Planning and designing test fixtures



Planning and designing test fixtures

Designing a test fixture is a methodical process. You have to start with an understanding of what the circuit you are going to test does in the game, then find a way to emulate those operations on the bench. To list an example of the things to be done:

Power requirements

What kind of power is required to run the assembly being tested? What kind of power is required by the tester itself?

Input circuits

What kinds of signals are required by the input side of the assembly being tested?

Output circuits

What kind of outputs from the assembly being tested do we have to interface to?

Special circuits

What other kinds of circuits are required to make this assembly function as though it were in a game? If this requires a microcontroller-based design we need to assess the requirements for a microcontroller or microprocessor based interface.

Once we have an idea of what we need we need to look at component requirements. To do this we need an understanding of what components are capable of. This is where a knowledge of electronics beyond just changing parts comes in. You have to understand what various components do, what characteristics they have, and how the components are rated. If five minuites I can show you how to change a transistor. It takes much longer to teach you what transistor you need for an application you are designing.

Understanding what components you have available and what they are capable of you design each section, one step at a time.

Software planning

Software planning is little different than hardware planning. You evaluate what must be done in the game, and conceive software that accomplished that step by step.

Coin Comparator tester as an example

Let's take the example of something we should already be familiar with and design a test fixture to test it.

For a coin comparator (CC-16 12 V DC Inhibit, IGT) we can design a tester for it by observing requirements first.

Power

+13 V DC, at about 350 mA (max), well regulated.

With power alone we can power up the coin comparator and make it work. We have one input and one output to deal with that is not covered by putting the device under power alone. We have an Inhibit line that must be pulled low before the coin comparator will actually accept coins. When it accepts a coin we have an active low output "Sense*" line that should go low telling the game a good coin has been accepted. These are both open collector type signals, working off of the +13 Volt line.

Inputs

"Inhibit / Enable*" When this line is pulled low the coin comparator should be able to accept coins. When left to float high accepting coins should be disabled. We can do this with as simple a circuit as a simple switch.

Outputs

"Sense*" A low on this line means the coin comparator has recognized a good coin and accepted it. This is an open collector output, NPN transistor, medium power. It is capable of about 50 mA out, so we can drive an LED directly from this output. The problem here is that it is hard to see a 25 ms pulse from an LED. Not impossible, but not reliable.

Normal operation

Normally the coin comparator is disabled from accepting coins. When the game wants to accept coins from the player it turns on the Insert Coins lamp and pulls the Inhibit line low. The Sense line is monitored. When a coin is inserted it passes through a Sense coil. This coil has one primary and two secondary windings. A reference coin is positioned between the primary and one secondary coil. The incoming coin path is between the primary and the other secondary coil. The two secondary coils are wound in the opposite direction so that when the incoming coil is properly positioned the signals in the two secondary windings are equal and opposite so they cancel each other out and the Sense coil signal disappears for a short time, causing a null in the output. Circuitry inside the coin comparator senses this null and generates a pulse. When a coin is accepted by the coin comparator it pulses the Sense* line low for about 25 ms. In most applications this same Sense* signal is what pulls the Accept coil in. In the 12 V versions this Inhibit input is actually on the Emitter circuit of the transistor that drives the Accept coil. In the IGT 13 Volt versions the Inhibit is a logic input a few transistor stages before the Accept coil driver.

When the player his the Spin button the MPU disables further coins by releasing the Inhibit line so it floats high, Inhibiting the Accept coil from being pulled. Coins can still pass through the Sense coil and generate a Sense pulse, but the Accept coil will not be energized.

A minimal test fixture can be built with a 13 Volt power supply (okay it will work just fine on 12 Volts), a switch and an LED with a 1,000 ohm resistor to limit current. What we can't see is how long this Sense signal actually is. If the Accept coil is misadjusted we can get the condition where a coin causes a Null condition just as it approaches the sense coil and again as it leaves the sense coil, giving us two Sense signals for each coin entry. This may not be caught with our simple test fixture.

To give us the ability to check this double nulll condition we can watch it closely with an oscilloscope or we can monitor it with a microcontroller and catch the problem even if it only happens occasionally. These are not the only alternatives, just two possibilities.

If we go with a microcontroller-based design we need to consider software requirements also. The method is basically the same.

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