I have conquered the Motorola 6820/6821 peripheral interface adapter!

Last week I got another batch of prototype PCBs and one of them was the latest version of my Bally/Stern CPU socket based daughter board that adds a microcontroller connected to all of the CPU pins needed to let the microcontroller run in parallel with the original CPU or halt it and interface with the RAM, ROM and PIAs on it’s own. The ultimate goal is to have a system which adds as much RAM and ROM as the original CPU (Motorola 6800) can address and be programmable by a PC via USB. At that point it will be possible to write far more complex game rules to run on the original CPU or new game rules that will run in the microcontroller making the Motorola 6800 CPU unnecessary. This RAM and ROM will be usable by either the original CPU or the microcontroller because the microcontroller is connected across the whole address and data bus.

Currently I’m working on writing the microcontroller operating system. The first step was to interface with the PIAs and get them doing my bidding. I first read the 6821 datasheet years ago and I understood the basic concept of how to read and write to the PIAs but I had to go over the datasheet pretty extensively to get the full picture of how they work.

I’m happy to say that at this point I have code to read and write to PIAs. That is a huge step toward reading switches and controlling lamps/solenoids. It seems like something so simple but it was pretty exciting when the LED on the MPU flashed under my control for the first time.

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If you are ever in Las Vegas you owe it to yourself to visit the pinball hall of fame. I was lucky enough to visit the pinball hall of fame on 2 occasions over the 4 days that I was in Las Vegas. I played about 90% of the machines that were available and had a blast. My wrists were sore the following days from playing so much. It was great! I also gave a couple of my circuit boards to owner Tim Arnold and talked with him for a few minutes about pinball boards. You can tell that he is focused on providing great playing games and I really admire the time he commits to what is basically a charity operation. You can read all about Tim and the pinball hall of fame on their web site.

I took a bunch of pictures. I’ve split them up over 2 posts. You can click on any of the pictures to see a larger version. Here is the first set:

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Part 2 of the Pinball Hall Of Fame pictures:

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Update Aug. 21: Looks like you can replace the original 470pf capacitors on -17, -13 and MPU-100 MPUs with 220pf to convert them to the MPU-200 clock speed. I will try to get an oscilloscope image of this conversion but it’s 2 less components to replace.

I was inspired by this thread on RGP to do some hacking on the Bally/Stern MPU clock circuit with the intention of converting -17, -35 and MPU-100 MPUs to the MPU-200 clock circuit. This in conjunction with the 5101-6116 conversion would effectively convert these MPUs to an MPU-200. You would also need to convert the MPU to use either 2732 EPROMs or a single 27c256.

The difference between the 2 circuits is actually pretty minimal. Only 2 resistors and 2 capacitor values need to be changed. Here are the steps to convert a -17, -35 or MPU-100 clock circuit to MPU-200:

1. Replace R4 and R10 with 13.7K 1% resistors (1/4 watt)
2. Replace C14 and C15 with 100pf capacitors (at least 6V)

This will convert the MPU to the MPU-200 circuit. Alternatively you can just replace C14 and C15 with 220pf capacitors.

The following clock circuit schematic shows values for all Bally/Stern MPUs.

Click “Read more” for more detail on the Bally/Stern clock circuit with some examples of this conversion in action.

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Recently I have been experimenting with methods to replace the EPROMs on Bally/Stern MPUs with a single EPROM. This is the direction I’m planning to go with the more advanced version of my MPU daughter boards. Initially I was planning to use a 2764 EPROM to replace the original 2x 2732 EPROMs but the Motorola 6800 is capable of addressing a lot more memory than what is used on the original Bally and Stern MPUs. With that in mind I decided to increase the available address space for program ROM and RAM with my boards. I accomplished more ROM quickly using Oliver Kaegi’s great work. I am tempted to use this addressing scheme in the next version of my Bally/Stern MPU daughter boards because it’s so simple to implement with basic logic but ultimately I would like to use programmable logic for the address decoding so I can increase the available RAM dramatically. The NVRAM I’m using is 8kx8 so I can easily increase the available RAM by more than 16x and ROM by more than 8x using a 27c512 slapped across the whole address space. With that kind of silicon available even a hack like me can write some deep and cool game rules for these old games!

Below is a picture of the MPU I’m using to test these ideas. It’s starting to look pretty hacked. I like to think of it as SUPER CHARGED!

Click “Read more” for a more in depth review of Oliver’s ROM addressing method and how to use it with any game.

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I’m finally all moved out of my warehouse. I had to leave quite a bit of stuff behind but I was able to move all of my pins and pinball parts as well as pretty much everything I actually wanted to keep so in the end it was not a big loss. It was actually kind of nice to shed so much stuff all at once. Here are a couple of pictures of one of the loads of monitors. These are mostly vector monitors.

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NOTE: Procedure was updated July 30, 2010.

While looking at the Stern MPU200 schematic trying to figure out how I was going to add RAM to my MPU daughter boards I realized how easy it would be to install a 6116 SRAM in one of the unused EPROM sockets to replace the 5101. This replaces both 5101 RAM chips on an MPU200 with a single 6116. If you intend to use this conversion with game code originally using only one 5101 RAM you will need to tie D0-D3 of the 6116 to +5V. I tested it with Ninball and Stingray ROMs but this whole conversion has not been thoroughly tested. I’m surely not the first person to come to this conclusion but I haven’t seen this conversion discussed on the web. It seems so obvious once you look at it.

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A lot of my projects are finally starting to come together. These boards are another step toward a dedicated lamp driver board testing system. These boards connect to J1, J2 or J3 on the lamp driver board and connect an LED to each SCR output. There is a chart screened on the board that shows which SCR is associated with which LED based on which connector the board is attached to. They can be used by themselves to troubleshoot problems with the Bally/Stern lamp driver board while it’s installed in the machine. There is a wire soldered to the board with an alligator clip on the other end that connects to the switched illumination bus. Alternatively the boards can be used with my lamp driver tester board to test lamp driver boards out of the machine.

Here are some pictures:

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A while back I designed some boards to replace the EPROMs on Bally/Stern MPUs. Once I started using it to test boards I realized that I needed to design another board geared more toward testing. What you see below is the result of this effort. It’s basically the same board but I’ve added a test LED connected to CPU address line A6, a reset LED (on when reset is high), a reset switch (which actually resets the CPU rather than triggering a non-maskable interrupt like the button on the MPU) and a jumper selectable on-board reset circuit (using Dallas DS1811 150ms reset).

These boards turned out really well. I’ve already used them to repair a couple of MPUs and I won’t consider working on a Bally/Stern MPU without one ever again. I will have a limited number of these for sale at $40 each including shipping within North America. Email me at lrupertus@gmail.com if you would like one.

Recently the company that I’ve been renting my warehouse space from needed to use the space for their own storage so I was forced to move out. Most of the pinball machines I had stored there are now being stored in my basement and I’m planning to restore and sell 50+ machines over the next few years. This is good news for readers of this blog but it has been a lot of work to get everything moved and get back to a state where I can actually work on games again.

Here are some pictures of the games being moved. I couldn’t have done it without the help from some great friends and fellow collectors. We moved 5 truck loads the first day and another 3 truck loads about a week later.

There is still a lot of shuffling of stuff going on but I should be back in restoration mode by the end of July 2010.

Enjoy the pictures.

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