Test Connections

Field Programmable Gate Arrays (FPGAs) and their older generation cousins – PALs and GALs have been part of the electronics landscape for over 20 years. During this time, design engineers have readily embraced the power and flexibility associated employing programmable logic arrays for a range of digital and mixed-signal board designs. The use of field programmable devices which was once the exclusive domain of design engineers has now grown to include users in the applications and test engineering fields.

Today, with the on-going increases in performance and capabilities of FPGAs, coupled with the enhanced design tools now available for programmable devices, test engineers can now readily design custom instruments for a range of digital and mixed signal applications. PXI offers the ideal platform for creating custom instrumentation using an FPGA based design. Current generation PXI FPGA cards such as Geotest’s GX3500 offer several key attributes to help simplify and ensure success with an FPGA based design.

These features include:
Full set of software tools /drivers to interface to the FPGA device with all interfaces and functions predefined, minimizing the need to create custom drivers or code
Availability of royalty free FPGA development tools such Altera’s Quartus II web-based tool set, minimizing a user’s upfront investment costs while offering the user the ability to work with a native tool set. Additionally, if more advanced features are needed, the user has the option to use Altera’s advanced tool set, providing a robust and full-featured native tool environment without the need to use an intermeidiary or 3rd party tool set.
Full access to all PXI resources via the FPGA for supporting the PXI triggers, local bus, clocking, etc.
User accessible phase lock loops which can be used for synchronous digital designs, clock generators, etc.
Multiple digital I/O outputs. For example, the GX3500 supports 160 digital I/Os (4 banks of 40 I/O pins) which are accessible via  the external interface connector. Direction control is provided on a per pin basis and isolation is controlled on a per bank basis providing a high degree of flexibility and control when interfacing to external interfaces or an on-board expansion board
Flexibility to download the FPGA code via the PXI interface or to load the device on power up via an on-board configuration memory

Figure 1 details a block diagram of the GX3500’s architecture, note that the user FPGA is completely dedicated for user applications and is not required to support the PXI control interface.
Figure 1 – GX3500 Architecture

Enhancing the FPGA Card’s Capabilities
FPGA cards by themselves do not satisfy all applications. Many times, specialized logic levels such as differential TTL, LVDS, buffered TTL, ECL, etc., may be needed to interface to the device under test. This is  probably one of the major differences when using an FPGA for a test application vs. a board design application. Other examples may include the use of an FPGA with a mixed signal application where the incorporation of analog circuitry is required.

Geotest’s GX3500 uniquely addresses these “extended” requirements by providing for the addition of an expansion board or daughter card to the FPGA card. The mechanical design allows for the addition of this card to the GX3500 without requiring an additional PXI slot or additional space outside of the PXI chassis. The result is a design and implementation that offers a compact, self-contained solution without the use of external plug-ins which can be cumbersome and difficult to integrate into a system.
Figure 2 presents a block diagram of the GX3500 with the expansion card architecture. Note the use of the isolation switches which are software controlled – providing the user with the ability to control access to the expansion card’s I/O interfaces.
Figure 2 – GX3500 and Expansion Board Architecture
Geotest currently offers several standard expansion cards for use with the GX3500 which can address a range of special applications:
GX3501, 80 channel TTL buffer expansion board. Configurable direction on a per pin basis
GX3509, 80 channel differential TTL expansion board. Configurable direction on a per pin basis
GX3510, 80 channel MLVDS expansion board. Configurable direction on a per pin basis
GX3540, ECL expansion board featuring 20 differential inputs, 20 differential outputs with selectable terminations to -2 or – 5.2 volts
GX3571, Video generator expansion board, supports VGA, composite and S-video formats

To learn more about FPGA test solutions, click here to read about using an FPGA card to implement a video generator.
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