pulses about 400ns long,
running at
375kHz.) The line runs through a flat cable of modest length, along
with other signal-carrying lines. (No, we were not smart, and didn't
put ground lines between each pair of signal lines!)
Oops!
Could cross-talk cause this kind of noise? We would have thought that
you'd only get spikes, associated with the rising and trailing edges
of a signal in a parallel wire, not a whole square-wave. During the
constant-current period in the middle of the pulse, there shouldn't be
any cross-talk? Is there some mechanism I/we don't understand that could do that?
Transmission line theory applies. Adjacent lines see the electric and magnetic
fields nominally seen along transmission lines. Some would say it this way, you get
induction from one line to another based on how those wires are routed and terminated.
Its only 375khz... No, its pulses with rise and fall times in the Nanosecond region with
bandwidth of hundreds of Mhz.
(My guess is there's a leakage path in the
circuitry on one end or the
other, not cross-talk in the cable, but...)
Nooooo. You have to treat those wires as transmission lines ( like coaxial cable or
parallel pair) for signals. Its not DC leakage. You send a pulse (or a train of them)
down a transmission line and if the line is not terminated the pulse energy will be
reflected rather than absorbed. Is there is a signal line next to it it will see the
resulting fields from the currents flowing.
Add to that your ground for the SD card is remote so there will be a current flowing on
that lead as well from circuit ground and the actual ground pin.
This is why people do not remote SD cards (unless someone is forcing it). Its input
looks like a capacitor at the end of a transmission line and incorrectly handled you get
reflections and ringing. Just like backplanes and all sorts of other media.
Allison
Thanks!
Noel
30 Mar
30 Mar
10:22 a.m.
I'll offer a suggestion that if your SD card *must* be a significant
distance from its host, that you employ a small MCU at the SD card and
use a more noise-immune protocol to transmit data to the host.
Small MCUs today are very inexpensive.
--Chuck
1:21 p.m.
On 3/30/2017 11:22 AM, Chuck Guzis via cctalk wrote:
I'll offer a suggestion that if your SD card
*must* be a significant
distance from its host, that you employ a small MCU at the SD card and
use a more noise-immune protocol to transmit data to the host.
Small MCUs today are very inexpensive.
GO ANALOG ... ONLY A LITLE BIT OF A PROBLEM. :)
--Chuck
Well that does not solve the ring, but off loading
the SD card is a good idea, if you have the software
time for a new cpu. Most of the time upper managment
drags thier feet, unless they want it yesterday.
Ben.
1:59 p.m.
On 03/30/2017 01:21 PM, ben via cctalk wrote:
Well that does not solve the ring, but off loading the
SD card is a
good idea, if you have the software time for a new cpu. Most of the
time upper managment drags thier feet, unless they want it
yesterday. Ben.
Well, Noel has stated that this will not be the situation in the final
stage, but my suggestion was done in the spirit of having a well-behaved
low-impedance linkup with the host; not something that resembles an antenna.
--Chuck
2:45 p.m.
I didn't look clearly at the trace but it could also be
a reflected un-terminated line and not cross talk.
Dwight
________________________________
From: cctalk <cctalk-bounces at classiccmp.org> on behalf of Chuck Guzis via cctalk
<cctalk at classiccmp.org>
Sent: Thursday, March 30, 2017 1:59:00 PM
To: General Discussion: On-Topic and Off-Topic Posts
Subject: Re: Cross-talk square-wave?
On 03/30/2017 01:21 PM, ben via cctalk wrote:
Well that does not solve the ring, but off loading the
SD card is a
good idea, if you have the software time for a new cpu. Most of the
time upper managment drags thier feet, unless they want it
yesterday. Ben.
Well, Noel has stated that this will not be the situation in the final
stage, but my suggestion was done in the spirit of having a well-behaved
low-impedance linkup with the host; not something that resembles an antenna.
--Chuck
29 Mar
29 Mar
1:56 p.m.
There are few things that come to mind here. The op
seemed to indicate the lines are terminated. If they are not terminated in the
characteristic impedance of the source and the transmission line, it is very unlikely he
would be seeing nice square waves at either end. The reflections would distort the square
wave. Given the reported squareness and that the op indicates terminated line, I do not
think impedance mismatch is the issue here.
