Vampire 500,600,1200 V2 JTAG connector
stands for Joint Test Action Group and it is standard for verification and testings PCB after manufacture. Over the years become widely used within FPGA devices.
To simplify explanation how this works it is enough to said that with FPGA using JTAG you can actually see how specific FPGA I/O pin "see" signal or combination of multiple signals on PCB traces. I must also say that JTAG is also very dangerous tool who could give you lot of info about specific circuit which drives to cloning if system is not protected by other mechanisms.
In our case with Vampire accelerators JTAG gave big advantage over the others because suddenly we had device who could be final product and same time debugging device. This is something that saved us lot of money and time because it completely excluded need for expensive Logic Analyzers. We could actually see what's going on in Amiga, we could trigger specific event under determined conditions and in real time see what is going on. We could recreate that event with code, modify it, improve it or completely override it by other.
For me personally using JTAG created some kind of excitement and joy that is beyond explanation. Using it I felt that it I m truly entering Amiga world and often I trigger some specific Address on Amiga bus and watch how Amiga suddenly become alive, ticking, how chips communicate to each others. How they ask each others for permission to do something. Oh damn! Best definition for JTAG on Amiga, "pure P000RN!". I had to say it!
Vampire 1200 V2 debugging using Altera Quartus 13.1 SignalTap II LogicAnalyzer
Running roto demo using JTAG Debug server tcl script