Automatic test switching

Automatic test system switching' test equipment allows for high-speed testing of a device or devices in a test situation, where strict sequences and combinations of switching must be observed. By automating the process in this way, the possibility of test errors and inaccuracies is minimized, and only systematic errors would generally be encountered due to such as an incorrect programmed test condition. This eliminates error due to human factors and allows application of a standard test sequence repetitively. The design of a test system’s switching configuration is governed by the test specification, which is derived from the functional tests to be performed.

A typical test system would involve the connection of input and outputs of the device under test to the test equipment, which is usually controlled by an electronic program generated by a computer or a Programmable Logic Controller.

The simplest definition of a switch is “a device that opens or closes a circuit.”

A relay is an electronically operated switch. Three relay types are commonly used in automated test system switching:

The ideal switch:

It’s important to recognize, however, that real-life switches are not ideal, so when calculating the overall system accuracy, the effects of the switch itself and all the switching hardware in the system must be factored in.

As a signal travels from its source to its destination, it can encounter various forms of interference and sources of error, so whenever a signal passes through a connecting cable or switch point, it may be degraded. For example, in low current and high resistance applications, unshielded cabling can introduce leakage currents that will degrade measurement accuracy. Unshielded cable can result in noisy readings for low current and high resistance applications, especially if the cabling runs adjacent to equipment generating electromagnetic interference.

Three terms are used to describe the configuration of a relay: pole, throw, and form.

Pole refers to the number of common terminals within a given switch. Throw refers to the number of positions in which the switch may be placed to create a signal path or connection. Figure lA illustrates a single-pole, single-throw normally open switch (SPST NO). Figure 1B shows a single-pole, double-throw (SPDT) switch. One terminal is normally-open (NO) and the other is normally-closed (NC). Depending on the state of the switch, one or the other position is connected to the common terminal (COM). One signal path is broken before the other is connected, so this is referred to as a break-before-make configuration.

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Wikipedia