Author Topic: 16 channel digital probe for HP54645D MSO?  (Read 5780 times)

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Offline carl_lab

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16 channel digital probe for HP54645D MSO?
« on: September 27, 2016, 04:49:48 pm »
Where can I get a 16 channel digital cable and probes for HP54645D MSO for a reasonable price?
Are there clones available?
Can I make one DIY?

« Last Edit: September 27, 2016, 06:54:16 pm by carl_lab »
 

Offline alank2

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Offline carl_lab

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Re: 16 channel digital probe for HP54645D MSO?
« Reply #2 on: September 28, 2016, 07:29:29 am »
Thank you!

The cable looks like a normal floppy drive/HD flat cable.
I thought the original was a woven/twisted pair type flat cable? Am I wrong or doesn't it matter?

What do I need for 54645D scope?

Pod 1 Data + J Clock,
Pod 2 Data + K Clock,
Pod 3 Data + L Clock,
Pod 4 Data + M Clock,
Pod 5 Data + N Clock or
Pod 6 Data + P Clock Logic Analyzer Probe?

What's the differences?


 

Offline alank2

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Re: 16 channel digital probe for HP54645D MSO?
« Reply #3 on: September 28, 2016, 12:02:47 pm »
The cable looks like a normal floppy drive/HD flat cable.

It is.

I thought the original was a woven/twisted pair type flat cable? Am I wrong or doesn't it matter?

You are right; I found this one to work fine, but signal integrity is likely better on the woven/twisted one.

What do I need for 54645D scope?

The kit I listed above comes with the Pod 4 Data + M Clock one, but I'm not sure what the differences are.
 
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Offline carl_lab

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Re: 16 channel digital probe for HP54645D MSO?
« Reply #4 on: September 28, 2016, 01:27:55 pm »
...I'm not sure what the differences are.
Maybe just the numbering (for "real" logic analyzers with more than 16 channels)?  :-//

The PCB is just an extension without any components on it?



« Last Edit: September 28, 2016, 01:39:48 pm by carl_lab »
 

Offline alank2

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Re: 16 channel digital probe for HP54645D MSO?
« Reply #5 on: September 28, 2016, 02:00:10 pm »
The PCB is just an extension without any components on it?

That is correct, no components, straight through.
 
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Offline Ice-Tea

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Re: 16 channel digital probe for HP54645D MSO?
« Reply #6 on: September 28, 2016, 05:06:35 pm »
To be clear: it's not just floppy cable.There's  resistor network in the head (see the teribly bad picture) which makes the input impedance 100k. Otherwise you'd have reflections up the yazoo.

Offline alank2

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Re: 16 channel digital probe for HP54645D MSO?
« Reply #7 on: September 28, 2016, 05:22:09 pm »
Ice-Tea is correct, the HP part of it has a resistor network in it.  The floppy cable and adapter boards however are straight through.
 
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Offline Someone

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Re: 16 channel digital probe for HP54645D MSO?
« Reply #8 on: September 28, 2016, 10:23:12 pm »
This comes up over and over on the forum, with the same misinformation each time. Agilent produced a lengthy document on the cabling including how to design your own interfaces to it, bizarrely its still on the Agilent site:
http://cp.literature.agilent.com/litweb/pdf/5968-4632E.pdf
The document is "Probing Solutions for Logic Analyzers" should people need to find it in the future.
 
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Offline carl_lab

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Re: 16 channel digital probe for HP54645D MSO?
« Reply #9 on: September 29, 2016, 07:21:33 am »
Yes, thank you!

I think, this is what you mean:



 

Offline carl_lab

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Re: 16 channel digital probe for HP54645D MSO?
« Reply #10 on: September 29, 2016, 07:48:17 am »
What do you think, do I need the original woven ribbon cable?
What about bandwidth?

Could this cable be a reasonable compromise ($1.85/ft = $6.10/m)?
http://www.surplussales.com/wire-cable/ribbon.html
http://www.surplussales.com/wire-cable/pdf/wrc-468-2829-050.pdf
« Last Edit: September 29, 2016, 04:02:43 pm by carl_lab »
 

Offline georgd

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Re: 16 channel digital probe for HP54645D MSO?
« Reply #11 on: September 29, 2016, 09:28:43 am »
What do you think, do I need the original woven ribbon cable?

