Knockoff "Saleae" analyzer DC offset

[staze]

Hi All,

Question for people. I'm trying to use one of these ancient knockoff Saleae's (actually labeled as a usbee ax pro) to look at an SMTPE signal, but I'm not triggering at all. I hooked up my scope, and I noticed there's a 3V DC offset that goes away when I unhook the LA.

First I found that my previous USB cable didn't have continuity between the shell at both ends, but then I switched to another cable that did, and it STILL had the offset. Looking at the circuit, I noticed the resistor and cap that tie the LA's GND to the USB GND were also missing, so I added a 100R resistor. Same result still.

I can literally see the offset if I hook a DMM between any of the channels and gnd on the analyzer.

Has anyone else seen this? Is there something I'm missing? I've checked a few channels, all have the same result.

Thanks!

[hamster_nz]

Might be a 'feature':

The original Logic is also the only Saleae device to use pulled up inputs. All other Saleae devices have inputs that are pulled to ground. This could cause issues with a circuit that relies on very weak pull-down resistors or when the circuit needs to correctly detect a high-Z state. In these cases, modifications to the circuit may be required. For instance, an extra pull-down resistor could be added to counteract the original Logic's pull-ups.

https://support.saleae.com/hc/en-us/articles/208667766-Connecting-a-Logic-Analyzer-to-My-Circuit-Appears-to-Interfere-with-Its-Operation

What sort of signal are you looking at? Is the signal AC coupled? If so, a LA might not be of much use....

[Mechatrommer]

to look at an SMTPE signal, but I'm not triggering at all. I hooked up my scope, and I noticed there's a 3V DC offset that goes away when I unhook the LA.

the knockoff saleae inputs are floating, you need to add pull down resistor to each input (100K - 1Meg ohm each), but when you connect the input to other low impedance pin (output) of other digital circuit during probing, it should trigger just fine even without pull down. we dont know how do you connect those pins and what type of saleae knockoff we talking about.

[staze]

It does appear to be AC coupled (±1V around 0V). It's Linear Timecode (SMPTE specifically). I was hoping the Saleae knockoff would be able to decode it (I found a decoder someone wrote, compiled it, and added it). Sadly, doesn't seem to work.

Good info about the floating. I don't recall running into that years ago when I first tried the unit. But does make sense that it shouldn't care. It probably just isn't sensitive enough to pick up the 1V (though it'd probably only see half the data).

Trying to figure out how else I can troubleshoot this.

Thanks for the help!

[Gyro]

to look at an SMTPE signal, but I'm not triggering at all. I hooked up my scope, and I noticed there's a 3V DC offset that goes away when I unhook the LA.

the knockoff saleae inputs are floating, you need to add pull down resistor to each input (100K - 1Meg ohm each), but when you connect the input to other low impedance pin (output) of other digital circuit during probing, it should trigger just fine even without pull down. we dont know how do you connect those pins and what type of saleae knockoff we talking about.

The knockoff Saleae inputs are pulled up to 3V3 by 100k resistors. You'd want to pull them down with ~10k if you want them to go low.

https://sigrok.org/wimg/6/67/Usbee_ax_clone_pcb_front.jpg

[Mechatrommer]

Mine is usb blaster knock off cypress chip iirc, modifiable to saleae knockoff. If you connect disconnect to low or hi state it will stay at low or hi, hence floating... ymmv, no picture at OP..

[Gyro]

Interesting, I hadn't seen those Cypress Ez-USB based USB Blaster clones. There also seem to be CPLD based ones in the same package (although there's no chance that they would work with Saleae or Sigrok LA). I found a Youtube teardown...
 https://youtu.be/M0K9FYcF-T8

They do indeed seem to be missing the pullup resistors of the ('genuine' ) Saleae clones (including the OP's AX pro marked one), although they do retain the HC245 buffer. I'm not sure if the USB Blaster clones include direction direction control of the HC245, It's hardwired to 'input' on the Saleae clones. I don't know what other differences it has in comparison, it's obviously close enough to work, excluding the input float behavior. The original genuine 8 bit Saleae didn't include the buffer and, I think, had pulldowns instead of pullups.

Ymmv indeed, if in doubt pop the lid off, you may end up with a CPLD based USB Blaster clone version instead!