Don't presume all thermal printer mechanisms are the same - you can only presume they operate in a similar manner! I've worked with Epson mechanisms before. The print head has it's own drivers and is loaded serially alaong with a print signal. As the others have mentioned, this is where you don't want to go wrong. The datasheet for the mechanism is usually quite specific (at least the Epson ones are). The other gotcha is the peak current the printheads draw - this can be quite a bit. The main killer with the Epson mechanism I used was the requirement for a flexible printed cable - quite expensive in small quantities unless you can source a supply of them. Personally, unless you're going to build a large quantity - just buy a mechanism with an interface. Job done.