Patent paper with best description what is it an why need to use:

https://www.google.ch/patents/US4777326

Quote from the patent:
A type of cabling method used in the prior art required the use of a coaxial cable with a resistive center conductor for each signal to be probed. For example, a logic analyzer having 16 probes would require 16 coaxial cables tied together from the logic analyzer to the system under test. This tended to greatly improve the performance of the probing system, since the shielding was greatly improved, bandwidth was increased, and crosstalk was reduced. However, the result was an extremely bulky, costly, and heavy cable, making it difficult for the user to operate with a typical system under test.

On short distances up to 20 cm and on frequencies below 100 MHz an ordinary flat cable should to work.

Georg
Georg
 
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Offline JFJ

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Re: 16 channel digital probe for HP54645D MSO?
« Reply #12 on: September 29, 2016, 11:37:01 am »
I thought the original was a woven/twisted pair type flat cable?
The original cable is woven, but its signal and ground conductors are not in twisted pairs. Each signal conductor is separated by two, thinner, ground conductors. This parallel signal/ground conductor arrangement is intended to reduce the capacitive coupling between adjacent channels, to avoid crosstalk. Flat, 80-conductor, Ultra DMA hard drive cables used a similar technique to reduce crosstalk.

A twisted pair cable would be of no benefit, because your scope's digital inputs are single ended (all the signal grounds are common). A differential input would be required to reject the common noise pickup of a twisted pair.

Pod 1 Data + J Clock,
Pod 2 Data + K Clock,
Pod 3 Data + L Clock,
Pod 4 Data + M Clock,
Pod 5 Data + N Clock or
Pod 6 Data + P Clock Logic Analyzer Probe?

What's the differences?
When purchased new, the pod connectors are all the same (unlabeled). A sheet of self adhesive labels is included, so that the user can assign each pod's function.
 
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Offline carl_lab

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Re: 16 channel digital probe for HP54645D MSO?
« Reply #13 on: September 29, 2016, 04:58:24 pm »
Thanks georgd and JFJ.

A twisted pair cable would be of no benefit, because your scope's digital inputs are single ended (all the signal grounds are common). A differential input would be required to reject the common noise pickup of a twisted pair.
I think, this is correct, but regardless there are twisted pair P-ATA/UDMA HDD cables:



AFAIK P-ATA/UDMA is single ended, unlike differential SCPI, so why they use twisted pair cable for P-ATA/UDMA then?




« Last Edit: September 29, 2016, 06:04:34 pm by carl_lab »
 

Offline timb

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Re: 16 channel digital probe for HP54645D MSO?
« Reply #14 on: September 29, 2016, 05:48:27 pm »
Thanks georgd and JFJ.

A twisted pair cable would be of no benefit, because your scope's digital inputs are single ended (all the signal grounds are common). A differential input would be required to reject the common noise pickup of a twisted pair.
I think, this is correct, but regardless there are twisted pair P-ATA/UDMA HDD cables:



AFAIK P-ATA/UDMA is single ended, unlike differential SCPI, so why they use twisted pair cable for P-ATA/UDMA then?

Because, it's easier to route these cables inside a computer case, makes things look overall neater and helps with cooling. This happened in the late-90's/early-2000's.

Originally, 80 conductor (P)ATA-66 cables were flat, just like their slower 40 conductor ATA-33  counterparts. Case modders would take Exacto knives and cut little slits between every few conductors and then zip tie them together. This led to "round" cables being commercially produced; they're not actually twisted pair in the true sense (though the entire bundle may be twisted together).
Any sufficiently advanced technology is indistinguishable from magic; e.g., Cheez Whiz, Hot Dogs and RF.
 
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Offline carl_lab

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Re: 16 channel digital probe for HP54645D MSO?
« Reply #15 on: September 29, 2016, 06:03:53 pm »
This led to "round" cables being commercially produced; they're not actually twisted pair in the true sense (though the entire bundle may be twisted together).
I don't think so, this one looks to me like a perfect multiple twisted pair cable.
Each pair of wires is twisted separately, not only the whole bundle...



Anyway, now I don't think twisted pair cable will have any benefit, because the patent paper's sketches show, the special cable is NOT twisted pair.


Another point:

The linked patent paper also says:

"... is constructed from wire and fabric woven together to form a flat ribbon cable, having resistance wires for signals alternating with standard lossless wires for ground to form a lossy transmission line."

But standard ribbon cable uses high conductible copper wire... This will cause reflections as mentioned and reduce bandwidth.
I don't know, if I need the full bandwidth so using a standard cable maybe OK.

Does anybody know, if there will be a signal level problem at lower frequencies, when you use a standard copper ribbon cable instead of the original resistance wire cable?

