A Z80 Computer Kit

Visit the finished Z80 Computer Kit website

This page is about the development of a Z80 computer kit for sale. I have had many visitors email me expressing interest in a kit, and I seem to have a little more energy these days, so I thought I would go ahead and do this. I have never made a printed circuit board before, so it is an interesting learning experience for me. I will update this page as I make progress with the kit. The kit will be essentially the same design as the computer described in the Z80 page on my web site, which is taken largely from Steve Ciarcia's Build Your Own Z80 Computer, Byte Books/McGraw-Hill 1981. I am willing to modify the design if customers want something different. Just let me know.

The kit will consist of printed circuit boards (PCBs), for both the main board and the display, and the components. I think I will be able to sell the main board kit for about $50, and the display kit for $30. The prices might be less if I can sell more kits. The kits will have all through-board parts (no surface-mount components) so it should be easy for a novice to solder together. The kit will include a pre-programmed ROM.

November 8, 2011:

I have finished the PCB layout and routing. I used the KiCad open-source software to create the boards. It worked well. Here are images of the boards:

Now I have to get a PCB fabrication shop to make the prototypes. Then, I will assemble and test the prototypes. Once that is done, I will get a production run, and set up a store on eBay to sell the kits.

November 11, 2011:

I finished the design and layout of the computer and display boards, and uploaded the plot files to the BatchPCB site for making the prototypes. I also decided to include third board, for a logic probe. It will only add a few dollars to the cost, and it is an important instrument to have to make sure the kit is being assembled correctly. I should get the prototype boards back from the fabricator in 2 to 3 weeks.

December 1, 2011:

I just received the prototype printed circuit boards from the fabricator. They look very good. I will solder up the prototypes this weekend, if I have the time. If they work, I will look for a fabricator to make the kit boards to sell.

December 18, 2011:

I completed soldering the prototype main board and display. They work fine. I will start to get quotes on a small production run and sell the PCBs. Here is a photo of the prototype:

December 21, 2011:

Here is a video of the prototype in action:

January 3, 2012:

After more testing I was able to confirm that the system works well. However, I made a mistake in the circuit design that resulted in the input port switches being backwards. That is, the leftmost switch of each port controls data bit 0, and the rightmost switch controls data bit 7. I will have to re-do the circuit board traces to correct this. Since I am changing the traces, I am also making a few other changes. I am removing the single-step circuitry and switch, because it does not work reliably. Although the Z80 CPU is static by design, it is not tested at slow clock rates, and cannot reliably be single-stepped. If the CPU is out of reset (running), and if it does not receive a clock edge in 1 or 2 seconds, it usually goes into a state that looks like reset. The slow clock works fine, and I will not be changing this. I will add an LED to indicate that there is power to the board. I will add some more labels on the board. I will try to make the board a little smaller, which should make it cheaper to produce. There is a trade-off though; if I make the board a lot smaller, it will be harder to solder, and I want to make the kit as easy to assemble as possible. Finally, I am going to take out the 1K RAM, which is made of 2 4-bit by 1K chips, and replace it with a single 2K static RAM chip. This is because, over the years, the 4x1K RAM chips have become very expensive ($2.25 each, or $4.50 to make an 8-bit by 1K RAM). A 8-bit x 2K static RAM chip is only $1.15. Being a hobbyist, I like cheaper.

The bottom line is I have to make a new prototype, and test it. This will take 4 to 6 weeks. If all is well, I will order a small production run. I am still hoping to have the kit ready by March. Thanks for your patience.

February 4, 2012:

I got the second prototype board and soldered it up. It works well -- except for one thing. When running on the slow clock, and connected to the display board, the CPU bogs down after a few cycles then stops. It is behaving the same way it did with the single-step clock (see the post dated Jan 3). The computer works on the slow clock when it is disconnected from the display board, and it works well on the fast (2 MHz) clock whether or not it is connected to the display board. But with the slow clock and the display board it did not work properly. This is a serious problem, because the slow clock and the display board are important educational features of the kit. What is going on? I think I know.

Running the Z80 at a few hertz is hard on the CPU. It doesn't damage it, but the Z80 gets unstable at very low clock frequencies. As mentioned above, I am unable to get it to run when single-stepped, and now with the slow clock while connected to the display board. The Z80 is designed as a static circuit, that is, it should in theory be able to run with a single-stepped or very slow clock. However, according to the Z80 data sheet, it is not tested at clock frequencies below 5 KHz, and only guaranteed to run at clock frequencies above this limit. So, in trying to run it slower (in my case much slower) I am exploring unknown territory.

