Bally 6-Digit Displays

This page will help you understand and repair your Bally 6-digit displays. The information is focused on the AS-2518-21 display. Since the AS-2518-15 display is interchangable with the "-21" display, everything mentioned here will apply to both displays. 7-digit displays behave the same too, so this information will be helpful for those too. The only real difference is an additional digit enable signal, and some more electronics on the board to drive the 7th digit.

If you could care less how they work, but are looking for information on how to repair them, then Click Here and I'll show you how. I'll also show you some ideas on how to keep your displays working properly.

A healthy AS-2518-21 6-digit display
AS-2518-21 AS-2518-15

You can see that the two displays are a little different in the way the components are layed out. Regardless of the differences in appearence, they both perform the same function and are interchangable with each other The original display (at least the one mentioned in my Power Play Owner's Manual) is the "-21". The two shown here were upgraded with 1/2 watt 100K ohm resistors. A modification that you should do on all your displays too.

If you're looking for information on how to repair these displays, and are not interested in learning how they work, then just Click Here and go right to my repair page.

Here are some other displays that will work in place of the two above

Stern DA100 Stern unknown


The way the display circuitry works is really quite interesting. Although the human eye can not detect it, at any given moment, each display is only showing one digit at a time. The program that runs on the machine's computer is changing the digits so fast that you can not tell. If you were to film the displays and play the film back in slow motion, you'd see all the displays showing the same digit, and it cycles through all six, from left to right. It just cycles so fast that your brain thinks the whole display is lit all the time.

As you can see, each display has six digits, and if you look closely, you can see that each digit consists of seven segments. This is important stuff to know in order to understand how these displays work, and most important, how to fix them cheaply!

If you study the display's schematic, you can see that there are 4 main "parts" of a display assembly: The glass display itself, and the display driver consisting of the input decoder, six digit driver circuits, and seven segment driver circuits. There is one digit driver circuit for each of the six digits, and one segment driver circuit for each segment of a digit. How the actual glass display does what it does in order to light various digits and segments is beyond the scope of this tutorial, so we'll ignore that. The decoder takes a number from 0 - 9 as input and determines which segments need to be energized in order to represent this number. The digit drivers are responsible for applying the proper voltages to the proper pins of the display to tell it which digit to light. The segment drivers are responsible for applying the proper voltage to the proper pins of the display to tell it which segments of the digit to light. It is the MPU's job to supply the proper signals to the display driver to make it do all this stuff.

The decoder is a small integrated circuit (MC14543LE) called a "BCD To Seven Segment Decoder". This decoder happens to be a "latching" decoder, which means it latches on to it's inputs and keeps them, even if they are no longer applied, until the decoder is told to release them (blanked). The decoder also has an input called a strobe. When strobed, the decoder will read it's four inputs and latch on to them. A strobe signal is usually a quick off/on/off pulse. There is also an input for the blanking signal.

BCD stands for Binary Coded Decimal and is a fancy term for storing a number from 0 - 9 in a half byte of storage (four bits). Using BDC encoding, each byte of memory can store two digits. Anyway, the input of the decoder is a BCD number from 0 - 9, and the output of the decoder is seven signals. These seven signals are either on or off, and relate to the seven segments of a digit. Each of the seven output signals go to a MPS-A42 transistor, which is part of a circuit called a segment driver. This transistor acts like a switch to turn the segments on or off. The outputs of the seven segment drivers go to the seven segment pins of the display glass. So this is how the computer tells the display driver which segments to light. The MPU has a four-signal data path that goes to all five displays (or seven for Six Million Dollar Man). These four signals provide the 4-bit input into the decoder, and remember, all four signals go to ALL of the displays. Below is a diagram of how the segments are labeled, and a truth table showing the 15 possible inputs and outputs to the decoder. For those of you that don't know about binary arithmetic, you can get 15 possible combinations of on/off with 4 digits. (e.g., "0000", "0001", "0010", ..., "0111", "1111"). This is also how you count in binary, or base-2. Remember that the display driver is only interested in 10 of the 15 possible combinations, the ones that represent the numbers 0 - 9, or "0000" - "1001". Any other input combinations will result in unpredictable outputs from the decoder, so we label these as "don't cares", since we know they will never happen under normal circumstances.

Inputs Outputs Display
D0D1 D2D3   abcd efg
0000   1111 110   0
0001   0110 000   1
0010   1101 101   2
0011   1111 001   3
0100   0110 011   4
0101   1011 011   5
0110   0011 111   6
0111   1110 000   7
1000   1111 111   8
1001   1110 011   9
1010   XXXX XXX   X
1011   XXXX XXX   X
1100   XXXX XXX   X
1101   XXXX XXX   X
1110   XXXX XXX   X
1111   XXXX XXX   X
1=ON, 0=OFF, X=Don't Care
(Only interested in 0 - 9)
Segment Labels Decoder Truth Table

The digit driver circuit consists of an MPS-A42 transistor and a 2N5401 transistor connected in a circuit that acts as a switch. Normally with no input signal applied, the switch is off, keeping the high voltage supplied by the HV Regulator away from the display. There are 6 digit signals provided by the MPU, one for each digit. The MPU will enable one signal at a time, telling the display driver which digit to operate. This signal will then turn on the "switch" for the digit, allowing the high voltage a connection to the proper pins of the glass display to energize the desired digit. These signals from the MPU are simply 6 wires and the MPU will activate one of them at a time. Like the segment signals, the six digit signals go to all of the displays in a dasiy chain fashion.

