In just about any test system, there must be a way to connect the tester to the Device Under Test (DUT) or the Unit Under Test (UUT). A well designed Test Interface (TI) can make a world of difference when it comes to how well the overall test system works.

Contact Nyno Technologies for expert guidance and advice on Test System Interfaces

Test Interfaces

When putting together a new test system, for example a production line tester, it's wise to think through the Test Interface as well as the off the shelf tester components such as digitizers, digital IO and switches. This may seem obvious, but some times test engineers get wowed by the test equipment vendor's product line and purchase component for a new test system to find that they could have optimized their investment if they had put more thought into the interface.

Some things to consider when designing a Test Interface

• Impedance differences between the DUT, cabling and the test equipment. A mismatch can cause big problems and can literally render a test system unusable (this is not good)
.

• Physical connections to the DUT and how they are going to be connected to the test equipment (connectors, probe card, pogo pins, optically, etc.)
.

• Access to signals for debugging. Nothing is more frustrating than not being able to debug a system due to inaccessibility of key signals (VERY IMPORTANT!)
.

• Connection diagnostics - If a connection goes bad, can you detect it? The worse case is a bad connection causes you to pass bad parts (not good). On the other hand, bad connections that cause false fails can be a pain. Making provisions for testing the tester should be considered
.

• Isolation - Are there voltages on the DUT that could damage the tester instruments? In many production environments, the DUT is being tested to make sure it's made properly. Is it possible that a mistake on the assemble line could inadvertently route high voltage to a low voltage connection?


• Consider calibration requirements for the system and how the Test Interface will play a role in it
.

• Adaptability - How many different devices will be tested with this tester? The Tester Interface can act as the adapter so that one tester can be setup to test many different devices.


• Change Overs - If using multiple Test Interfaces for a tester, consider ways to ease the change over process. Consider having a way for the tester to poll the Test Interface for information such as an ID or model number. This will allow the test software to check for a correct test setup.
  
  

The Test Interface can also be an opportunity to add custom electronics to optimize the overall test system. For example, let's say there are 10 single ended analog signals that must be read from a DUT for a certain production test and these signals don't have to be read simultaneously, they can be read sequentially. The brute force way to handle this is to simply run 11 wires (10 signal, plus a ground) from the DUT to the tester as shown below.

This is a very simple tester consisting of just one DAQ card. The TI could just be a simple screw terminal adapter that connects the lines of the DAQ card to a discreet wired connector for the DUT. The test software can read the analog voltages from the DAQ card one at a time. This is a great solution if this is the only DUT you'll need to test and there won't be any changes to it. However, change always happens.

Let's say that the company had great success with DUT1 and they made a new product with twice the capacity and some new features and now the tester has to read 20 analog signals and control 2 digital outputs and read 2 digital inputs. You can see where this is going, we either have to buy and install another DAQ card or switch matrix to handle the extra lines, or do something with the TI. By the way, we still have to be able to test DUT1 as well as the new DUT2. Let's look at a solution using an additional switch added to the system.

With this implementation, the tester cable grows to 25 conductors and we had to add in some sort of tester side interface to route the switch to the DAQ card, plus we have the 4 digital lines. This solution could work quite well, but there could be some issues with it like when the tester has to be switched back for testing DUT1, the cabling would have to be switched out as well. Another issue could be that if a production line wanted or needed to have a number of these testers and they wanted the ability to use any tester for DUT1 or DUT2, they would all have to have the switch installed on each tester, even though not all of them needed it. Also, you would need to have have both sets of cables for all testers in order to meet the requirement for all testers to be able to test both products. Not all test departments will face some of the requirements or changes we talk about here, but some do. If you're faced with some of the issues we talk about here, let's look at how a custom designed TI board would help solve some of these issues. Let's say we came up with a TI board that looked like the one below.

This is actually 2 boards, the main TI board and a noseboard or personality board. The noseboard is configured to mate with a particular DUT so there would be a different noseboard for each product, however, in this case, the noseboard would be the only hardware that would have to be swapped out to test another type of DUT since the TI board can handle either DUT. The TI board utilizes a simple serial interface to the tester. In this case we use the digital IO lines of the DAQ to communicate to the TI board, this way we can just use the DAQ card for all tests for both boards. Advantages of this configuration are:

• Can add ID lines so the test software can load the proper test or check for errors


• Protection from high voltages can easily be added


• Greatly reduced the number of wires needed for the test cable


• The same test cable can be used for all testers

The test system now looks like:

With this configuration, we added a simple Tester Adapter that routes our 5 tester cable lines to the appropriate connections on the DAQ card. We now have a 5 line tester cable and switch overs can be accomplished by just changing out the noseboard. So this system can now test either DUT and use a much smaller cable plus, it has extra protection for the DAQ hardware. There are many other consideration this example didn't cover such as power requirements, high speed situations, calibration and debug features, but hopefully it demonstrates how a test system could benefit from using a custom Test Interface board.

• High model mix of products


• Long cables between the tester and the DUT or test head


• Space considerations where bulky terminal boards won't work


• Where there's a number of testers on the production line, but each has slightly different hardware


• There are special termination or impedance matching needs