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VNAccess (va-nak-sess) was developed to be an API (application programming interface) for the N2PK VNA. It encapsulates all of the details concerning the clocking of the chips through the parallel port (printer) interface. It also manages the various timing issues present in setting the DDS chips and reading the ADC. VNAccess calls are made in terms of the hardware registers present in the major integrated circuits. If you are using the LTC2440 ADC chip, then you need to use VNAccess version 2.00, or greater. Versions under 2.00 were written with the LTC2410 in mind, and communicated with the 2410 in a way which does not work with the 2440. The newer version of VNAccess works with both chips. VNAccess assumes that it can directly access the parallel I/O port on the computer. On Windows 95, 98, and ME, this is the default mode of operation. On Windows NT, 2000, and XP, the operating system normally disables direct application program access to the hardware as part of the overall security and protection model. On these platforms, direct access to the hardware can be obtained through the use of a driver which selectively turns off the protection mechanism. That driver is not part of VNAccess. We have been using UserPort 1.0, one of the many available packages for enabling direct access. VNAccess is provided in a DLL (dynamic link library) form. The code has been written so as to minimize dependence upon additional packages, features or software. Although it is written in the C++ language, and was developed using the Microsoft Visual Studio version 6, the library can be used by standard C programs, and other popular languages such as Visual Basic. VNAccess does not include direct support for the computation of electrical parameters such as reflection coefficient, or impedance. The data returned by VNAccess is the raw input to that next step. The VNAgra API supports those calculations.

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