In the early 1970s designer Ed Krynski at D. Gottlieb & Co. designed several single player games with baseball themes (*). A main objective of these games is to score runs by hitting various targets that represent base hits. These games have a standard set of Score Reels for points like other games of the era but they also have an extra pair of Score Reels to keep track of runs scored. Keeping track of base runners as various base hits are registered is much more complicated and interesting than adding points.
(*) Gottlieb baseball games that track base runners include 1970 Baseball, 1970 Batter Up, 1971 Playball, 1971 Extra Inning, 1971 Home Run, 1972 Grand Slam and 1972 World Series.
The rules for determining which bases have runners seem pretty straightforward at first glance:
To determine whether a given base has a runner you just need to evaluate the rule for that base. But much like a computer program you can't evaluate all of the bases simultaneously; they need to be evaluated one at a time, or sequentially. Evaluating them simultaneously would require extra circuitry without any providing any benefit.
Things get more complicated when you realize that the order in which you evaluate the bases matters. If you evaluate the bases in order the results might surprise you. As an example assume that the bases are empty and the batter hits a single base hit:
Evaluating the bases in order results in runners on all three bases which is incorrect. The problem with evaluating the bases in order is that the rules can't tell when the base runner arrived at the previous base. To avoid this problem, the bases should be evaluated in reverse order. So with the same scenario (bases empty, batter hits a single) the results are correct:
The circuits to calculate the base runner positions described here are based on the circuits from Gottlieb's 1972 Grand Slam pinball machine. The circuits have been simplified some for clarity but are essentially those used in the game.
1972 Gottlieb Grand Slam
The main components of the circuit are a Step Unit, Simple and Interlock Relays, a Score Motor and switches. The Step Unit is used to coordinate the order in which the circuits are evaluated. The Score Motor generates the pulses that drive the circuit.
Three Interlock Relays are used to keep track of whether each base has a runner. Interlock Relays have two coils (trip and reset) that change the state of the relay. Once a pulse briefly activates the trip or reset coil the Interlock Relay it holds its position indefinitely until the next pulse. The brief video below shows the two coils of an Interlock Relay being pulsed alternately to change the state or position of the relay. Notice how the armatures move just enough to open and close the switches.
A Gottlieb Interlock Relay Whenever a base hit happens the Interlock Relay for each base must accomplish two things. It must reset or clear the base runner it may have held and then reevaluate whether the hit puts a new base runner on the base. But each base must also hold on to its previous runner until the next base has finished its calculations. So for all the calculations to complete correctly the six actions must complete in this order:
The animations below will illustrate how these calculations are done in this order.
The circuit animation below shows the simplest case, a single base hit with no base runners. Only the circuits used to evaluate first base are shown:
Single base hit with bases empty
On the left is the Motor Switch Timing Chart and timing diagram that indicate when devices activate as the Score Motor turns. In the middle are the circuits for the light bulb under first base and the relay that activates when the first base target is hit. On the right are the circuits for the 1st base Interlock relay coils and the solenoid that advances the Step Unit, called the Hit Unit in this circuit. Note that the Hit Unit appears twice in the schematic. Each instance represents a different pair of wipers on the Step Unit.
When the Single base hit target switch closes, the Single relay activates which activates the Score Motor and starts the animation. Using the Score Motor angle as a key, the important events in this animation are listed below. (Tip: After playing the video above, drag or click on the timeline at the bottom to move to any frame in the video.)
The result of the simulation is that the Man on 1st Interlock Relay is left in its tripped position and the light under first base is illuminated. Here's what that looks like in the game:
Single base hit with bases empty Single base hit with the bases empty
If you listen carefully you can hear the chime and the Hit Unit taking five steps as the evaluations are done. This is what the Hit Unit does each time there is a base hit:
Here the second base circuitry has been added to the schematic and the animation shows what happens when the Double base hit switch closes.
But before playing the animation notice how the second base circuitry compares to the first base circuitry. On the left side the circuits are essentially the same but on the right side the Man on 2nd Interlock Relay reset and trip coils are wired to positions on the Hit Unit used just before the positions used by the Man on 1st circuits. The is because the Man on 2nd Interlock Relay reset and reevaluation must complete before the Man on 1st Interlock Relay changes.
Notice also that a third wiper was added to the Hit Unit. This third wiper is needed because there are two situations or circuits where a runner can end up at second base: a double base hit (through Hit Unit B) or a single base hit with a runner on first base (through Hit Unit C).
Double base hit with bases empty
The key points of this animation are:
The result of the simulation is that the Man on 2nd Interlock Relay is left in its tripped position and the light under first base is illuminated. Here's what that looks like in the game:
Double base hit with bases empty Notice that the 2nd base light comes on slightly earlier than the 1st base light in the previous video.
The animations below include circuits for all three bases. The first animation illustrates the events when the triple base hit switch closes and the bases are empty. Just as the second base evaluations have to occur before the first base Interlock Relay resets, the third base evaluations must happen before the second base Interlock relay resets. So the third base circuits use earlier Hit Unit positions than the other bases.
Also because there are three situations when a runner can reach third base (described earlier), there are three different circuits that end at the Man on 3rd Interlock Relay trip coil.
Triple base hit with bases empty Significant events in this simulation include:
At the end of the simulation the 3rd base light is illuminated, shown below as it happens in the game:
Triple base hit with bases empty Triple base hit with bases empty
The 3rd base light comes on earlier that the other two lights in the previous videos.
When there are runners on base before a base hit the calculations do not take any longer. Different paths through the circuits may be exercised but the basic sequence of resetting and then evaluating the bases in reverse order will always arrive at the correct result. A few examples are shown below.
Notice that whenever there is a base runner on a base at the beginning and the end of the simulation the light for that base goes out briefly. That indicates when the Interlock Relay for that base has been reset just before being reevaluated.
Single base hit with a runner at 1st base Single base hit with a runner at 1st base
Single base hit with a runner on 2nd base Single base hit with a runner on 2nd base
Double base hit with a runner on 1st base
Double base hit with runner on 1st base Double base hit with a runner on 1st base
Notice that the second and third base lights go out briefly when their Interlock Relays reset just before they turn back on after the reevaluation.
Double base hit with the bases loaded
Double base hit with the bases loaded
These games tend to be noisy as they are being played because each time a hit is registered all of the bases have to be reevaluated by advancing the Hit Unit stepper and resetting and reevaluating the Interlock relays. This is what it looks like inside the game:
Hit Unit and Interlock Relays The Interlock Relays labeled A, B and C are the Man on 1st, Man on 2nd and Man on 3rd relays. The Interlock relays move very little but if you look closely might notice that they twitch in reverse order (C, B, A) as the Hit Unit steps through its five positions.
Follow these links to other animated schematic diagrams: