MSO Scopes - Do digital channels allow protocol decoding - Page 1
Forum section: electronics / testgear

author: rfdes
author_rank: Regular Contributor
timestamp: October 29, 2017, 12:15:16 am
content: I'm MSO shopping and it is not clear, but I am under the impression that many of the low cost MSO scopes do not support protocol decoding when using the digital channels. Only the analog channels will allow protocol decoding. Is this correct or have I misunderstood?In fact, just how much functionality does the logic analyzer part of the scope have?Your opinions are appreciated.Jim

author: rhb
author_rank: Super Contributor
timestamp: October 29, 2017, 01:06:56 am
content: I *think* the Instek MSO-2000E series will decode in the LA. I'm about to buy one, so I'll know fairly soon. The manual says it will display a table of bus events in binary or hex. So rather "flint knives and stone axes" but I'm hoping "good enough". I haven't read hex dumps in anger for 35 years, but I'm sure I can if needed.You might want to look at the GW Instek MSO-2000E & MSO-2000EA user manual starting about p 124.I strongly suspect that proper decoding will require operating the scope via the SCPI interface. But Keysight and Tektronix charge as much for a software key as Instek charges for a scope with all the available features enabled.The PC software is OpenWave-GW and is a collection of Python scripts that you can download from github.In any case, I *strongly* urge you to read all the manuals for anything you're considering buying and compare what those say to the spec sheet. There are a lot of errors no matter who the OEM is. Then ask the OEM sales staff questions.

author: JPortici
author_rank: Super Contributor
timestamp: October 29, 2017, 06:36:25 am
content: keysight MSOX-2000 cannot decode from digital channels. ()keysight MSOX-3000 onwards can.All Lecroys with digital channels i've seen can decode/trigger from digitalwell, even if you can't decode, having at least 16 lines to look at parallel ports interfaces is neat, control signals in the analog channels..(msox-2000 has only 8 channels)

author: Howardlong
author_rank: Super Contributor
timestamp: October 29, 2017, 06:49:53 am
content: The Rigol MSO1074Z decodes and triggers on digital channels, analogue channels, or a combination of both. Just be aware of the decoding limitation of just what’s on the display at any one time, and that the decoder aggressively subsamples, making the event list feature almost completely useless. The trigger appears to be done in hardware and so isn’t affected.The MDO3000 also decodes and triggers on any combination of digital and analogue channels, and assuming you’ve set up the acqusistion depth appropriately, it will fully decode as expected, except it is horrendously slow on deeper memory. Also note that as soon as the digital channels are switched on, irrespective of whether you’re using decoding, the update rate drops to barely a few dozen wfm/sec.

author: simone.pignatti
author_rank: Frequent Contributor
timestamp: October 29, 2017, 08:11:58 am
content: While reading the data sheet of the units of your interest take a look at the specs under logic channels, you will notice that sampling rate and memory are different (lower) than analog channels. Also don’t forget to get a MSO unit with logic probe included as standard.

author: MrW0lf
author_rank: Frequent Contributor
timestamp: October 29, 2017, 09:13:59 am
content: Picoscopes can decode digitals, analog. Multiple decoders at time, different thresholds. Full memory decode etc.

author: bson
author_rank: Unknown
timestamp: October 29, 2017, 10:13:02 pm
content: Quote from: Howardlong on October 29, 2017, 06:49:53 amIt's worth noting here that "on display" means the actual decimated trace data, not the acquisition record gated by the display window. This means there are many transactions it's totally incapable of decoding, such as a 512 byte write to an i2c eeprom or determining if it was properly concluded (ACK-NAK). There's simply not enough display trace points for it. It also lacks segmented memory, so you can't tell it to go capture say 10 transactions and then take a look at what you got. So a device that gets accessed every few seconds will pose a problem if you want to analyze a sequence of transactions - you need a timebase high enough to capture the traffic, but don't have enough buffer to keep going to 30 seconds. A PC LA will handle it without issue, but then you have a different problem if you (the OP) want to also capture analog inputs and outputs (say for I2C ADCs and DACs) alongside the protocol traffic. If you suspect a DAC output glitch is caused by rail glitches for example you may want to capture protocol traffic using digital channels, analog output pre-reconstruction filter, post filter buffered, any voltage reference (input or output depending on whether internal), and the rail voltage. You may even want to use a complex trigger that includes both protocol traffic and rail voltage if it's really rare - then leave it running overnight for up to say 99 segments or until memory is full to see what it picked up.

