https://www.pinwiki.com/wiki/index.php/Williams_WPC#Switch_Problems

I am struggling to understand how you are executing the tests to produce the results you are reporting. If you unplug all the switch connectors from the CPU board, to close a column 1 switch you would need to ground the pin on the header. How can you close another switch when you've already grounded that pin on the header? Please help me understand.

The key thing to think of here is that you can only interpret results if the tests are meaningful. A less polite way of expressing this is "garbage in, garbage out". I'm not saying your test is "garbage", I'm saying that you have to have a meaningful test to draw a meaningful conclusion.

Ok,

Go into switch edges test.

On the bottom of the CPU, disconnect the plugs at the bottom of the CPU board. You should have nothing connected to J205, J206, J207, J208, J209, and J212.

Put an alligator clip on J207, pin 6.

Touch the other end of the alligator clip to J209, each pin in sequence, and you should get switch 61, 62, 63, 64, 65, 66, 67, 68.

Move the alligator clip to J207 pin 5

Touch the other end of the alligator clip to J209, each pin in sequence, and you should get switch 51, 52, 53, 54, 55, 56, 57, 58.

You can do the same thing down J207, pin 4, pin 3, pin 2, pin 1.

I think that's what you are doing, right?

The purpose of this test is to make sure that the cpu can register all the switch rows, and all the switch columns.

However, if you were to connect two different connections DIRECTLY, without the switch diodes being in the circuit, normal behavior would be for it to register switches along the lines that you've directly connected inappropriately.

The test on the bottom of the CPU board for the switch rows and columns is not useful in combinations of connections... only test one connection at a time.

Multiple connections require the diodes on the switches to prevent the circuit from triggering switch registration inappropriately.

By the way, you've revealed why all the switches in pinballs have diodes attached.

If you remove J205 then you cannot enter the diagnostic tests. J205 is only related to the direct input switches and not the switch matrix switches.

I know you specify the order here as:

1. Enter switch edge test.
2. Unplug connectors.

The operation must be done in that order if you remove J205. You cannot remove J205 and then enter switch edge test.

Just being nitpicky and detail oriented to prevent a post saying "I removed all the connectors and now I can't get into tests". Also note that it is safe to remove the connectors with the power on since the switch matrix runs on 12V. You can certainly unplug everything except J205 and power on but doing so will potentially result in a "check fuse" message since SW24 is not detected.

Appreciate this!

I didn't think deeply about this, and I was concerned about getting everything out of the switch matrix entirely except for the jumper wire alligator clip.

This is a really good example of people who do some things, but don't fully understand the circuits involved.

I always learn things when you step in.

Appreciate the extra help!

A couple of points.

• Writing "removing the switch connectors" is not specific enough. You need to specify which connectors you removed. When I read what you wrote, I assume you only removed J206/J207/J208/J209 and did not remove J212. It might seem nitpicky but it's important because J212 is part of the switch matrix.
• The test you are doing is not standard. Just connect a single wire from one row to one column. It should register only one switch. You only need to do 16 combinations. One column to all rows and expect a single switch to register. One row to all columns and expect a single switch to register.

The CPU (software) only sees rows and columns. It works in a very simple way. It grounds column 1 and reads the state of the 8x rows. It then grounds column 2 and reads the state of the 8x rows. Do this for all 8 columns and repeat. It really is as simple as that.

switch_matrix_theory_of_operation.jpg

Nope, if you plan on testing 2 columns at the same time , you MUST use a diode at the end of your jumpers. You can only use a jumper when testing a single row/column combination.