Diagnostic Interface Fabrication
OSSA's original $800 USD diagnostic hardware/software utilized a computer's RS-232 (+/- 12V serial communications) port. However, since the ECU requires logic-level (5-volt TTL) communications signals, the original diagnostic hardware shifted those RS-232 signal levels to TTL levels.
As computers transitioned from RS-232 ports to USB ports, many would-be tuners were forced to employ a USB to RS-232 converter which then connected to the original RS-232 to TTL level-shifter. This is complicated wiring for a simple task (a bit like sending a letter from the US to the UK via Australia).
Direct USB-to-TTL converters are available for a few dollars on eBay. I started with a converter that used a Prolific 2303HX chip, but it suddenly stopped working after a Windows 7 update. Apparently, the Prolific product was being counterfeited in China. Prolific's solution was to write new drivers that not only invalidated the cloned chips but their own older products as well! There was no way to roll back to old drivers. Prolific wanted everybody to buy new hardware. Not wanting to reward behavior like that, I got a converter based around the Silicon Labs CP2102 chip. It has been working fine despite many subsequent updates to Win7. Such a converter is shown below.