Category Archives: Arduino

Apple II + Arduino + SD Card + SdFat = A2SdFat!

Doing Something Useful

After building the basic framework for interfacing the Apple II to an Arduino over the game port, something useful had to be created with it. I had an Ethernet Shield lying around that also happens to have an SD card slot.

Arduino with Ethernet and Proto Shields

Arduino with Ethernet and Proto Shields

There are already plenty of Ethernet products available for the Apple II, but few SD card options for transferring individual files. FloppyEmu, from Big-Mess-O-Wires is a great product and I highly recommend it, but for my project I wanted something closer to a hardware version of CiderPress: the Apple disk image utility.

I’ve worked on a similar project with Rodney Ross to bring the Apple II Pi’s virtual disk interface to the Super Serial Card using an Atmel processor and the SdFat library. I thought I would take the same basic framework but create a different use: interact directly with the  FAT filesystem on the SD card by way of the SdFat library. This wouldn’t be a virtual drive (that would be easy enough to implement), but a direct interface to the FAT filesystem on an SD card. Also, many new laptops come with an SD card reader built in, so it makes for a great way to sneaker-net files back and forth. Both individual files and floppy disk image files can be read and written to Apple connected storage. ProDOS hard disks, CFFAs, and such can copy files using the CiderPress file naming convention to retain the ProDOS meta information. Floppy disk images can be read and written directly to physical floppy drives. The challenge is to make the data transfer over the game port fast enough to support copying large files back and forth. Sound good? Want to build one yourself? Read on…

The Hardware

One of the goals when creating this project was to use easy-to-obtain parts so that anyone with a little time could replicate the hardware and software themselves.  The parts list is nice and short:

  1. 1 Arduino Uno
  2. 1 Logger Shield or Ethernet Shield + Proto Shield
  3. 16 wire Ribbon Cable
  4. 2 IDC 16 pin DIP Plugs (for both ends of the ribbon cable)
  5. 1 16 pin DIP Socket
  6. 6 Wires
  7. Optional – 1 SPDT PCB Slide Switch (II & IIGS functionality)

Along with a soldering iron and some solder, you will be good to go. You can see my build using the Proto Shield on my previous post. One modification I made was for the Apple IIgs, which lacks the $C040 Strobe signal, was to connect the AN2 pin as the SCLK signal. I used the slide switch to select between the $C040 Strobe and AN2 going to the Arduino’s Digital Pin 6. Using AN2 for the SCLK is slower than the $C040 Strobe, but the only option on the IIgs. Here is the pin connections I use:

 Apple II        Signal         Arduino
 --------        ------         -------
   AN0            MOSI        Digital Pin 8
   AN1             SS         Digital Pin 3
   PB0            MISO        Digital Pin 7

Apple II/IIe     Signal         Arduino
------------     ------         -------
 C040 Strobe      SCK         Digital Pin 6  

Apple IIgs       Signal         Arduino
----------       ------         -------
    AN2           SCK         Digital Pin 6

Apple II game port diagram is available from:

Arduino Uno pin diagram is available from:

The Firmware

Once the hardware is connected (built seems a little extreme), the software to give the Arduino its purpose needs to be programmed. Three items need to be installed on your computer (modern, not retro):

  1. Arduino IDE
  2. SdFat Library
  3. AppleSlave Project
  4. RTC Library (optional)

The Arduino IDE will create a working directory for your projects. Inside this directory, you should create a sub-directory called AppleSlave and place the AppleSlave.ino file there. From the SdFat download, move the SdFat/SdFat/ directory into the libraries/ directory in your working Arduino directory. You should be able to load the AppleSlave.ino project into the Arduino IDE and build it, bringing in the SdFat library. Depending on which Arduino shield you are using for the SD card interface, you may need to edit the AppleSlave source. Near the top, you will find a line that looks like:

const int sdSSpin = 4;

This is the default when using the Ethernet Shield. If you have the Logger Shield, this line should be changed to use 10 instead of 4. If all is in the proper location, you should be able to plug the Arduino into the computer with a USB cable, and build/upload.

The current firmware is still a work in process. There are a few more functions to implement, but it is enough to support the software below.

The Software

There are currently five basic utilities to interact with the files on an SD card.

  • FATCAT – display catalog of SD card
  • FATGET – get a file from the SD card
  • FATPUT – put a file onto the SD card
  • FATWRITEDSK – write an image on the SD card to floppy
  • FATREADDSK – read a floppy to an image on the SD card

These programs are written in PLASMA, a mid-level programming language developed for writing new applications using modern syntax and efficient execution. You can download a disk image, SDFAT.PO, containing a bootable ProDOS floppy with these programs on it, ready to go. Now you have a chicken and egg problem: getting the image written to an actual floppy will require a program like ADTPro (or maybe a friend can create a floppy for you).

Using A2SdFat

Using the utilities involves the PLASMA command line interface. It is a very simplified environment, but not hard to use. You can get a ProDOS catalog by entering ‘C’ and return. To run one of the A2SdFat program, use a ‘+’ to signify it is a PLASMA module, as in:


The other commands take additional parameters. You will get a quick synopsis by just typing the command. But, they are:

+FATCAT [directory]
+FATGET <FAT filename in CiderPress format>
+FATPUT <ProDOS filename>
+FATWRITEIMAGE <FAT filename> [drive (1*, 2)]
+FATREADIMAGE <FAT filename> [drive (1*, 2)]
* = default value

That should be all you need to get lots of floppy disk images read and written, as well as copying ProDOS files around. Hopefully, more utilities will be written to provide menu driven and batch mode copy operations, but this is enough to get going.