There's certainly some ringing there but it's not the spike followed by
an exponential decay that I'd expect from an induced signal. However,
maybe I just don't know what an induced signal can look like so that's
the question.
I also agree that an induced current in an adjacent
line would not be square. So I agree with the op's thoughts that this signal is
getting on this line in some other fashion, I don't believe this is an issue of cross
talk. However, some pictures of some waveforms would be interesting to see
Ask and receive. See: http://pdp10.froghouse.org/qsic/pic_24_1.gif
The bottom signal is the one in question and the top signal is the clock
I'm sending to the SD card (which isn't plugged in at this time). I
wanted to see if it was the clock single I was seeing coupled here and
it's obviously not, though you can clearly see the clock inducing some
noise in the CS signal.
The other thing I'm seeing from this trace that I hadn't really noticed
before is that 0 is not 0. The 0 for the bottom trace is where the 2 is
on the left side. This line is suppose to be going from the 270k
resistor to ground on one side across the ribbon cable to an FPGA pin
which is set to high-impedance on the other. Clearly something else is
going on here.
2:14 p.m.
And I think this picture is the smoking gun.
http://pdp10.froghouse.org/qsic/pic_24_2.gif
Again, the bottom trace is the CS signal in question and the upper trace
is now one of the QBUS DAL lines (after the bus transceiver and level
converter) that's running across the ribbon cable near the CS signal.
It does appear that induction can make a fairly clean square wave.
Well, the purpose of a prototype is to learn and this has been one
learning experience after another.
2:35 p.m.
On 3/29/17 2:14 PM, David Bridgham via cctalk wrote:
And I think this picture is the smoking gun.
http://pdp10.froghouse.org/qsic/pic_24_2.gif
Again, the bottom trace is the CS signal in question and the upper trace
is now one of the QBUS DAL lines (after the bus transceiver and level
converter) that's running across the ribbon cable near the CS signal.
It does appear that induction can make a fairly clean square wave.
simple thing to try is split the ribbon cable between the two signals
5:33 p.m.
270K is not a transmission line load.
As I recall ribbon cable is around 100-150 ohms
impedance some place.
The signal does look nice and square.
I doubt is is inductive coupling, with that high a
load, I'd say it was capacitive.
inductive coupling requires current flowing.
Dwight
________________________________
From: cctalk <cctalk-bounces at classiccmp.org> on behalf of Al Kossow via cctalk
<cctalk at classiccmp.org>
Sent: Wednesday, March 29, 2017 2:35:54 PM
To: cctalk at classiccmp.org
Subject: Re: Cross-talk square-wave?
On 3/29/17 2:14 PM, David Bridgham via cctalk wrote:
And I think this picture is the smoking gun.
http://pdp10.froghouse.org/qsic/pic_24_2.gif
Again, the bottom trace is the CS signal in question and the upper trace
is now one of the QBUS DAL lines (after the bus transceiver and level
converter) that's running across the ribbon cable near the CS signal.
It does appear that induction can make a fairly clean square wave.
simple thing to try is split the ribbon cable between the two signals
6:48 p.m.
I am still not convinced it is coupling at all. You would expect the affected line to show
a signal like dV/dt , no? I just don't think you can get square waves from square
waves. That's not to say the input of some logic somewhere isn't getting triggered
by unintended coupling and then getting "squared up" in some gate to produce the
square we see.
Oh, and 270K surely is a transmission line load if the source has a characteristic
impedance of 270K. Granted, that seems unusual and I don't know what the circuit looks
like, but yes, most probably it is not a matched load. Having said that, given such a
probable mismatch, then it is even harder to believe one could successfully couple a
square wave onto such a transmission line unless the signal is actually being asserted on
the line at a low impedance (ie the intentional output of a gate somewhere).
Looking at this picture
http://pdp10.froghouse.org/qsic/pic_24_1.gif
this shows exactly what I would expect to see with cross talk the little glitches on the
CS line that correspond to edges on the clock signal. Classic dV/dt. scenario.
