FAP reborn – Backplane

Previous Post: FAP with a Keyboard

Next post: CPU board, memory board, video card

Github repo

Can’t believe it has been more than 9 months since my last update. Work picked up, and although I was still working on FAP on and off, I just didn’t bother update the blog until I made some progress.

Last time I was here I just finished the I/O board. The mode 2 interrupt and port read was working, keyboard was working, so did the serial input. So I started coding some simple print routines in assembly, stuff like putc and puts. It quickly turned out that my programs would only work intermittently, sometimes goes into NMI for no reason, sometimes resetting by itself, sometimes jumps to the wrong address. And faster the clock speed, more often those happen. It was getting extremely frustrating since when it goes haywire I wouldn’t know if it’s a bug in my program or just FAP being unhappy.

As you probably have guessed, It was the noise problem from having no ground planes,  hundreds of wires bunched together cross talking like crazy, and maybe even one or two cold solder joints. I thought I could get away with it but I should have known better, just look at this spaghetti shitshow running at 8MHz, it’s amazing it worked at all.


Clearly, in order for FAP to work, I had to ditch hand-assembly and go full PCB. This way, it will look much more professional, and I don’t have to cut, strip and solder miles of red wires. That’s exactly what I did. And now, after 9 months of hiatus, FAP rises again, stronger than ever.

First things first, I need to design a brand new backplane. The original one is basically a stripboard with 4 dozens wires soldered from an STM32 dev board. Since I’m doing it again, I’m going to do the whole thing properly. The uC used on the old FAP was an STM32F103VCT6, 3 problem with it: it’s a 100pin part which is too much for this application, it’s a relatively old member of the STM32 family, missing a lot of nice features, and there is a 32KB code size limit in uVision5. So back to the parts pin it goes, instead I’ll using STM32F072R8T6. It’s a F0 so no code size limit in uVision, it has a lot of new features that F1 doesn’t (32-bit timer, build-in USB pull-up, swappable RX/TX, just to name a few), it’s a 64pin so it’s easier to solder, and in the end it’s cheaper too.

Now comes the problem with the size of the backplane. The board size limit of EAGLE free version is only 8cm x 10cm so that won’t do. Luckily EAGLE offers a educational version, free as well if you have a .edu email, that has a 10cm x 16cm limit. That’s what I ordered and as a result 10cm x 16cm will be the dimension of the FAP backplane.

Next part to reconsider is the bus connector, in the old FAP it was a double row female pin header with 2 rows of 40 pins, so in theory it should have 80 signals. But because it was on a strip board the two rows were connected, only 40 signals were actually available. And because I had to cram most of the CPU signals on there, few control signals had to be omitted and only one pin was used for GND, resulting in the non-existent noise immunity. This time I decided to still use double row female pin header, this time 38 pins wide because of the board size. But because it’s on PCB I have all 76 signals so every single Z80 signal is on the bus this time. Here is the pinout of FAP’s new bus connector.


As you can see most of the first row is GND to reduce noise, the clock signal is surrounded  by 3V3 to for that reason too. The signals is arranged in the form of control outputs, control inputs, data, and address.

With those problems out of the way, here is the finished design of FAP’s new backplane:


5 bus connecters spaced 2cm apart, the microcontroller on the right side of the board, a USB connector provides both power and communication, and a 3.3V regulator and 6 buttons round up the design. I’m also putting the LCD above the uC instead of letting it dangle off the side of board.

Here is the assembled board, much neater than the old one, and hopefully a lot less noisy too.


The LCD is secured via 4 mounting holes, and the microcontroller hides below it.


The backplane firmware needed an update too. On the old backplane communication was via a serial port, this time we’re using USB, which is much faster. But overall not much has changed. You can find the up-to-date resources on the Github repo of this project.

Previous Post: FAP with a Keyboard

Next post: CPU board, memory board, video card

Github repo


3 thoughts on “FAP reborn – Backplane”

  1. Aesthetic choices I don’t get here…don’t know if I want to look back and see why it is called FAP…; super archaic industrial header connectors (what, in case you decide to choose the 42V family instead of 3.3V?) But no per-board FAULT lamps or speaker/buzzer debug monitor ports. Staying at 8MHz as sysclk instead of the clock controlling Little.Big ARM transitions. Not just picking a part that muxes multiple Z80 as PIC peeps were doing in the late ’80s. Not adding an instructions/clock whirligig. No Voight-Kampf bellows. And every other little daughtercard seems to be a microcomputing platform? Maybe you plan to put a Dalek in one of cards, or the IIgs 2017 (kidding!)

    Then of course there’s…I mean why wouldn’t you just slave the 8088-based display controller rather than feed it some derivative signal requiriing VGA or NTSC tracking? That scrambly output was wonderful! If you could only mux it back in. Best of luck with JTAG ports.

    Maybe exercise ball energy recovery modules in when you’re planning the console, screen manager and joystick I/O? Backend debug for my the backend!


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