Optimize Manufacturing Test Throughput Using Measurement Hardware Synchronization

Nearly all manufacturing test systems incorporate switching hardware, which is usually the most cost-effective method of channel expansion. But as any test engineer knows, switching between points typically accounts for a substantial portion of test execution time. For many test systems, test execution time translates into increased expense, and techniques that reduce the time-to-test can translate into tremendous financial savings. In this paper, we will introduce techniques of hardware synchronization between switches and other measurement instruments. By implementing these advanced techniques, which are supported by a wide-array of measurement hardware, you can reduce your test execution time by as much as 50%.

Hardware Scanning versus Software Scanning

Simply put, scanning is cycling through a list of operations on a switch module. This is done by downloading a scan list onto the switch module and cycling through the list using triggers. Without scanning, the system remains in immediate mode, wherein the driver makes a single call to the switch and the hardware performs that task without delay. In this mode there is no synchronization with any outside trigger.

Scanning can be either software-timed or hardware-timed. Software-timed scanning uses triggers generated by a software call, and hardware-timed scanning uses triggers generated by signals connected to the hardware trigger lines. Software scanning is faster than a sequence of immediate mode operations because the scan list is already downloaded onto the hardware. However, there is still significant software dependency and overhead because software-scanning systems need software intervention to advance the trigger for each reading. The timing of these scans is subject to the performance of the software system because the software shares resources with all other applications that require processor time.


Manufacturing test throughput is therefore maximized by implementing hardware-timed scanning, which removes the software-dependency. There are two methods of hardware-timed scanning, synchronous scanning and handshaking.

Hardware Scanning - Synchronous Scanning

Synchronous scanning takes advantage of the output trigger available on many instruments. The switching hardware can use this trigger as the scanning trigger line, which ensures that the instrument completes the current measurement before the switch moves to the next channel. However, measurements may be inaccurate if the instrument trigger does not compensate for the switch settling time between each reading. In this case, time delay must be added programmatically to ensure that the switch has settled and is ready to switch to the next channel. To minimize data loss, this programmatic delay is usually set to be relatively large because of the unknown switch settling time, which decreases overall throughput and efficiency.

Hardware Scanning - Handshaking

Hardware handshaking improves upon synchronous scanning by taking advantage of triggers from both the instrument and switch. Instead of adding software delay to allow for the switch settling time, the instrument and switch interact by sending triggers to each other when complete or ready. The instrument sends a trigger signal when the reading is complete, and the switch sends a trigger once it is settled and is ready for the next measurement. All trigger interaction is hardware-controlled, which minimizes time wasted between measurements and guarantees maximum throughput. Hardware handshaking between the switch and the instrument is completely independent of the software environment, and is not affected by software.

Application Example of Hardware Handshaking

An example of hardware handshaking is shown below. In this example, the NI PXI-4070 FlexDMM communicates with the PXI-2530 high channel multiplexer switch. After the first channel is closed, the switch trigger tells the DMM to take a measurement. When the DMM completes the measurement, its trigger tells the switch to move on to the next channel, and the cycle repeats without software intervention.

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Because of this determinism, additional software activity (such as adding extra data processing) does not affect the results of the hardware handshaking test as it does when software scanning is used.

The execution time improvement of hardware handshaking can be seen in the table below. These are benchmark results for 1000 Switched DMM Scans using a PXI 2501 FET Switch and a PXI 4070 DMM. The use of hardware handshaking increased execution time more than 46%. Also note that the hardware-handshaking use case is more resilient to extra software processing tasks, which severely slows execution of software scanning.

Execution Time Comparison for 1000 Switched DMM Scans

	Standard Run	With Extra Processing	% Change
Software Scanning	3021 ms	3840 ms	27%
Hardware Handshaking	1640 ms	1655 ms	0.9%
% Throughput Increase Using Hardware Handshaking	46%	57%

Using this hardware handshaking scanning method, the determinism and throughput of the system is maximized. Additionally, if PXI instrumentation is used, then all trigger signals are passed over the PXI synchronization backplane, eliminating the need for any external wiring and simplifying setup.


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Implementation

Support for the techniques discussed in this document ship with National Instruments switching products, and is fully documented with the National Instruments Switch products driver: NI-SWITCH. As shown below, a complete chapter of the NI-SWITCH documentation is devoted to Scanning with NI Switches and NI Digital Multimeters. Example programs are included as well. NI-SWITCH ships with all NI Switching products, and is also available for download at the link below.

See Also:
NI-SWITCH: Current Driver

Conclusion

The automated test equipment engineer is primarily concerned with increasing the throughput and reliability of the manufacturing test system. The hardware handshaking form of scanning improves system throughput and determinism by allowing direct communication between the switch and the instrument without any software intervention. National Instruments’ PXI switching and measurement hardware allows the manufacturing test engineer to make deterministic, hardware-timed measurements without any external wiring, thereby improving the throughput and ease-of-use of the test system.
Related Links:
PXI Tutorial
Next Generation Switching
Browse and Compare NI Switches

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