As a quick follow-up, I’ve received the cheap Chinese order from BangGood of the GeekCreit Uno and Data Logger Shield. Not nearly the quality of the original, but the price is right. Three knock-off Unos and Data Logger Shields cost less than one original Arduino Uno. The Data Logger Shield is a nice shield that has a full size SD card port and a Real Time Clock chip. Here it is being built up with a socket for the game port cable:






The Arduino/AppleSlave project has been updated to use the SlaveSelect pin on pin 10 and add the RTCLib from AdaFruit.

Also, note that you will need to download a USB serial port driver to use the Arduino IDE under Windows or OS X. So not quite plug and play.  I’ve created a quick video showing off the new parts:

Feel free to leave feedback on the PLASMA GitHub site, or drop me a line.


Apple II, meet Arduino

Having received a package in the mail last week containing Andrew and Ivan Hogan’s GamePort I/O Board, it got my creative juices flowing. The GamePort I/O Board is a great interface to big, external devices. It’s meant to control things like electronic sprinkler heads, relays, that sort of thing. I wrote a PLASMA sketch to simplify the game port interface, and generally had fun with it. But, as I sat there looking at the game port specification in the Apple II Reference Manual, I noted a utility strobe signal that would produce a 1/2 micro-second pulse when referenced. Hmm. In years past, I had tried building an SPI (Serial Interface Bus) connection using a 6522 and external shift register so I could adapt an Arduino Ethernet Shield to the Apple II. It almost worked, but not quite. And not too long ago, Charles Mangin of Option8 fame had brought up for discussion a method of talking to an Arduino over the Apple II’s game port, and a KansasFest presentation to match. All of this must have sunk into my sub conscience, because I actually had a dream about how to bit-bang an SPI interface from the Apple II to an Arduino over the game port.

Why SPI? Well, it is a nicely defined protocol that has some interesting properties. More information can be found on WikiPedia, but it boils down to having a master talking to a slave in a synchronous manner. Data travels both ways: a bit is read as one is written. So you read and write at the same time. Not all data need be relevant, however. SPI doesn’t define a high-level protocol, just a transfer mechanism.  And it only requires 4 wires: SlaveSelect, Clock, MasterOut/SlaveIn, and MasterIn/SlaveOut.

The Apple II game port isn’t the greatest interface for implementing a fast bit-banged protocol, but it isn’t too bad, either. The master is responsible for the clock, which is where that utility strobe comes in. Along with the strobe, it has 4 general purpose digital outputs and 3 general purpose digital inputs. Programming the outputs is a little strange, and somewhat time consuming on a 6502 if you want to scan out a series of bits. For SPI, only two outputs are needed and one input, if the strobe is used as the clock. This is where the strobe signal really helps out with the bit-banging approach. The clock signal doesn’t have to be symmetrical, the slave just wants to see the edges, and it doesn’t have to adhere to a consistent rate (although I assume most hardware implementations are). So that takes care of the Apple II side of things. What about the Arduino?

SPI on the Arduino is officially only supported in master mode. If I was going to bit-bang the interface on the Apple II, why not the Arduino. One of my goals was to leave the hardware SPI interface on the Arduino intact so I could use the Arduino as a proxy for the Apple II when talking to the myriad of Arduino shields out there – many of which talk SPI. On top of that, there are some nice libraries that implement higher level interfaces such as TCP/IP and FAT file systems. Now the Arduino Uno (well, the ATmega 328P) has some nice features for interfacing to all sorts of things, and the Arduino IDE is a great prototyping environment, so it did’t take long to whip up a connection between the Apple II game port and the digital pins on the Uno. It did take a few iterations to identify the best way to interrupt the 328P when the Apple II was initiating a transaction. Software polling was going to be slow, and potentially miss the quick clock pulses. So I connected the clock signal to an interrupt to make sure I didn’t miss any. However, there was enough potential jitter in servicing the interrupt that bits could get lost. So I ended up interrupting on the assertion of SlaveSelect, and went into a tight loop watching the clock signal and updating the shift registers. Because of the tight constraints during data transmission, I skipped the Wiring library and went straight to the I/O ports on the 328P. By choosing my pins wisely, it made the software particularly simple to shift data in and out over the wires.

Testing out the wiring options was made easier by using a breadboard to connect the game port to the Arduino’s header.

Breadboard ConnectionSwitching the pins around was a matter of moving the wire.

Finally, I came to a conclusion as to the best wiring connections:

 Apple II        Signal         Arduino
 --------        ------         -------
   AN0            MOSI        Digital Pin 8
C040 Strobe       SCK         Digital Pin 6  
   AN1             SS         Digital Pin 3
   PB0            MISO        Digital Pin 7

Once I was satisfied the interface was solid, I soldered up my one and only Arduino Proto Shield, which was waiting for just the perfect project to commit to.

IMG_2322I soldered the wires to the back to keep it nice and clean.

IMG_2325The game port does have a 5V supply, but it is very limited in current: 100mA. A base Arduino without much load should be able to keep under that requirement, but when connected to a host PC for programming, I didn’t want my 5V supplies fighting each other, so I added a jumper for the game port’s 5V.  The Arduino site says this isn’t recommended. Yeah, whatever.  Finally, it all came together and I ran my test program on it.

IMG_2323The LED lights up whenever a key is pressed on the keyboard that has the LSB set, or turns the LED off when it doesn’t. The Arduino’s on-board LED was somewhat covered up by the Proto Shield, so I plugged another one right into the header so you could see it. You can find the Arduino sketch and the PLASMA sketch on GitHub.

Going forward, I would like to leverage the FAT filesystem library for SD cards so I can read and write files to and from the Apple II directly from the card. That will greatly simplify my file transfer workflow.