With advances in technology, ATE systems are becoming more widely used across a range of industries including manufacturing, avionics, aerospace, military, and defense. ATE systems are efficient and can be incredibly useful, allowing quick and accurate testing that communicates across a set of devices. However, it can become a complicated task to properly set up an ATE system to achieve the user’s desired outcomes.
When multiple test instruments are required to work in conjunction with one another to acquire a measurement or sequence of measurements, the ATE architecture becomes even more complex and difficult to design. For example, a test may require triggering a digital multimeter (DMM) to take a measurement when a relay is closed, triggering a switch module to close a relay when a signal is successfully generated, or triggering a DIO to output a signal after the DMM takes a measurement. Engineers can choose from different triggering methods to find the best approach to achieve their goals. Each triggering method has its own level of efficiency as it relates to execution time and implementation simplicity. Some more deterministic triggering methods can decrease program overhead and latency by allowing devices to communicate directly, outside of the program.
There are various methods for LXI instruments to communicate with one another in a test sequence. An engineer can pace the sequence of events in an ATE system through his or her application code (software triggering). This is very simple to implement in code; however, there is a high degree of dependency on the host to manage the test sequence.
Alternatively, the LXI specification provides various means through extended functions for handshaking between instruments to reduce the amount of host intervention required, thereby minimizing any latency attributed to host-instrument communication.
LAN events can include trigger messages that are sent over the Ethernet wire from one device to another, indicating events such as when an instrument has completed an operation. A second LXI device can be programmed to interpret that message as a trigger event for it to take action.
Another approach is to use the LXI wired trigger bus, which is an 8-wire hardware trigger bus analogous to the PXI or VXI backplane trigger bus. The LXI wired trigger bus can be used as a vehicle to send physical signals from one device to others in order to pace the test sequence.
Test developers can take advantage of modular switch and I/O instruments, with embedded scanning capability and an internal analog backplane to reduce external cabling requirements, making test systems operate more efficiently.
Triggering Method Setup
Each triggering method — software, LAN event, and LXI wired trigger bus — can be easily set up and efficiently used by LXI instruments to perform accurate and timely measurements in an ATE system.
Software Triggering. In comparison to other triggering methods, the software approach is the easiest, but slowest and least efficient method to trigger an action among LXI instruments. The user simply sends a command through a program to trigger an action, and repeats these actions in software until all measurements are recorded. The flow chart in Figure 1 describes a switch/measure sequence that is paced by the application code where a single measurement device is used to make multiple measurements through a multichannel switch. Due to the number of commands given by the program, and particularly the need to add a software delay, latency can become an issue.
Another way to illustrate the sequence is shown in Figure 2 using a DMM as the measure device with an integration time of 1.67 ms, and an electromechanical switch module settling time of 5 ms.
Removing all variables, and considering an example in which 400 measurements are made through the switch, the best-case scenario for execution time is (1.67 ms + 5 ms) 6.67 ms per measurement. At the rate of 150 readings per second, the test will complete in approximately 2.7 seconds. However, due to the variables in time consumed in sending commands across the host-instrument communications bus, as well as the variability in wait statements, the efficiency in execution of a software-paced program is much less than the theoretical maximum.
LAN Event Triggering. Of the triggering methods available, the LAN event approach stands out as one of the easiest and most efficient ways to perform triggering. Since all LXI instruments are connected and communicate using an LAN cable, LAN events can be generated without extra effort spent on cabling. Triggering can be performed automatically after a condition is met. To ensure that LAN event triggering is achieving its highest level of efficiency, the test network should be isolated from any nonessential traffic. Application code is required to create the handshaking mechanism using LAN events between a DMM in one instrument mainframe and a multi-channel switch housed in another mainframe.
LAN event triggering is less intuitive than using a hardware trigger bus or simple software pacing; however, it does reduce the bus traffic and requires no external hardware. When the DMM has completed taking a measurement, a backplane trigger is generated, and the instrument processor sends a LAN message across the Ethernet cable. Any LXI device can be programmed to listen and respond to this message.
A switch system is set to close the next channel in sequence and open the previous channel when the DMM has issued a LAN message indicating a measurement is complete. The LAN message is converted to a backplane signal that the switch module interprets as the trigger to close the next switch in sequence and open the previous channel. This cycle continues until all measurements have been made (Figure 3).
LXI Wired Trigger Bus Triggering. While LAN event triggering can achieve accuracy in milliseconds, the LXI trigger bus method can achieve accuracy in nanoseconds, making it the most precise way to communicate between instruments. In order to perform triggering using the wired trigger bus, the user must physically connect multiple chassis with an LXI wired trigger bus cable and terminating connectors (Figure 4). Also, the LXI domains on all the chassis must be the same. The LAN event must be modified so it uses the LXI trigger bus to trigger all of the actions instead of LAN events.
Summary
Pacing test sequences when using LXI instruments in an ATE system can be straightforward and analogous to other familiar test platforms such as GPIB, VXI, and PXI, while adding the flexibility to trigger devices over Ethernet. Different methods are available to choose the best approach to reach the user’s specific goals, and smart instrumentation can be used to increase accuracy and efficiency. If time is not an important factor, software triggering methods can be easily used. However, if accuracy and time are critical to meet the user’s ATE system requirements, using extended functions is recommended.
LAN event triggering can be efficient for distributed applications, and LXI trigger bus offers backplane-like performance. These LXI instrument extended functions can be used to overcome the impact of program overhead and reduce host intervention in an ATE system in order to create more efficient test programs.
This article was contributed by VTI Instruments, Irvine, CA. For more information, Click Here .
