To unlight the display segments so it goes blank?

Hi. My understanding (and I might get corrected by someone else) is that the blanking signal is a circuit reset mechanism. It fires high only for a moment, something like 0.1s on startup, then goes low and stays low until the machine is turned off again.

The blanking signal goes into the pin 3 ENABLE of the BCD decoder. When it's high digits are displayed, when it's low the displays are blank. The 555 forces it low if it isn't reset by the CPU in time (because the CPU is locked up). Are you sure you have the blanking and diagnostics plugs in the right order? If so, try unplugging the 1J3 and just jumpering 5V to the blanking wire. The blanking signal on 1J3 is also tied to all the outer outputs on the driver board. With it low, none of the solenoids should fire either, for instance. (or at least that's the wy it is on Sys6, you haven't specified which game) If your solenoids work then your blanking signal is probably fine, and you should check that your connector wires and pins are good and that there's continuity from the MPU to the master display board

Nevermind the two plugs thing, that's Sys3-7. On Sys11, 555 pin 3 should be high during normal usage for blanking. It should also be high everywhere on the display board. As it's high going into the U55 inverter, it should be low coming out. The blanking for the displays comes from 1J3 pin 12, not 1J21.

That is not how the blanking signal behaves.

Blanking will be on, from initial power on, until the MPU begins executing. Initially, blanking is attended to by some discrete circuitry adjacent to the 555. After the MPU begins to execute, the 555 timer takes over. It begins to count down. It will be reset each time the display PIA loads data on a particular line (U51, pin 4, see image below. The signal that T's off and goes South will lead to the 555 timer.) If the display data isn't loaded before the timer counts down, then the timer will invert the blanking signal and stop most game functions to protect the game hardware.

Bottom line...blanking doesn't really toggle. It's more like the reset signal, beginning in one state, toggling once, then remaining there.
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image_(resized).png

Some muddy info in this post.... Lets get the core idea out there of what the blanking does and how it works.

The blanking signal is used as a safety protection to make sure the computer is running. One of the display PIA outputs also goes to a timer IC. The way the timer is set up, as long as the display PIA signal going to the to the timer input toggles high/lowfast enough (the time interval is set by a cap/resistor values) the timer output (the blanking) will stay HIGH.

The blanking being HIGH assumes that the computer is running OK and the display is turned on. If the display is not operating, the display PIA that stimulates the blanking timer is not pulsing so the blanking stays LOW.

Now that we know how the blanking goes high and low. How does the computer handle that as a safety feature to turn off lamps, solenoids, etc? Through the use of 7408 AND gates. 7408 is a two input AND gate. That means each gate has two inputs and one output.

download_(resized).png

Here is our AND symbol. Pretend this gate is in the solenoid section of the mpu/driver. Lets call 'A' the solenoid PIA signal and 'B' the the blanking signal.

You can see the truth table there. The blanking must be high and the solenoid pia must go high for the 7408 output 'X' to go high and turn on the 2n4401 predriver to fire the coil.

The same principle is applied on the lamp columns, AND gated with blanking/PIA using a 7408 to predriver a 2n6427

^^^ That was a very informative post! Thanks for taking the time to explain that in detail. Learned something new today!

Thank you for the explanation. I always wondered how exactly the blanking circuit works. So forgive me for my ignorance, I know just enough to get myself in trouble. What would happen if the blanking circuit was not integrated into the circuit? Would that mean there would be a possibility of coil lock ups at start up?

Yes, devices could lock on at start up. Also, without a working blanking circuit, "devices" (lamps, coils, displays) could lock on if the CPU stops execution. The Blanking Circuit protects the devices from turning on when not under CPU control which includes initial startup and failure conditions.

As noted earlier, devices are blanked via "default circuitry" (a circuit that works merely by having power applied, no intelligence) at power on. After the CPU has tested memory and is able to talk with the peripheral devices (6821 PIAs) to determine that the system is healthy, the 555 timer circuit comes into play and the rest operates as Andrew described above.
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Chris Hibler - CARGPB #31
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Does this explain why coils briefly kick when turning on an old system11?

Yes. The coils could sort of "pulse" for small fraction of a second. They won't always, but sometimes do.
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Chris Hibler - CARGPB #31
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A perfect explanation. Nice.

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Chris Hibler - CARGPB #31
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http://webpages.charter.net/chibler/Pinball/index.htm
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I've noticed it doesn't always happen. Why is that inconsistent? Depends on how long the CPU takes to do its checks?

The CPU doesn't start actively controlling blanking until it has completed initial checks. Prior to that time, the same reset signal that holds the 6802 in reset causes the 555 to blank the system. Sometimes there is a "sort of" a race condition, conducted at 186,000 miles per second that results in that slight pulse. Age of components, temperature, etc can play into it.

I said that the 555 timer "counts down" earlier. That's not really correct. I tend to think of things digitally and use the words interchangeably. A 555 timer is an analog device (actually, everything is). It doesn't actually "count down". The length of time it takes to change the state of pin 3 is "programmed" via an RC circuit (resistor/capacitor).

Good explanation of how the chip operates is here: http://www.dummies.com/how-to/content/electronics-components-how-the-555-timer-chip-work.html

Most times, it's not necessary to know exactly how this IC works...they just work.
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Chris Hibler - CARGPB #31
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http://webpages.charter.net/chibler/Pinball/index.htm
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I thought the blanking signal was something my wife sent out when I brought home a new machine.

All of them really.