What is happening, I think, is that high-frequency noise on the power supply lines (the +5V Vcc and GND) is interfering with the CPU circuits. The Z80 seems to become more sensitive to this noise as the clock frequency decreases. What is needed is a bypass capacitor. I have not needed bypass capacitors in my projects up to now, perhaps because I was lucky in picking Z80s that were not so sensitive, or perhaps because there was enough natural capacitance in the wire-wrap systems that this was not an issue. However, as many digital electronic hobbyists know, bypass capacitors are often necessary. Steve Ciarcia also wrote about the need for these in his book (see above). Bypass capacitors are small, usually 0.1 or 0.01 microfarad ceramic caps connected between Vcc and GND near sensitive ICs. They allow high-frequency noise to pass to ground, and so protect sensitive ICs like the CPU from disturbing sharp power spikes.

So, I placed a 0.01 uF capacitor between +5V and GND at the site of the display board connector, and the behavior disappeared. The system ran fine with the slow clock and display board. This means I will have to put a place for this capacitor on the display board PCB before I make a production run. This won't delay the project much. I am glad I saw this problem before I shipped any kits. Interestingly, I makes me think that I might be able to single-step the Z80 if I use appropriate bypass capacitors. If I can, I will re-introduce the single-step clock in a later model. But for now, I will go to production with the current version. I am still hoping for March.

March 10, 2012

I have received the first production circuit boards from the manufacturer, and I have ordered parts for two-dozen kits. I am writing instructions for building the kits. I will build a computer and display using the production boards once the parts get here. If everything is OK I will start selling the kits. The prices will probably be a little less than I estimated before. Also, I have decided to sell the kits from this web site, because an eBay store is too expensive. I have also designed a basic logic probe, and I will sell that also. By the end of March everything should be ready to go!

March 26, 2012

I think it's going to be a few more weeks before I am ready to sell the kits. I have built the computer, display and logic probe from the production boards and the parts I bought to go with the kits. It all works great! But, I want to do a good job with the instructions, to make building and operating the computer a good educational experience. I also want to clean up the EPROM code. I will also need to do the HTML for this web site so people can buy them through PayPal. It is taking a little longer because I have a very demanding day job, and only have an hour a day or so during the week, and a few hours on the weekends to work on this. If you can't wait any longer, email me, maybe we can work out an early sale. But if you can wait to the end of April, I think I will have a decent instruction manual for you. Thanks for your patience!

April 27, 2012

I have finished a first draft of the kit instructions. Please have a look at them, and let me know if you find any mistakes or if there is something in them that you can't understand. They lack a table of contents, and index, and page numbers. I will add these, and make additional corrections in the next few weeks. But, I think the instructions are adequate for building the kit. I will be out for two weeks. I plan to start selling the kits when I get back, about May 15.

May 23, 2012

Here is the final draft of the kit instructions. There is a table of contents, and page numbers. The image pixel dimensions have been trimmed so the document file is smaller. The EPROM programs have been modified (added PEEK and POKE programs). I would start selling kits now, except I damaged my home-made EPROM copier (connected the +25V programming voltage to the +5V input and fried some chips). I will get new chips next week, and then I can start burning the EPROMs with the updated program.

May 30, 2012

I fixed my EPROM copier. I burned a bunch of 2716's, and I am ready to sell the kits. The prices are $42.00 for the computer kit, $24.00 for the display kit, and $8.50 for the logic probe kit. If you need a power supply (+5V DC regulated, 2000 mA) I have them for sale for $10.00. If you have a power supply, but need a plug with leads that fits the power connector on the computer board, I have these for $1.70. Here are the finished kit instructions.

To buy, send me an email telling me what you want. If you are one of those who already sent me an email, I will contact you, and you will have priority. Once I know what you want to buy, I will create a PayPal invoice for your purchase. PayPal will send you an email with a link, and you can pay by credit card, or PayPal transfer if you have an account. If you live in Maryland, I will have to add sales tax. If this will be an international sale, tell me your country, and we will try to work something out.

August 25, 2012

I have sold and shipped 13 computer kits so far, and a number of display and logic probe kits. Feedback has been positive. I have heard back from 5 or 6 customers, all built the kits successfully. I have not heard from anyone who was not able to build a kit. There have been a few bumps in the road. I shipped 3 display kits with 1 meg resistors instead of the 470 ohm resistors. One customer built the kit with his own 470's, and I replaced the resistors for the other two. I am, of course, now being more careful when packing the resistors! One computer kit had a bad Z80. I replaced it for free of course, and now I am testing all the Z80s that I ship. Hopefully this won't happen again.

There has been some confusion among customers regarding the power supply vs. power connector:

The power connector is really just leads and a plug that fits the computer kit power jack, for hobbyists who have there own power supplies. The power supply is a "wall wart" with a US-style plug for those who don't have their own power supply. The power jack on the computer kit board is Jameco part number 137673.

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