Marcus Jenkins has written an article on Serial Protocol debugging with Ubuntu & Bus Pirate v4:
You can see how the Bus Pirate can get you up and running with a new serially-connected peripheral pretty quickly. You can get started with configuring the peripheral in no time to suss out your initialisation and wiring. The next level would undoubtedly be to drive the Bus Pirate from a script so that you can develop some richer tests for your peripheral without quite so much pain since the device (that I have) doesn’t have much memory for the command history.
You can get a Bus Pirate v4 for $40, including worldwide shipping.