Active Device Test (A.D.T)

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Theory of Operation

The Active Device Tester feature is an Active test. This means that it will only operate correctly with the UUT powered up and running.

It will work only with the additional hardware plugged into connectors A and B and hence no processor emulation can be performed while it is in use.

For any digital logic device, at any given point in time, the pins will be set to a particular pattern depending on the device in question. This will always be the case however if the device is operating and running at particulary high frequency, some signals will be on the change over so the ‘snapshot’ must be performed at a slower rate. The ADT will do this and for most devices in most circuits, an accurate ‘snapshot’ will be taken. This is then applied to stored truth tables to determine if the device is performing correctly or not.

The tester is self-learning so a device that is not known to it will fail for a large proportion of the time until enough tables have been learnt and stored. Each time it fails and the results deemed correct can be stored, and it will remember this for next time.

The more results stored per device will result in an accurate In Circuit Device test.

Occasionally a false result will be given as explained above so before any results are stored, it is important to check them carefully using known data. If a false result is stored, then the tester will pass it next time and potentially faulty devices will be missed.


Connecting the Hardware

To set up the tester for ADT, simply unplug any link cables connected to the front connectors A and B. Plug in the ADT unit to connectors A and B.

A red LED on the ADT test box should be on at this point. If its not, disconnect and check connections.

The ADT uses a 20way test clip which should be clipped over the device in question.

NOTE:clip test clip over device so the LEFT GND pin is always connected even if the chip is shorter than the CLIP.

Turn power off to the UUT before any connections are made.

Once connected, apply power and the UUT should operate as normal without any affects from the ADT. If testing devices in clock circuit with crystals, then some issues may occur as the additional impedance and capacitance of the cabling may halt a sensitive clock.


Running a Test

To Start the ADT test, CLICK the BLUE ADT button in the Special Funtions buttons at the bottom of the Main Test Screen. The following screen will be displayed.

ADT

To quit the ADT, click the top right corner of the screen. The ADT screen will close and the Main Test Screen will be displayed again. Note if running on an external HDMI monitor, both screens will be displayed. DO NOT RUN THE MAIN TEST SCREEN OPERATIONS WHILE ADT IS OPEN.

To run a test, the device in question needs to be entered into the ADT screen. Click the DEVICE box and type in the device such as 74LS00 etc. Note its case sensitive so if testing a device make sure you use the same upper of lower case letters. The Pre-stored devices supplied are using UPPER case only.

After entering the device name, click the PINS box and type in the TOTAL number of pins for the device. Maximum is 20 and minimum 8.

Once this information is entered, then testing can start. Click TEST button to get a test. You will notice that the tester will set the display to show the device size and also display its image if one is present. Once a test is performed the tester will show the pin states in either RED or GREEN for high or low and also display if the device passes or fails. If it fails, a short bleep will occur and the TRUTH OK button can be clicked if the results are checked to be OK. DO NOT click the button if a false result is given.

As many tests as you want can be done on the device and each time a different set of pin states will be shown. This is normal since the tester relies on the device running in the circuit and each time a ‘snapshot’ is taken, the results will be different. This is why it is important to have plenty of stored tables so the tester can check all states.

There are two other tests that can be performed by either clicking the STUCK CHK or SHORT CHK buttons.

The STUCK CHK will try to identify pins that don’t change state. This can indicate that they are tied high or low but also they could be faulty and stuck in one state. Click the STUCK CHK button to run the test. It will take longer and the pins identified as potentially stuck will be shown in PURPLE. Any pins that change state will be blanked out.

The SHORT CHK button will look at pins that change state together. It will only check adjacent pins so for example it will check pin 2 and 3 as they are next to each other. If both these pins change state the same time and with the same states, then they could potentially be shorted together. Any pins found will be highlighted in YELLOW. Note a stuck pin check is also performed as part of this test so the resultant display could have RED, GREEN, YELLOW and PURPLE pins. Pins in WHITE are not tested.


Adding Truth Tables and GIF Files

All the files for the ADT are stored in the /home/pi/Desktop/DEVICES folder. If a device is new to the tester, a new file will be created once TRUTH OK is clicked. This file will be named as the same as the DEVICE name that was entered on the screen. The file will be text file and will contain data for each stored truth table. The more tests stored the bigger it will get.

If a file exists by the same name but with a .GIF extension, then this will be loaded by the tester and displayed each time that device is tested. GIF files can be added manually by storing one in the folder under the device name plus extension. So for example a 74LS00 device will have two files: 74LS00 and 74LS00.GIF

Any GIF file can be used but the size is important. Set the size to the following:

Length = 110 pixels, Height = (number of pins per side*20) pixels.