Let's Make Robots!

$10 logic analyzer

After a lot of umming and ahhing and general procrastination I decided to dig this out of the cupboard to give it a whirl again.

I replaced the 74LS00 with a max232 but it still didn`t work. A little bit of fiddling later and I found that I had forgotten to write the fuse bits which is why it didn`t work. Being an impatient sort and eager to get this thing working I accidentally wrote the wrong fuses and somehow locked myself out of the chip. DOH! I tried a few different crystals and also an external clock but just can`t bring the chip back. And to cap it all off the new shuttle PC I got doesn`t have any serial or parallel ports. I can see the purchase of an STK500 and usb to serial cable in the near future.

I`m not sure what I could have done wrong but this project is on indefinite hold until I get a new IC or a HV programmer to reset the fuses.

Lesson learnt.. triple check fuses before programming.

 


 

So lacking a scope and the money to buy one but wanting to see what was happening to the I2C bus for Ozzy, I thought I would give this cheap DIY logic analyzer a go.

Most of the parts I already had on hand, but I did buy some IC hooks from sparkfun to use. They are small and fairly cheap feeling but should do the trick fine.

It`s finished, as per the circuit diagram on the original site, but something is up with my serial port. It doesnt output a positive voltage until it finds a proper device (or something I dont understand). I found a small program called pwon to run which forces power out of the serial port, and that works OK but then when I run the program it can`t connect to the device.

I don`t know if running this pwon program makesthe PC think that the serial port is busy, or if it`s a problem somewhere else. I`ve got to try using external power first, and if that doesn`t work swap the 74LS00 for a real rs232 IC.

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Rudolph's picture
Check out this guy's post about using an Arduino to save borked-fuse chips without having to buy a high voltage programmer. I don't know jack about the mega16 but I imagine it might be finagled to work with a bit of code bashing.
Myc Holmes's picture

I was loking for the number of samples it stores and the baud rate it send the data over the serial port. There is nothing in the files

Any experience with these numbers? 

It might be p;ossilbe to PICAXE this. Of course, not at a 1 mHz sampling rate but, still high enough with a 20 mHz 40X1 to be useful.

Myc 

Zanthess's picture

Wow, that is cheap! I think I might have to build one :)

I've been oogling the 10 channel analyzers. Don't think I'd be able to max it out... but it would be tempting :3

OddBot's picture
The only other thing I can think of is the bios / hardware configuration settings for the serial port.
OddBot's picture

I looked at the project and schematic on the website. The 74LS00 isn't your problem. Where are your diodes that convert the power from the port to 5V?

If they are there but hidden in the photo then try adding a 100uF capacitor between the +5V and 0V of your processor.

ezekiel181's picture

The diodes are on the far left, in a hard to see place. Theres a diode to stop negative voltages, and a zener to clamp it down under 5v.

I will try adding the cap you suggested. Hopefully it helps cause I dont have the motivation lately to pull it apart to check wiring.

Rudolph's picture

Coolness. Let us know how it works. I've been planning on building a cheap one, and that's one I'd looked at. I finally decided on one using TFLA-01 (cause I have a linux machine on my workbench, and it's got a parallel port). I'm still waiting on my parts from (blankity #@$%) futurlec though. All I need is the flippin 74HC245... I could probably pick one up at the electronics store downtown, but it'd cost six times as much...

I really would prefer one of these though.