author: rhb
author_rank: Super Contributor
timestamp: October 30, 2017, 12:09:13 am
content: Quote from: bson on October 29, 2017, 10:13:02 pmDo you have any knowledge of the Instek MSO-2204EA in this regard? The use case you describe is exactly why I'm buying an MSO. I've been reading manuals and datasheets until my eyes are bleeding. The Instek *seems* the most capable without spending $5k. If I've got to spend $5k I'd like to find out without having to pay return shipping on an Instek.

author: TK
author_rank: Super Contributor
timestamp: October 30, 2017, 12:26:30 am
content: Quote from: rfdes on October 29, 2017, 12:15:16 amThere are few serial protocols that DSO's can decode that require more than 3-4 signals. If you get a 4 channel scope, you will be able to decode all serial protocols (UART, SPI, i2c, CAN, LIN, etc) using analog channels.

author: nctnico
author_rank: Super Contributor
timestamp: October 30, 2017, 09:08:54 am
content: Quote from: TK on October 30, 2017, 12:26:30 amThe point of using digital channels for decoding is that you are often interested in analog stuff as well and for that you need extra channels.

author: Pinkus
author_rank: Frequent Contributor
timestamp: October 30, 2017, 09:35:50 am
content: And don't forget that you might need a scope where you can check / trigger on two different protocols at the same time!Example: what I frequently need is monitoring a CAN controller (driven by SPI) and the CAN data traffic. For this you needMISO to check what the CAN Controller sends to the µCMOSI to check what the µC sends to the CAN ControllerCLKCSINTandCAN H and/or CAN L to check the CAN trafficAnother example:or you want to see a SPI bus data traffic with two partners (e.g. display and flash) where you need MOSI / MISO and CLK and DC and CS1 and CS2 = 6 channels.Thus important is:1) decoder on digital channels (analog channels only is often not enough)2) Be sure your scope does have at least 2full duplex decoder!(beware of the scopes like the Rhode&Schwarz scopes where you need to use one protocol decoder for MISO and you need to waste the second one for MOSI (because they did not integrate them as full duplex decoder) - so no chance to decode two different SPI bus partners (or to decode CAN output in my first example).However: all modern scopes with decoders of the last 4 years such as all Rigols and Siglents usually does have full duplex decoder, no problem there - it is only the R&S where you need to be careful.

author: Howardlong
author_rank: Super Contributor
timestamp: October 30, 2017, 01:35:31 pm
content: Quote from: bson on October 29, 2017, 10:13:02 pmISTR I calculated it as sampling at twice the display resolution, but it was some time ago. Either way, what you're saying is what I was alluding to about it "aggressively subsampling", I just wasn't as clear as you!

author: JPortici
author_rank: Super Contributor
timestamp: October 30, 2017, 05:56:39 pm
content: Quote from: Pinkus on October 30, 2017, 09:35:50 amthis is most important!

author: Keysight DanielBogdanoff
author_rank: Unknown
timestamp: October 30, 2017, 07:56:38 pm
content: In general, yes.Quote from: JPortici on October 29, 2017, 06:36:25 amSome context on this. When the 2000 X-Series was originally released there were no decoding capabilities at all. It got added in a few years ago as a software upgrade, but the hardware didn't change. The MegaZoom chip does all of our decoders and digital channel stuff, but it can only do one of those at a time. In the 3000T X-Series and up we put in multiple MegaZoom ASICs to handle this, but for the 2000 we couldn't make that hardware change without significant engineering effort.