Looking at this one
http://pdp10.froghouse.org/qsic/pic_24_2.gif
again, I don't know the circuit, but I have personally witnessed this behavior on my
PDP-8/e where a 7474 flip flop chip was bad. The input looked great and the output was
"half baked"
Here is a screenshot of what I was seeing (look for "Line OUT E29 pin5" on the
scope screenshot...)
http://w2hx.com/x/VintageComp/PDP-8e/M8650/LecroyScreen14.png
(post on that subject is here:
http://www.vcfed.org/forum/showthread.php?55171-PDP-8-e-Project/page6 )
Anyhow, I dunno. My jury is still out on this one.
Eugene
-----Original Message-----
From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of dwight via cctalk
Sent: Wednesday, March 29, 2017 8:33 PM
To: Al Kossow; General Discussion: On-Topic and Off-Topic Posts
Subject: Re: Cross-talk square-wave?
270K is not a transmission line load.
As I recall ribbon cable is around 100-150 ohms
impedance some place.
The signal does look nice and square.
I doubt is is inductive coupling, with that high a
load, I'd say it was capacitive.
inductive coupling requires current flowing.
Dwight
________________________________
From: cctalk <cctalk-bounces at classiccmp.org> on behalf of Al Kossow via cctalk
<cctalk at classiccmp.org>
Sent: Wednesday, March 29, 2017 2:35:54 PM
To: cctalk at classiccmp.org
Subject: Re: Cross-talk square-wave?
On 3/29/17 2:14 PM, David Bridgham via cctalk wrote:
And I think this picture is the smoking gun.
http://pdp10.froghouse.org/qsic/pic_24_2.gif
Again, the bottom trace is the CS signal in question and the upper
trace is now one of the QBUS DAL lines (after the bus transceiver and
level
converter) that's running across the ribbon cable near the CS signal.
It does appear that induction can make a fairly clean square wave.
simple thing to try is split the ribbon cable between the two signals
7:55 p.m.
On 03/29/2017 08:48 PM, W2HX via cctalk wrote:
I am still not convinced it is coupling at all. You
would expect the affected line to show a signal like dV/dt , no? I just don't think
you can get square waves from square waves.
Yes, you can. The capacitance of
typical cables is about 35
pF per foot. Given a couple feet of cable and essentially
infinite resistive load, it would be quite likely to give a
near square-wave result. If you were to load down the line
with 100 Ohms, then you'd see tiny, short pulses at the edges.
That's not to say the input of some logic
somewhere isn't getting triggered by unintended coupling and then getting
"squared up" in some gate to produce the square we see.
Oh, and 270K surely is a transmission line load if the source has a characteristic
impedance of 270K. Granted, that seems unusual and I don't know what the circuit looks
like,
Well, in fact, it is impossible to make a transmission line
with such impedance. The impedance of free space is
supposed to be 277 Ohms, IIRC.
Jon
8:16 p.m.
The impedance of free space is supposed to be 277 Ohms
377 ohms but you are right about the max impedance. I forgot about that.
Yes, you can. The capacitance of typical cables is
about 35 pF per foot. Given a couple feet of cable and essentially infinite resistive
load, it would be quite likely to give a near square-wave result.
Except for the fact that in the same cable, the edges of the square wave of the clock is
creating short pulses on the CS line. Why would the CS line now behave totally differently
if theoretically coupled to the QBUS line? In theory maybe you are correct, but in the
practice of troubleshooting this specific issue, I would have to say it is unlikely that
an adjacent square wave clock line would cause spikes and a similarly adjacent square wave
QBUS signal would cause a square wave. They would both really be expected to have the same
effect as long as the CS line hasn't changed change in characteristic.
My 2c
Eugene
-----Original Message-----
From: Jon Elson [mailto:elson at pico-systems.com]
Sent: Wednesday, March 29, 2017 10:55 PM
To: W2HX; On-Topic and Off-Topic Posts
Subject: Re: Cross-talk square-wave?
On 03/29/2017 08:48 PM, W2HX via cctalk wrote:
I am still not convinced it is coupling at all. You
would expect the affected line to show a signal like dV/dt , no? I just don't think
you can get square waves from square waves.
Yes, you can. The capacitance of
typical cables is about 35 pF per foot. Given a couple feet of cable and essentially
infinite resistive load, it would be quite likely to give a near square-wave result. If
you were to load down the line with 100 Ohms, then you'd see tiny, short pulses at the
edges.
