USB Confusion

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23 Apr 2006
Posts:

Hi,

For my application I need to collect a large amount of data into the scope and then transfer back to the PC to analyse in my software. We are currently using some Pico 3206s for this but would really like more than the 1 Meg memory it offers (you can never have enough memory!).

The cleverscope looks ideal as I should be able to use the drivers to collect 4 Meg of data into my application (written in C). I am a bit confused however about the USB specifications - in your comparison table you say that the cleverscope has a USB2.0 interface and the 3206 has a USB 1.1 port, yet reading through these forums it seems as if this is the wrong way around and the Cleverscope is USB1.1 (the 3206 certainly is USB2.0). As I need to transfer the entire buffer each acquisition the transfer rate is important so could someone please clarify.

Thanks
CW

24 Apr 2006
Posts: 398

Hello Prof Warner,
Both the Pico 3206 and the Cleverscope CS328 have a USB 2.0 compatible USB port. Currently both systems transfer data at 12 Mbit/sec, using the USB Full Speed rate (the terminology is very confusing). However, beginning in about 8 weeks time, we will be delivering the Cleverscope CS328A, which operates at the USB 2.0 High Speed rate of 480 Mbit/sec, achieving about 40 Mbyte/sec throughput.

So currently the transfer time will be about the same for both. Unfortunately, if you want the higher speed from Cleverscope, you will have to wait for the updated model release.

24 Apr 2006
Posts:

Thanks for the reply - I will wait until you have the CS328A so that I can use the 480 Mbit/sec USB link, fast download is important to me as I need to collect so much data. I was still a bit confused about the 3206 USB transfer rate so did some checking - just to correct you, the 3206 does communicate at 480 Mbit when plugged into a PC with a USB2.0 port. My software application downloads 1 million samples in around 25 milli-seconds which would not be possible with USB1.1.

I also asked Pico support to confirm if the data rate is 12Mbit or 480Mbit and their reply was ""the 3206 (like all our USB oscilloscopes) was designed from day 1 to be a true USB2.0 device and communicates with the PC at the 480 Mbit data speed you mention""

I look forward to your new product.
Thanks
CW

27 Apr 2006
Posts: 398

Thanks very much about the Pico information - it seems we were confused too (base on website information we have interpreted). We will let you know when the CS328A is available. A question for you - the CS328 and CS328A have provision for using RAMS twice as big - giving 8 MSamples of memory. In your opinion would the Cleverscope be more valuable with the higher quantity of RAM, at the expense of a higher price?

2 May 2006
Posts:

For my application, the more memory the better so I would be willing to pay a premium for more memory. If doubling the memory doubled the price it would be a bit steep, but if the increase was 20 to 30% then we would be interested.

Another question - will the new USB2.0 version of the scope still be powered from the USB port for easy field use with our laptops?

Thanks
CW

5 May 2006
Posts: 398

We would expect the increased memory would add about US$50 to the price.

No - we are not powering the scope off the USB port. The main reason for this is safety. Our power supply connects the front panel BNC earth to the mains earth. We strongly recommend all test stations that are involved in measuring high voltages (such as mains powered or variable speed drives) include a ground fault interrupter (or RCD) to turn off power if a ground fault is detected. In any case we expect a fuse in the mains power system. If a user inadvertantly connects the scope probe earth to a live terminal, the ground current will either trip the ground fault interrupter, or blow the mains fuse, and operator safety is not compromised.

If there is no ground connection (for example the oscilloscope is powered from the USB port), the PC will become live, representing a serious threat to the operator. We could not determine a solution to this problem other than fully isolating the input BNC's - which can be done, but at quite considerable cost.

For applications where people want to make non-ground referenced measurements, we supply a differential probe, rated at 1300 VAC isolation.

If you want to battery power the cleverscope, you can do this from an external battery (such as vehicle power), or we can sell a battery pack (same enclosure as the cleverscope, which sits under the cleverscope, and contains rechargeable batteries, charged by the standard Cleverscope power supply). This way of doing it also lengthens the PC battery life.

Thanks again for your comments.

20 May 2006
Posts:

BartSchroder said...
No - we are not powering the scope off the USB port. The main reason for this is safety. Our power supply connects the front panel BNC earth to the mains earth.

If there is no ground connection (for example the oscilloscope is powered from the USB port), the PC will become live, representing a serious threat to the operator. We could not determine a solution to this problem other than fully isolating the input BNC's - which can be done, but at quite considerable cost.

So if I understand your comments, the BNC common connects to earth ground through the power supply, but does NOT connect to the PC ground (because that would cause a hazard). How much differential can exist between PC ground and BNC common? Is there any internal protection in case I accidentally brush the scope probe's ground lead across an earth-referenced power supply in my circuit while troubleshooting (for example +12v)?

26 May 2006
Posts: 398

Hello JMB,
There is a very fat ground plane between the front panel BNC's and the power supply earth. It is capable of conducting many Amps (I don't know exactly how many, but at least 30). There is no fuse, because this would compromize the safety aspects. If you brush the ground clip against a 12V power supply, the outcome will depend on the 12V power supply. It will either go into current limit, or blow it's fuse. Imagine if this was a 240V phase connection instead - you would expect the phase supply to be turned off, to remove the hazard (by fuse or residual current detector). The USB ground is still present, but with higher impedance than the mains ground (in includes a low current common mode choke and a small series resistor.
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