Logic probes can be anything from useful to critical depending on the project, but if you don’t have one already, why not try building one? Instructables has a guide to building a logic probe using a PCB, an assorted lot of resistors, capacitors, regulators, DIP sockets, a 9V battery clip (and 9V battery), and a hex inverter. Instead of using an LED like other simple logic probes, though, this kit uses a 7 segment LED display to show whether logic high (H) or low (L). The schematic makes putting this together only slightly more complicated than snapping Legos together. Of course, the resulting device is not as versatile as a LogicPort logic analyzer, but it is still useful.
As promised earlier, the Near Future Laboratory has published an overview of the Logicport Logic Analyzer. They’re using the Playstation 2 analysis as an example. The Logicport uses “interpreters” to define protocols. It has I2C/TWI, SPI, RS232, and CAN 2.0A/2.0B, but you can build your own interpreter based on these. You can specify bit order and the format you want the data in. Slave interpreters can be used for specific tasks: with the PS2 they were used to just show the fifth byte, which is the actual button press.
“Triggers” are used to signal specific activity. On the PS2, one was attached to the falling signal on the slave select line. This event means the master is about to start sending data.
The final area worth exploring is “measurements”. These can be frequency or arbitrary time intervals between events. The Logicport has multiple ground connections to eliminate noise from the signal and you’ll have to play with sample rate and logic level to get things running smooth. It’s nice to see how-tos written from the perspective of someone just getting started with the tool.
The people at Near Future Laboratory are trying to spoof the behavior of a Playstation 2 controller using just a microcontroller. They wrote some initial code using the controller info found here, but decided the best way to figure out what was going on, was to probe the interface. They’re using a Logicport, which features 34 channels plus two clock channels. They only need six channels because the PS2 implements an SPI protocol plus an ACK line. The post is just a preliminary investigation, but will give you a little insight into how the Logicport works and why you might find it useful.