What is the resistance over one signal wire of the original cable?
« Last Edit: September 29, 2016, 09:33:08 pm by carl_lab »
 

Offline JFJ

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Re: 16 channel digital probe for HP54645D MSO?
« Reply #16 on: September 30, 2016, 02:19:52 pm »
What is the resistance over one signal wire of the original cable?
There's only direct access to signal wires at the probe end of an original cable (the plastic shell at the scope end is riveted). The resistance between scope connector and the other end of one signal wire is 190 ohm:



Anyway, now I don't think twisted pair cable will have any benefit, because the patent paper's sketches show, the special cable is NOT twisted pair.
Another problem with using an 80-conductor PATA cable is that one of signal wires used by the HP54645D, for channel 9 (I think), would be internally shorted to ground, by the connector:



It would probably be easier to modify a used woven cable from a different HP Agilent logic analyzer - e.g. a 16510-61601 or 16510-61602 16-channel cable (they usually sell for a tiny fraction of the price of a used 54620-61601 cable).
« Last Edit: September 30, 2016, 02:24:41 pm by JFJ »
 
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Offline carl_lab

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Re: 16 channel digital probe for HP54645D MSO?
« Reply #17 on: September 30, 2016, 08:52:34 pm »
I tried to make a simulation by LTSpice
a) using a line resistance of 190 Ohms (original cable)
b) using a line resistance of 0 Ohms (standard ribbon cable)

Ri calculation: Ri = (100k+370-90.9k-250-190) Ohm = 9030 Ohm
Ci is roughly estimated 10 pF.

« Last Edit: September 30, 2016, 09:02:23 pm by carl_lab »
 

Offline carl_lab

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Re: 16 channel digital probe for HP54645D MSO?
« Reply #18 on: September 30, 2016, 09:00:47 pm »
The amplitude response does not look very flat in both diagrams...

I'm sure my model is not absolutely correct (I'm neither a RF specialist nor a LTSpice specialist)...

How can I simulate the distributed resistance and distributed capacitance of the woven special cable more realistic?

« Last Edit: September 30, 2016, 09:16:24 pm by carl_lab »
 

Offline tautech

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Re: 16 channel digital probe for HP54645D MSO?
« Reply #19 on: September 30, 2016, 11:38:20 pm »
IMO you're over thinking it.
Digital channels probes do not offer the same response and accuracy as a analogue scope probe or need to.
Keep it as simple as possible and give a DIY probe a try.
In a real use situation you're only looking at timing correlations between channels and an indication of amplitude.
Even if decoding waveforms can look gawd awful and the scope will still decode.

Real bus line or logic performance is best left to a normal or active scope probe.
Avid Rabid Hobbyist
 
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Offline timb

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Re: 16 channel digital probe for HP54645D MSO?
« Reply #20 on: October 01, 2016, 09:42:36 am »
This led to "round" cables being commercially produced; they're not actually twisted pair in the true sense (though the entire bundle may be twisted together).
I don't think so, this one looks to me like a perfect multiple twisted pair cable.
Each pair of wires is twisted separately, not only the whole bundle...



The particular cable in that picture *may* use twisted pair, but there's no electrical benefit to it, since the signaling isn't differential. The cable in your picture also has a copper-braided shield that doesn't even appear to be connected, so it looks more "X-Treme Modder" BS than just functional.

Like I said, originally we would just take flat 80-conductor ribbon IDE cables and cut slits in the ribboning so it could be zip tied (or heat shrunk) into a bundle. It worked absolutely no differently than the original flat cable. This was commonly done from ATA-66 right up to ATA-133 speeds.

Any sufficiently advanced technology is indistinguishable from magic; e.g., Cheez Whiz, Hot Dogs and RF.
 
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Offline Someone

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Re: 16 channel digital probe for HP54645D MSO?
« Reply #21 on: October 01, 2016, 11:13:29 am »
The particular cable in that picture *may* use twisted pair, but there's no electrical benefit to it, since the signaling isn't differential. The cable in your picture also has a copper-braided shield that doesn't even appear to be connected, so it looks more "X-Treme Modder" BS than just functional.
Twisted pair still rejects induced noise and improves crosstalk when used with single ended signalling:
http://www.ti.com/lit/an/snla164/snla164.pdf
 
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Offline carl_lab

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Re: 16 channel digital probe for HP54645D MSO?
« Reply #22 on: October 01, 2016, 02:35:31 pm »
Thanks to all!
I will simply test different types of cable, when I get the oscilloscope and the "HP Pod Data+Clock" module incl. probe leads.
« Last Edit: October 01, 2016, 02:43:29 pm by carl_lab »
 


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