Score Reel
The click-click-click an old pinball table makes when you hit something good is not a sound effect. It is the arithmetic, happening at the speed of a motor, because the machine has no other way to count.
New to computer arithmetic? Start here
Numbers in a fixed-size box
A fixed-width representation has a finite set of values. Some answers lie outside its range; others fall between adjacent values. Range and precision are different limits, and both depend on the representation.
An unsigned integer stored modulo a power of two wraps when it passes its largest value. Floating-point arithmetic can overflow to infinity or a finite limit, depending on the rounding rule. Values between representable numbers are rounded by that rule. Each machine here states the format and behavior it uses.
The machine for this idea on its own is Two's Complement, if you would rather press it than read about it.
The reels
Hit a target
1 Close: a target switch closes once, whatever the award is worth
2 Pulse: the score motor turns that one closure into as many pulses as the award needs on one reel
3 Count: each pulse steps a reel by one digit, at the motor's speed and no faster
4 Carry: a reel wrapping to zero fires the next coil if there is a higher reel; the last wrap is overflow
Scoring steps the reels one pulse at a time, and every award in this model ends in a zero because the units digit is painted on. That was a common arrangement and not a universal one: plenty of machines drove a real units reel and scored in ones.
Checked when this page loaded: 7 of 7 checks passed. Award pulses, carry impulses, wraps and overflow are counted separately. The controls and these checks use the same queue and stepping mechanism.
A machine with no adder
There is no separate register in an electromechanical pinball table holding a number that the drums then display. There is no accumulator and no arithmetic unit. What there is, is stranger and better: the position of each drum is one decimal digit of the stored score, so the display is the memory. There are drums with digits printed on them, and a coil beside each drum that advances it one digit when it is energised. That is the entire vocabulary.
So a target worth fifty points cannot send fifty anywhere. It closes one switch, once, and something else has to turn that single closure into five separate energisings of the ten-point reel. That something is the score motor: a motor turning continuously with cams on its shaft, opening and closing stacks of switches as it goes round. Ask it for five pulses and it delivers five, at its own speed, and you wait.
Why the sound is the sum
An award that needs several pulses takes time you can hear; its point value alone does not tell you how long. Five thousand points is five pulses of the thousand reel, and the motor will not be hurried, so the machine counts out loud while the ball is still in play. On a good table you learn to hear what you scored before you look up.
It is also why so many scores on these machines end in a zero. On many of them the units digit is not a reel at all: it is painted on the glass, because nothing on that playfield was worth less than ten and a drum that never moves is a drum you need not buy.
Carry is a switch on the ninth position
Every reel has its own coil, and a pawl inside the unit that the coil drives against a ratchet. What passes between reels is not that pawl: a reel sitting on nine closes a switch, and the next advance of that reel fires the coil of the reel to its left. If that one was also on nine, its own switch fires the next. A carry is a chain of switch closures running left along the row, one reel at a time, and the sound here adds one brief impact for each higher coil that fires.
That is a ripple-carry adder built out of steel and wire, and it has the same property the electronic kind has: the worst case is the whole width of the register. Five nines and one more award pulse move five reels: the award coil steps the tens, and four carry impulses step the higher reels. All five wrap to zero. The last wrap is an overflow, not another coil firing, because there is no sixth reel. Machines rolled over, and players kept count on their fingers.
These ran in this browser when the page loaded. Each claim, whether it held, and the number behind it.
| claim | held | measured |
|---|---|---|
| a 50-point target costs 5 award pulses, reading 50 | yes | five scheduled intervals at 150 ms: 750 ms of model time, not a measured motor speed |
| 5000 points take 5 award pulses to the thousands reel | yes | the denomination changes; the number of award pulses is still five |
| 90 plus 10 reads 100, with one award impulse and one carry impulse | yes | the tens coil advances its reel; its carry switch fires the hundreds coil |
| 999990 plus 10 reads 0: four carry impulses, five wraps, one overflow | yes | one award coil and four higher coils move five reels; there is no sixth coil beyond the display |
| the smallest award reads 10, because the units digit is painted | yes | five drums and a painted zero; every award ends in zero |
| 5 awards totalling 1660 take 13 award pulses | yes | 1.95 seconds of scheduled intervals at this model's cadence; each award keeps its denomination |
| 50 followed immediately by 1000 reads 1050 | yes | five tens pulses finish before the queued thousands pulse; accepted awards keep their order |
What is real here, and what is not
There is no table
No ball, no flippers, no playfield, no physics of any kind. What a target is worth, and whether you could realistically hit it, is the design of a particular machine and is not modelled. This is the counting, which is the part that is arithmetic.
