Monthly Archives: March 2011

Fraudulent SSL Certs & Revocation

I just read news that fake SSL certificates were issued by Comodo CA, but more interestingly, browser updates were issued to blacklist the certificates. Why this was necessary since we already have a protocol for doing just that?

I found out from this post on the torproject blog that talks about how OCSP is not properly implemented in browsers:

The browsers treat revocation errors as soft errors and a MITM is deadly for revocation. The browsers believe they have to treat them as soft errors because the CAs are failing to do their job properly and are almost entirely unaccountable.

Here’s how some other browsers fare when OCSP fails.

MIDI to USB (Serial) Converter

MIDI is actually just serial data at 31,250bps in 8N1 format that is transmitted over a 5-pin DIN cable. This means you can receive MIDI data from your musical instrument using a serial port, or an FTDI cable.

Receiving MIDI data over the FTDI cable doesn’t magically turn your USB serial device into a MIDI device – you need to be running a software bridge or a driver that pretends to be a virtual MIDI device emitting these messages. For this purpose, I shall use the Serial MIDI Converter from SpikenzieLabs.

I’m using Mac OS X 10.6.6 and the latest Java update, so I didn’t need any extra JAR files.

Wiring It Up

The circuit is relatively simple – you need the DIN socket, an opto-isolator, 2 resistors, and optionally a diode. In my case, parts came from a scrap bin, so I used a 330 ohm resistor instead of a 220 ohm for Rb and a 1K for Rd instead of 280 ohms. For the opto-isolator, element14 had some non-RoHS CNY17-2 on sale, so I just used that.

Note that the RXD output is only meant for interfacing with a TTL circuit like an FTDI chip/cable or MAX232 transceiver, not the RS232 serial port directly.

You can find the same circuit diagram (with different values & parts) in the official MIDI Electrical Specification Diagram.

Continue reading →

Optical to analog audio converter

The PCB I made recently was for this project that converts audio from a TOSLINK input to analog stereo.

My new TV has no analog audio outputs – only an optical one (and its speakers suck), and the “home theatre” system (a Sony DVD player with a 5.1 speaker system) accepts only analog stereo. If I bought a home theatre system I would have made sure it has an optical input, or at least “line in” inputs for the 5.1 so that I can use the amp. This little project was designed to bridge the 2 systems (with loss of audio quality).

The circuit is really simple – most of the heavy lifting is done by 2 main ICs – the DIR9001 digital audio receiver and a WM8762 DAC.

Continue reading →

iteadstudio PCBs

This is my first try designing and fabricating a PCB, so I decided to use iteadstudio.com since they are the cheapest available – US$12 for 10 pieces of the same 5 x 5cm board.

I tried my best to make the design error-free because I didn’t prototype the circuit first (not really a good idea). I’m still waiting for the components to come in, so I can’t tell if there are any problems yet.

I sent the gerbers, and they shipped the order in about 7 days. It arrived after 2 weeks (not sure if they were clearing the backlog from Chinese New Year).

A couple of things to note:

  • the silkscreen is white (on green) despite the site claiming it is black
  • the board will be routed to your outline

It was said that seeedstudio uses the same board house, so I used the DRC file and the CAM job file available in their web store where they offer the Fusion PCB service. The DRC helps you ensure that the board meets the fab house requirements. iteadstudio does not provide any such files. As the CAM file description states, the drill holes will not line up with the other layers, but the board house seems to have no problems with it.

Boards that are E-tested will have a marking. This marking can be seen here on the top edge. Oddly only 4 boards are marked, even though the site states that 5 boards will be tested.

Here are the front and back of the PCB. The silkscreen is slightly skewed, as you can see from the IC3 rectangle. I designed the PCB shape to fit into a case I bought locally, which unfortunately did not have any technical drawings. The good news is the board fits in perfectly.

The vias are all 0.4mm and were tented. I thought this was an error, but I re-checked the design and found that the seeedstudio DRC will not generate a soldermask stop for holes smaller than 100mils.

I love the QR code – it looks really nice against a green board, though I think it would look even better on red (like SparkFun boards) or blue (like Arduinos).

I will talk more about what this board does, as soon as I assemble and test it.

Update 12-Mar-2011: Apparently Dave Jones has also used their services and they mucked up his board by reducing copper pour around holes.  He talks about the problem in episode #155 (skip to 9:20 mark).