I hit a nasty little problem with I2C recently, and managed to solve it with a great deal of trial and error and not a little luck.
With an oscilloscope, it would have been a great deal easier, so I’m starting to look around for one.
I have practically no work or storage space, so it has to be a small USB scope. Has anyone had any experience with the Parallax and/or BitScopes (or any other suggestions?)
Cheers,
Matt.
I have a Parallax USB OptiScope, and have found it invaluable in diagnosing signal issues any number of times. There are a few little quirks to it, as compared to a “traditional” phosphor-based scope, but it has plenty of features found on more expensive storage scopes to make up for it. I don’t have the kind of budget to afford a stand-alone scope, but the USB scope makes for a nice dual-channel diagnostic tool (with trigger channel) which has been a very worthwhile addition to my toolkit.
The Parallax USB scope I heard is great for light duty stuff like reading PWM signals from a servo and serial comunications, etc. If you need to read high speed signals, then a USB scope will not be able to read these signals as it will be out of its range. (to slow)
If high speed signals is in your future, then a 1 or 2 Ghz O-scope is needed. Usually, the faster the more expensive the scope.
For hobby stuff, I think the Parallax USB scope is ideal. Andy has one and he likes it.
This is a good point, and one I had intended to put in my response, but forgot to mention.
One of the quirks of the USB scope is its capture speed - it’s adequate for looking at many types of logic signals, PWM outputs, etc., but it’s a bit too slow to handle some other types of signals. Still, it’s great for a lot of different uses, and the price is right. It’s a very useful addition to the toolbox, and while it may not have the ability to measure every signal you may work with, it’s still a great tool, and a whole lot better than trying to diagnose things with just a multimeter.
I completely agree, It is a fantastic tool for hobby robotics. I wish I had one when I was toying with my sensor board.
You can get high speed USB oscilloscopes, but like CRT scopes, you have to pay for it. They are available up to 250MHz at the moment as far as I know of (not sure if there are any faster).
Here is a full range of them. I’m not saying these are your best options, because they cost a lot more than say the parallax one for what you get, but if you need a high speed one, you might want to search around.
As SN96 mentioned, I have that same USB scope from parallax. The speed is acceptable for i2c as long as your problem still manifests when you slow your i2c clock speed down to a crawl. When it’s tooled all the way up (400K+) this scope is essentially useless.
That said, every time I’ve pulled the scope out for an i2c problem, I ended up solving the problem via software debugging rather than decoding the scoped signals.
Thanks everyone.
There have been quite a few times I really could have used one, not just with I2C, and where the multimeter just wouldn’t cut it.
I think in terms of bang-for-buck, the Parallax one seems to be the way to go for me at the moment, I’m really not looking to move to anything much over 100mhz at the moment.
I hope you meant 100kHz! 
kHz. Yes.
In twenty years I’ve only needed more than a 20mhz scope on a handfull of occasions. What on earth are you doing that you need ghz signalling?
you just can’t know the number of times in nearly 25 years where I’d have almost given up body parts to get away with needing only a 20MHz scope. 
of course that is at work… at home I’ve got an old tek hand held thingy that might actually be a 20mhz but I use it so rarely I’m not sure. It is much too easy to bring stuff to work and have something interesting to do at lunchtime (provided I’m not already in gEEk overload mode like I have been recently).
Actually, going over everything and doing a little more research, I think the BitScope is better value. It is 3x the price of the Parallax unit, but seems to have a good deal more in the way of features and still comes in under $500.
Its also had consistently good reviews, and all the software is open source.
I have no idea what speed signals Matt might be working with. I am not an O-scope guru so I was being broad in the speed sector. At my work 1 and 2ghz scopes are essential. We have down-hole modem boards (oil wells) that need to communicate in a timely manor. It depends on the application as to what speed/range is needed.
Matt,
That BitScope certainly does look better. The only good thing I’ll say about the Parallax one for robotics however is that it’s completely powered via USB. Might be a trivial distinction but in most cases it can be quite handy occasionally.
Just noticed the smaller one (310) can be USB host powered.
Thanks again to everyone for their replies.
Needing an external power supply isn’t too much of an issue for me, I work on the same desk the PC is on anyway, and have a few spare outlets there.
I’ve narrowed the selection down to the 50 and 310 models - they also both have waveform generators, which is a neat thing to have. Both have pros and cons, largely price and features.
I’m working with moderately slow speeds, and I don’t think that’ll change much. I2C from the stamp was running at 350kbit/s originally, though I’ve since slowed it down to 100kbit/s. Bumping it back up would be fantastic, especially as I want to add a few more I2C devices (3d accelerometer and an FPU).
I saw the Bitscope article in Circuit Cellar. They look to be the a good mix of features for the hobbyist. I think a new one was coming out…
Sorry to dig up this onld thread, but I am thinking of buying myself a better PC powered Oscilloscope. I currently have the Parallax USB one and it works ok for many of the things that I have tried to debug. However I often wish it had a much larger buffer so I could more easily capture what I am trying to see. Probably 90+ of the time I am interested in logical 0 or +5v logic levels (serial, I2C, servo output/inputs, etc). It is only ocasionally that I try to capture analog inputs.
So would should I look at the bitscope versions, or maybe VELLEMAN version that Jameco sells or are there others that you would recommend?
Thanks
Kurt
I have the parallax scope too. If I were to buy a diffrent one it would probably be a “picoscope.” You should be able to find this on google. They are a little more expensive than the parallax scope but seem to have much more capability.