The clicking is synthesised, and it is not a recording
Each award impulse sounds its denomination's chime. Each carry impulse adds a brief plunger impact beneath it, so a long carry has more impacts than a short one. Overflow adds no impact of its own: there is no coil beyond the last reel.
The pitch changes with the reel because a real table carried a chime unit: separate coils striking separate chimes, so ten points and a thousand arrive as different notes and a player knows which without looking up. Three chimes was the usual arrangement, so anything above the thousands here reuses the lowest rather than inventing a fourth that nobody had.
None of it was sampled. It used to be a filtered noise burst, and this ledger used to say that anyone who had stood next to a machine would know the difference immediately, which was true. It is now built from the three things that sentence named. The impact is the plunger arriving, three and a half milliseconds of broadband noise. The chime is a bar, because a chime unit is a solenoid throwing a plunger against a metal bar, and a bar free at both ends has transverse modes in a ratio that does not depend on its size or its tuning: 1, 2.76, 5.40, 8.93. Those ratios are inharmonic, and that is the whole reason struck metal sounds like struck metal. The cabinet is one low resonance under both. Measured after the change: each pulse rings for about 95 milliseconds instead of stopping inside a few dozen, and its energy sits on the chime's own frequency and the cabinet's instead of smeared across an octave. The upper modes are real in the model and brief in the ear; do not expect to pick them out. It is still synthesis and still not a recording, and the cadence, 150 milliseconds between pulses, is the part that carries the arithmetic.
The motor speed here is plausible, not measured
The pulse rate on this page is a reasonable one for the era. It is not taken from a specific machine, and real cadences differed by manufacturer, by model and by year, as did the number of positions on the motor's cams. Treat the rhythm as indicative and the arithmetic as exact. A browser may delay the scheduled ticks; that stretches the performance without changing which reel each pulse reaches.
Accepted awards keep their place in line
The buttons queue awards in the order you press them. Each keeps its denomination until all its pulses have arrived. This is a choice for operating the model, not a reconstruction of a particular table's relay interlocks. New game and winding to 999,990 cancel queued work, stop the current sound, and clear the event counters.
An earlier version kept one pulse count and one destination. Pressing 50 then 1,000 before the motor stepped redirected the pending tens pulses to the thousands reel and scored 6,000. Its verification table missed that defect because it scored every award through the tens reel, reporting 166 pulses for awards that need 13. The controls and the table now use the same denomination queue; separate tests check the arithmetic and rapid button presses.
The relay logic is missing entirely
On a real table the score motor also sequences the game reset, the ball-in-play and player-up steppers, the bonus count-down and the match at the end, all through ladders of relays and stacks of switches. None of that is here. This page models one thing the motor does.
Five reels is a choice, and machines varied
Six-digit displays with a painted units zero were common in the era this page describes, and that is what is drawn. Earlier machines scored in far smaller numbers with fewer reels, and later ones went further. The rollover shown here belongs to the machine as drawn, not to every machine.
Sources
- US Patent 3,390,255, Roman F. Garbark, assigned to D. Gottlieb & Co., filed 1965 — “Stepping mechanism for use in a pinball machine or the like”, which is the reel this page models, described by the people who built it as “an improved counter mechanism of the drum type suitable for tallying the score in a coinoperated amusement device”. The manufacturers' own schematic manuals would be better still and are not available: Gottlieb enforces copyright on them and they were pulled from the pinball database. A patent is what is left that the maker wrote.
- Fun With Pinball, the score motor circuits animated
- Home Pinball Repair, on score reels
- PinWiki, electromechanical repair