OK, before I explain how the computer makes all this work, let's sum up:
A display driver has four inputs from the MPU. The first is a collection of six signals used to tell the display driver which digit to energize (six digits in the display, six digit signals). Only one of these signals are on at a time. The second input is a collection of four signals that tell the display driver which number to display, in the form of segments. These four signals provide a binary pattern that is interpreted as a binary number from 0 to 9. The third input is a strobe signal, which tells the decoder to read it's inputs, and the fourth signal is a blanking signal, which tells the decoder to turn off all it's outputs. Also, but not mentioned above, are various voltages from the power supply and high-voltage regulator. Each display driver is supplied with +5VDC from the 5-volt regulator on the solenoid driver board. This is used to drive the logic circuits of the decoder. There is also +190VDC* applied to the display driver from the high-voltage regulator on the solenoid board. Finally there is a connection to ground, which brings all the voltages to the proper reference point.

* For brand new displays, this voltage should be at +190VDC in order to "burn in" the display. Once a display has become used, this voltage may be backed down to +170VDC, which will work just fine and will help prolong the life of the display.


OK, so now you know how the display works. The next thing to understand is how the computer in your pinball machine operates the displays. For this article, we'll refering to the AS-2518-17 Bally MPU Module, but all this also holds true for the "-35" MPU module as well. And again, I'm only refering to the Bally AS-2518-21 and AS-2518-15 Display Drivers. I've not studied the Stern style display driver, but I'd guess they behave about the same.

As mentioned above, there are four sets of signal lines that go from the MPU module to the display driver modules. The first set is the BCD data set which carry the display segment BCD data to all the displays on 4 wires. They leave the MPU module (A4) at connector J1, pins 25-28 and visit every display driver module at connector J1 and pins 16-19 (D4 - D0). The next set of signals is the digit enable signals. These 6 wires carry signals to all the display driver modules with information telling it which digit to light. The third set of signals are 5 latch strobe signals. There is one separate signal for each display driver, and it is the signal that tells the driver's decoder to read the decoder inputs, and output the proper segment signals. The final signal is a single single that goes to all the display drivers called Display Blanking. The signal tells the display driver's decoder to turn off all segment outputs, thereby blanking out the display, or turning all segments off. Below is a diagram of all the connections between the MPU module and the display driver modules that have to do what we're talking about.

MPU to Display Driver Connections

Once the machine has been turned on and has booted up, the processor on the MPU module is continuously running a program that is stored in the module's ROM chip(s). This program is responsible for controlling the game by reading all the switches, lighting all the lamps, activating all the solenoids, and controlling the displays. The program keeps a lot of information in RAM and uses this information to keep track of scores, switches, etc. An interrupt is a term for a section of computer program that interrupts the "main" program in order to execute a smaller program, sometimes refered to as a "service routine". We won't get into just how this actually happens, just be aware that the main program of a computer may be interrupted at any given time. And to make things even more complicated, interrupts themselves can be interrupted by higher priority interrupt service routines. There may be several different interrupts that occur in a pinball's computer program, but the one we want to study is the one that controls the displays. Keep in mind what was mentioned above, that at any given instant, only one digit is lit on any display. This is called multiplexing.

320 times a second, or once every 3-1/4 milliseconds (thousands of a second), the CPU is interrupted to service the displays. In memory, the CPU keeps track of all the information it needs to operate all the displays. This information includes a counter used to indicate which display digit is active, the BCD data for all the displays, etc. Here's what the display service routine actually does:

  1. Determine which digit was updated last time
    The MPU looks at the digit counter and adds 1 to this value. If the new value is 7, it is changes to 1, then the new value is stored back into memory. Let's assume the new value is 4, so we're going to update the 4th digit.
  2. Blank out all the displays
    The CPU raises the signal on the Blanking Line which causes all displays to go blank (the blanking signal tells all the decoders to turn off their segment outputs).
  3. Fetch the BCD data from memory
    The BCD data for the first display, 4th digit is fetched from memory.
  4. Send the BCD data to the display driver
    This BCD data is placed on the BCD data bus and display #1 is strobed. This will cause the display's decoder to latch onto the input signals (store them for future use).
  5. Do it again
    The previous two steps are repeated for the second, third, fourth, and fifth display.
  6. Enable the digit The MPU then enables the 4th digit and disables the other 5 digits by raising and lowering signals on the Digit Enable lines.
  7. Finally, turn the digit on
    The MPU lowers the signal on the Blanking Line, which causes the all of the decoders to output their proper segment signals and the 4th digit on each display is displayed.
  8. All done!
    The interrupt service routine then exits and control returns to the main program
As you can see, the display interrupt service routine only handled 1 digit for all displays. Every time it is invoked, it will process the "next" digit, resetting the counter back to 1 when necessary. The process of updating 1 digit for all displays takes about 500 microseconds, or 1/2 of a millisecond, to complete. Pretty cool, heh?

So, lets do some math. It takes 1/2 millisecond to update one digit, and since there are 6 digits, it takes 3 milliseconds to display all six digits. Since the interrupt runs 320 times a second, and it takes 6 interrupts to update the entire display, dividing 320 by 6 means that the displays are completely updated just over 54 times every second. That's fast enough to fool your eyes and brain into thinking the display is completely lit all the time. Also, since the interrupt routine takes about 1/2 millisecond to run, and it runs 320 times every second, that means about 160 milliseconds of every second of time is spent updating the displays, which is about 16 percent of the time.

OK, now that you know how the displays work, and you know all about all the signals between the MPU and the Display Driver, it should be easy to diagnose and repair your display problems. Click Here and I'll show you how. I'll also show you some ideas on how to keep your displays working properly.

If you don't feel up to repairing your display yourself, I offer display repair services too. Just Click Here for more info.

If you're looking to buy some used displays, check here as sometimes I have them for sale.

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Created 2/18/03 - Last Modified 1/19/10 - Steve Kulpa Mail Icon Nolensville, TN
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