Atari Missile Command
Prototype Marquee Page

last updated April 14, 2015



 

 

I always wondered why Atari Missile Command PCBs had space for more components but you never see them loaded... There are no details in the schematics alluding to what they might be used for either.

If you've ever seen images floating around the internet of a prototype or early field-test missile command machine, you will see they have this extra huge marquee panel above the monitor.

The extra panel of lights and annunciation really would make it look like you were at the controls of a missile interception system!

Sadly this was dropped, some say because of the distracting nature of the flashy lights...

But this explains the foreshortened height of missile command cabinets, here is a rare photo of one of the prototypes on the left, next to a production model.
........
So, this got me to thinking, flashy lights? knowing Atari they probably did something quite cool, not just random blinky. And i bet the empty spot on the pcb has something to do with it. When they changed the shape of the cabinet, they could do away with a lot of the complexitiy in production and cut costs at the same time. Cut the top off, simplify wiring and construction, and save a bit of cash on loading components on the pcb too.

So, whats missing? you can ignore the two spaces at F8 and H8, these are empty IC locations with no tracking, if you have empty space on a pcb you often just put in space for an IC. Well, Atari used to do that anyway instead of having empty fibreglass real-estate.

The spot below F9 in the photo shows that there's some tracks going to an unloaded position. And strangely, they didn't screen it with its location, E9.

Of course, there was also space for some more transistors. The layout is identical to the 2N6044's used to drive the coin counters which are 12v... So one can infer that the marquee panel lights must have also been little 12v incandescents.

so i crudely drew out the pinout and measured the tracks, seeing how it was connected in the circuit.

So what type of chip could it be? I went through my databook looking for anything that had outputs on pins 5,7,10,12. I came up with the 74174 (hex d-ype flip-flop), and conveniently the /RESET on pin 1 made sense, as did the clocking pin on pin 9.

So I put in a 74174 in a socket at E9.


I didn't load the transistor positions or the limiting resistors because right now i can't be sure there will be anything driving these outputs. I'll load them later if needed. (TIP111 can be used)

So how would the game control these outputs? The key here is the clocking pin 9 which goes to the 7442 decoder at E8-3. This decoder also allows the setting of outputs and reading of inputs.

So what do the schems say? Well, interestingly, Atari actually describes a signal that doesn't actually exist.. /OUT2 (Output 2 enable).

They describe it like /OUT0 (which is used) but its not actually labelled coming off of the decoder. Pin 3 would actually be /OUT2 if pin 2 was /OUT1 for example. And guess where this track leads? Right to our 74174 latch above.

So its a ghost of schematics passed... Someone forgot to remove this signal reference from the schems. You can find it on 1st and 2nd printing schematics, i haven't looked on later revs.

Now we know the latch chip can be addressed from this decoder, we need to work out how we can toggle these outputs from the CPU. Well, take a look at the adresseses decoded for the others

ColorRam is at $4B00, Watchdog at $4C00, those are on pins 4 and 5 of the decoder, so take one step back, and you can see that Pin 3 of E8 must be $4A00.

Now we know that the bi-directional databus feeds our latch chip with BD0-BD5 this gives us 6 bits to play with at $4A00... In actual fact, only the lowest 4 bits are used for our 4 extra outputs. This means that if you write $01 to $4A00 you can make Pin 7 of E9 go high. Write $0F and they'll all go high. Bits 4 and 5 (pins 2 and 15 respectively) are not hooked to external hardware, but they will respond all the same.

Now we can control the outputs of E9 from the CPU, how do they talk to the outside world?

If you look at the schems for the coin counter outputs, you will see that its a simple darlington transistor arrangement. There are LED start button outputs too, so depending on how you wanted to work it, you could drive little incandescent lamps (as i think was the original intention) or if you changed the values of the resistor and bypassed the transistor loading positions you could drive LED's instead, mind you, only at small current.

The edge connector P19 is where these outputs actually go into the cabinet wiring, I've traced them as follows :
P19 Edge Pin
Transistor position
N
A10 (Q7)
M
B10 (Q6)
11
C10 (Q5)
8
D10 (Q4)
Interestingly, as before, Q4-7 are absent from the pcb and parts list.. After Q3, the next transistor is Q8.. So logically following this pattern, the Darlingtons must be Q4-Q7.

 

CONCLUSION
Well, the excercise was to satisfy my own curiosity more than anything else.
Sadly.. The production Missile Command romsets that are out there (01, 02, 03) do not contain any code that writes to $4A00 that i have been able to determine. We will need the prototype code to see if that performs writes to $4A00 to infer anything more from it.
There are three possible ways that the lights could be controlled by the game...

1) there are just 4 lamps, or 4 strings of say 4 lamps together.

2) there are 16 lamps and there is a decoder in the marquee box that decodes the 16 possible static combinations (from 4 outputs)

3) there are 4 lamps in a matrix

Guess #2 is not very likely, you wouldn't use darlingtons on the game pcb to send 4 signals to the marquee box to be decoded. Guess #3 is possible, but this would require the programmer to be watching the timing of the row/column strobes and counting CPU cycles. While its not impossible, this is a lot of work, but it will allow you to control the lamp intensities if you vary the chosen strobe pulse-width.
Guess #1 seems the most likely. Maybe there were say strings of 4 lamps that corresponded to certain indicators on the panel, you could turn these strings of lamps on and off depending on what was going on in the game. I would imagine one light string for each base, one for something else? who knows...

 

 

 

EXTERNAL LINKS
Someone else looked into the possibilities with re-creating the prototype marquee too. This little examples lights up the marquee up in a random fashion.

http://www.youtube.com/watch?v=_QpXi3IuxHU

http://www.youtube.com/watch?v=mQvyrJcAmIs

Someone vectored the topper artwork from the crappy photos that exist online of it, so you can make your own!!

The photo of the prototype Missile command machine was found in Electronic Gaming magazine from the February 1983 issue, page 14, on their National Vanity Board page

You find the issue here and also here

 

SEND ME A MESSAGE IF YOU CAN HELP TO
OBTAIN THE PROTOTYPE CODE, OR WANT TO
WRITE YOUR OWN...

ENJOY!

 

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These pages are (C) Andy Welburn 1996-2002. I cannot be held responsible if the information supplied herein results in a blown monitor/power supply/house fuse/mind. Oh yeah, have a nice day :)