That's not to say the input of some logic
somewhere isn't getting triggered by unintended coupling and then getting
"squared up" in some gate to produce the square we see.
Oh, and 270K surely is a transmission line load if the source has a
characteristic impedance of 270K. Granted, that seems unusual and I
don't know what the circuit looks like,
Well, in fact, it is impossible to make
a transmission line with such impedance. The impedance of free space is supposed to be
277 Ohms, IIRC.
Jon
30 Mar
30 Mar
11:19 a.m.
again, I don't know the circuit, but I have personally witnessed this behavior on my
PDP-8/e where a 7474 flip flop chip was bad. The input looked great and the output was
"half baked"
I'll bet that flop was a Fairchild.
Dwight
________________________________
From: cctalk <cctalk-bounces at classiccmp.org> on behalf of W2HX via cctalk
<cctalk at classiccmp.org>
Sent: Wednesday, March 29, 2017 6:48:09 PM
To: General Discussion: On-Topic and Off-Topic Posts
Subject: RE: Cross-talk square-wave?
I am still not convinced it is coupling at all. You would expect the affected line to show
a signal like dV/dt , no? I just don't think you can get square waves from square
waves. That's not to say the input of some logic somewhere isn't getting triggered
by unintended coupling and then getting "squared up" in some gate to produce the
square we see.
Oh, and 270K surely is a transmission line load if the source has a characteristic
impedance of 270K. Granted, that seems unusual and I don't know what the circuit looks
like, but yes, most probably it is not a matched load. Having said that, given such a
probable mismatch, then it is even harder to believe one could successfully couple a
square wave onto such a transmission line unless the signal is actually being asserted on
the line at a low impedance (ie the intentional output of a gate somewhere).
Looking at this picture
http://pdp10.froghouse.org/qsic/pic_24_1.gif
this shows exactly what I would expect to see with cross talk the little glitches on the
CS line that correspond to edges on the clock signal. Classic dV/dt. scenario.
Looking at this one
http://pdp10.froghouse.org/qsic/pic_24_2.gif
again, I don't know the circuit, but I have personally witnessed this behavior on my
PDP-8/e where a 7474 flip flop chip was bad. The input looked great and the output was
"half baked"
Here is a screenshot of what I was seeing (look for "Line OUT E29 pin5" on the
scope screenshot...)
http://w2hx.com/x/VintageComp/PDP-8e/M8650/LecroyScreen14.png
(post on that subject is here:
http://www.vcfed.org/forum/showthread.php?55171-PDP-8-e-Project/page6 )
Anyhow, I dunno. My jury is still out on this one.
Eugene
-----Original Message-----
From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of dwight via cctalk
Sent: Wednesday, March 29, 2017 8:33 PM
To: Al Kossow; General Discussion: On-Topic and Off-Topic Posts
Subject: Re: Cross-talk square-wave?
270K is not a transmission line load.
As I recall ribbon cable is around 100-150 ohms
impedance some place.
The signal does look nice and square.
I doubt is is inductive coupling, with that high a
load, I'd say it was capacitive.
inductive coupling requires current flowing.
Dwight
________________________________
From: cctalk <cctalk-bounces at classiccmp.org> on behalf of Al Kossow via cctalk
<cctalk at classiccmp.org>
Sent: Wednesday, March 29, 2017 2:35:54 PM
To: cctalk at classiccmp.org
Subject: Re: Cross-talk square-wave?
On 3/29/17 2:14 PM, David Bridgham via cctalk wrote:
And I think this picture is the smoking gun.
http://pdp10.froghouse.org/qsic/pic_24_2.gif
Again, the bottom trace is the CS signal in question and the upper
trace is now one of the QBUS DAL lines (after the bus transceiver and
level
converter) that's running across the ribbon cable near the CS signal.
It does appear that induction can make a fairly clean square wave.
simple thing to try is split the ribbon cable between the two signals
2585
days inactive
2586
days old
16 comments
9 participants
participants (9)
-
aek＠bitsavers.org -
allisonportable＠gmail.com -
bfranchuk＠jetnet.ab.ca -
cclist＠sydex.com -
dab＠froghouse.org -
dkelvey＠hotmail.com -
elson＠pico-systems.com -
jnc＠mercury.lcs.mit.edu -
w2hx＠w2hx.com