author: KrudyZ
author_rank: Frequent Contributor
timestamp: January 12, 2018, 05:12:57 pm
content: Hi Daniel,what is the maximum SPI data rate supported by the MSOX3000A series?The Keysight 5989-5126EN publication, which seems to be for the 4000 series, mentions 25 Mbps, but is this a hard limit and does it also apply to the 3000?The R&S RTB2004 also has a 25 Mbps hard limit according to their tech support.This is really a bit limiting for modern embedded systems...Rudy

author: Howardlong
author_rank: Super Contributor
timestamp: January 12, 2018, 08:01:07 pm
content: Quote from: KrudyZ on January 12, 2018, 05:12:57 pmThe same 25MHz applies to the MSOX3000A I’m afraid, somewhere on the forum I had the same question and was a little surprised especially as it’s a hardware ASIC decode. ISTR from experimentation it flakes out at about 30Mbps on SPI.https://literature.cdn.keysight.com/litweb/pdf/5990-6677EN.pdf?id=2002874#page10Bizarrely, the Rigol DS1054Z will decode and trigger a 50MHz SPI signal.

author: KrudyZ
author_rank: Frequent Contributor
timestamp: January 12, 2018, 09:35:22 pm
content: Do you think this limit might be 25% higher for the 1GHz version, since it ups the sample rate from 4GHz to 5GHz?It would be nice to know since my current project I'm working on uses a 30 Mbps SPI bus.Anyone here that would be willing to test this?Thanks,Rudyp.s. If I remember correctly I think I saw mikeselectricstuff in one of his videos showing the RTB2004 still decoding at 50 Mbps, despite what their tech support told me...

author: TK
author_rank: Super Contributor
timestamp: January 12, 2018, 10:10:31 pm
content: Quote from: KrudyZ on January 12, 2018, 09:35:22 pmI think the Zeroplus LAP F1 can do SD 3.0 @ full speed. 40 channels, 1GHz sample rate. It should be able to do SPI up to 80MHz comfortably

author: Howardlong
author_rank: Super Contributor
timestamp: January 12, 2018, 11:21:17 pm
content: Quote from: KrudyZ on January 12, 2018, 09:35:22 pmI've just looked at the problem I had, it was on an MSO7104B.https://www.eevblog.com/forum/testgear/spi-speed-limit-on-agilentkeysight-dsomso-500060007000/I have just run a similar test on an MSOX3054A and MSO7104B at the same time. Certainly at 50MHz there is a problem, but the MSOX3054A seems to manage much better, but both lose bits. At 40MHz I can see the MSO7104B lose bits but not the MSOX3054A. At 35MHz things seem stable on both.This was on the digital pods with 3.3V logic level signals.Image is at 35MHz.

author: Howardlong
author_rank: Super Contributor
timestamp: January 12, 2018, 11:35:44 pm
content: Here's an example at 50Mbps where it misses a clock.

author: nctnico
author_rank: Super Contributor
timestamp: January 12, 2018, 11:50:59 pm
content: I had no problem decoding 125MHz SPI on my 200MHz GW Instek GDS-2204EThere is an MSO version as well but since I don't have that I didn't test it.

author: KrudyZ
author_rank: Frequent Contributor
timestamp: January 13, 2018, 02:49:47 am
content: Thanks, that's great to see.I am trying to decide between the Agilent MSOX-3000A and the R&S RTB2004.Rudy

author: ahbushnell
author_rank: Frequent Contributor
timestamp: January 13, 2018, 03:27:07 am
content: Quote from: Howardlong on October 29, 2017, 06:49:53 amI have the Rigol MSO1074Z and it decodes and triggers. I bought the options so if you don't get the options it will reduce the feature set. Read closely.

author: Howardlong
author_rank: Super Contributor
timestamp: January 13, 2018, 06:49:43 am
content: Quote from: ahbushnell on January 13, 2018, 03:27:07 amI’m not sure I follow? Yes, it decodes and triggers, but it has a limitation that on decode it only decodes what’s currently displayed, and subsamples that data, so only a few bytes can be decoded at any one time.

author: ahbushnell
author_rank: Frequent Contributor
timestamp: January 13, 2018, 04:40:25 pm
content: Quote from: Howardlong on January 13, 2018, 06:49:43 amI agree. I was just indicating my experience. But some of these features are add on's.