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Virtual Jtag use sld_hapi.dll file, but it's API function is unkown.
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Yes, that's discussed in the said and other previous threads. JTAG UART is one of many virtual JTAG functions. The basic virtual JTAG interface is documented by Altera, but the interface in the Quartus JTAG stack (particularly sld_hapi.dll) isn't. Of course, this doesn't mean, that you can't use it, but it's not easy.
P.S.: My personal opinion in this regard is, Altera should officially undisclose a virtual JTAG API, because it's a strong Quartus feature that can be utilized as a competitive edge.
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