Open and Short Circuit Testing

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Overview
Single Ended Measurements with Ground Reference
Differential Measurements

Overview

Embedded systems can be used to fault test other devices, systems or signals. A BrainStem is used to demonstrate simple methods to test and monitor various circuit conditions in another system.

This example explores utilizing general purpose analog and digital pins available on a BrainStem using C++ code to test for fault conditions. Source code and cross platform development projects are available through the Acroname Download Center in the "C/C++ Examples" download.

Note

This example is distributed in through the Acroname Download Center in revision 100419 and higher.

Contents

Single Ended Measurements with Ground Reference

A simple circuit could be used to test a device under test (DUT) for proper power connections before applying appropriate power to turn the device on. This may be a concern when the potential of grounded power planes, bad connections, or design mistakes. A digital IO pin can issue a small current to the DUT and the resulting voltage measurement can help formulate a decision to safely power up the device (or not).

The power delivered to the DUT can be minimized by using a current limiting resistor in series with the digital output.

Single ended test point in reference to Ground

Test point A will have different values depending on what state the DUT connection is at. For this example, a BrainStem Moto's power input connection was wired and tested. The following graph is a representation of three different tests: open, connected and shorted.

Testing of Moto power input using open short c++

BrainStem Moto Open/Short Test Results

Source code is commented and available in the C/C++ Examples download. The method call used to wrap up a single ended measurement relative to ground is shown in the following:

///////////////////////////////////////////////////////////////////// // Open/short test relative to ground // Measured value must be within the A2D allowable input range // Need to define which digital pin and analog pin to use // Also, specify the number samples to take void openShort_RelativeToGround(acpStem *pStem, aUInt8 module, aUInt8 digChannel, aUInt8 a2dChannel, aUInt32 nSamples)

Contents

Differential Measurements

Differential methods involve comparing two signals against each other to produce a meaningful result. This methodology is commonly used to eliminate noise on transmission lines and reduce electromagnetic interference. Differential methods are used in many communication protocols, such as RS-485, USB and Ethernet to name a few.

This method is not only limited to communication protocoals, but also safety checks on circuits. It may also be useful to know the whether two different points in a circuit are isolated from one another.

Take the following example. Two connectors should be separated on a circuit board, but connector selection design choices tend to make the connections bridged (joined) during manufacturing. The following schematic could be utilized to test the two connectors to be positive of isolation.

Detecting Differences Between Connections

Source code is commented and available in the C/C++ Examples download. The example's method call used to take a differential measurement between two points (test point B and C) is shown in the following:

///////////////////////////////////////////////////////////////////// // Open/short differentail test between two points // Measured value must be within the A2D allowable input range for // each test point. // Need to define which digital pin and analog pins to use void openShort_Differential(acpStem *pStem, aUInt8 module, aUInt8 digChannel, aUInt8 a2dChannelP, aUInt8 a2dChannelM)

Revision History:

2010-04-19: New Example