Daily Archive for January 17th, 2012

ACRIS Board Rework: Fixing Faulty Communication

A while back, I identified a major issue with ACRIS’s communication network. The MAX485 chips that I use can switch between transmitting data and receiving data. To switch modes, you simply pull two pins either high or low. I tied these pins directly to the ATMEGA so that I could allow the LED controllers to talk to each other in future firmware revisions.

However, when the ATMEGA first starts up, the state of this pin is unknown, so a blip in the logic power can cause multiple devices to want to transmit data. As a result, the entire communication network just sort of stops. 🙁

But, by adding a simple pull-down resistor to those transmission-enable pins, it will make the default state of the MAX485 to receive data.

The rework on my lights was pretty difficult because everything was pretty tightly packed. But, I was able to remove the main body in order to get at the MAX485 chip.

Afterward, soldering a 10K pulldown resistor was not hard.

Re-assembling was a pain in the neck, though. I kept losing the plastic spacers and doodads.

I Can Multimeter Things Again!

Okay, so I have a ton of multimeters — I think around 5. I’m not really sure why; I think almost all of them I’ve gotten for free except for my favorite one, a Sinometer VA18B, which has a ton of functions and can also send data via USB.

But, I left a battery in it and the terminals got so corroded that they were destroyed. 🙁 So I thought I could quickly solder a new connector to the board.

Rule #1 of electronics: DO. NOT. TAKE. SHORTCUTS.

I didn’t want to take the board out of the housing as shown above, so I thought I could just desolder the existing wires and feed the new ones in. I ended up lifting the pads, making those spots that I was supposed to solder completely useless. Furthermore, I accidentally had my iron temperature set too high, so I was melting a lot of the insulation (the high temperature also caused the pads to lift).

So, I fixed the problem by basically tacking the wires onto the sides of components that were electrically close to the original holes. Mechanically, this is a very bad idea; the connections are very brittle. Take a look at this mess:

And to top it off, the replacement 9V connector I had was too big for the plastic slot, so the battery cover didn’t fit on very well at all. Ugh.

So, morals of the story:

  1. Don’t leave dead batteries in your devices.
  2. If you’re going to do rework on a board, keep the iron temperature as low as possible.
  3. Take the time to undo the extra few screws to have better access to the board. Otherwise, it’ll take you more time in the end and you’ll end up with a worse result.
  4. On the plus side, my multimeter does in fact work again, so I’m happy about that. 🙂 🙂 🙂