So many combinations: Testing a switch-matrix board

A switch-matrix board that routes any of eight programmable voltage signals to any of 96 output pins. The board generates digital patterns up to 20 MHz and 30 kV and has 36 Mbytes of memory. Four field-programmable gate arrays (FPGAs) generate the patterns, control 842 relays in the switch matrix, and communicate with the main controller.

Perform an automated functional test of power routing and memory. Measure DC voltage at each pin with a DMM and pulsed-power waveforms with an oscilloscope. Test all memory locations. Automatically connect a DMM and an oscilloscope to any pin under software control. Design a text fixture that allows for easy removal of the board under test.

• A&F Fixture Products: reverse bed-of-nails fixture. www.affp.com.
• Boston Engineering: custom interface PCB that connects the PXI instruments to the unit under test. www.boston-engineering.com.
• National Instruments: PXI chassis, DMM, oscilloscope, switching cards, digital I/O cards, graphical programming language. www.ni.com.

Boston Engineering (www.boston-engineering.com) designed a switch-matrix board for use in an electrostatic-discharge (ESD) tester. The board routes signals from eight power supplies to 96 output pins. Senior controls engineer Eric Atherton developed an automated functional tester that reduced test time from 1 to 5 days to less than 1 hr while testing all power supply and pin combinations.

Prior to development of the automated tester, a technician would program each board under test by entering codes to the memory through a command-line interface. All connections to the board were performed manually.

With Atherton’s automated tester, the switch-matrix board under test routes any of the eight power supplies (0.7 VDC to 15 VDC) to any of the 96 pins located along the edge of the board. That’s 96⁸ possible combinations. The board’s 36 Mbytes of memory store sequencing data for all pogo pins.

An automated tester exercises and measures signals from a switch-matrix board.

The tester’s high-frequency interface board includes the power supplies, and the board routes data from four PXI digital I/O cards to the board under test. The digital I/O cards can load and read data from the switch-matrix board’s memory, and they can control the relays on the switch-matrix board during a test.

Testing starts with signal routing. The interface board puts the test system in direct control of the switch-matrix board’s switching capabilities. The first test involves switching each of the eight power supplies to each of the 96 pogo pins. The PXI DMM card measures the DC voltage at each pin, with signals routed through four 24-channel PXI switching cards. The software compares each measurement to specified limits.

Next, the digital I/O cards control the pin drivers at 20 Msamples/s, which produces a 10-MHz pulsed output at each pogo pin. The PXI switch cards then connect the oscilloscope to the pogo pins where the system captures each 10-MHz signal.

For the memory test, PXI digital I/O cards load the board’s 36 Mbytes of memory with alternating bit patterns (55 hex and AA hex). The cards then read back the data from memory.

Next, the tester exercises the FPGAs to verify that they properly control signal routing. The FPGAs decode data from the memory into control signals for the board’s relays, switch matrix, and pin drivers. The DMM and oscilloscope measure the output signals at each pin.

Reliable mechanical figures are an important part of any test system. Many test fixtures use pogo pins to connect test equipment to a board under test. In this case, the pins were located on the edge of and parallel with the board under test, extruding over the edge. Boston Engineering designed an interface board with connectors that mated to the board under test. The reverse bed-of-nails fixture holds the board in place while a mechanical lever raises and lowered the connector board, mating to the pogo pins.