PETdisk – operation

This guide should walk you through the basic operation of the PETdisk. Please contact me if you have any questions.

Cassette Interface

Connect the cassette interface board to the PET’s cassette port. Make sure the edge connector on the cassette interface lines up properly with the PET’s cassette port.

Saving a program:

Connect an audio cable between the “Out” or “Save” port on the cassette board to the audio in jack of your computer’s sound card. Using a sound recording application (I use Audacity), start an audio recording with the input level turned most of the way down. On the PET, do a “SAVE” command to begin saving a program to the cassette port.

You should see a square wave audio signal appearing in your recording. If not, check the connection and the input port setting of your audio application.

Loading a program:

Take the audio that you just recorded, and use your audio application to invert the waveform. In Audacity, you would need to select the entire audio recording, and then apply the “Invert” effect.

Connect the audio out from your computer sound card to the “In” or “Load” port on the cassette board. Do a “LOAD” command on the PET. Then play the inverted audio.

After a few seconds, you should see the message “found” or “found <name>” appear on the PET, followed by “ready” once the audio file has finished playing. Your program has now loaded into memory.

Why do you need to invert the waveform before loading? Honestly I’m not sure yet. But there will be a fairly simple hardware modification to remove the need for inverting the waveform before loading, in the form of a modified socket adapter for the op-amp chip. I will publish the details as soon as I get a chance.

One last note about the cassette board – what does the jumper do? The single jumper on the cassette board connects the output of the TL082 op-amp to the cassette read pin on the cassette port, and to one of the data lines on the USB jack. In a future firmware upgrade I hope to emulate cassette loading/saving directly to the SD card to .tap files, skipping the audio domain entirely. When this is ready, the jumper will be removed to select which cassette mode to use – audio, or direct-to-SD. As of now, leave the jumper on.

IEEE-488 Interface

The IEEE-488 board enables loading and saving programs to/from the PET via the IEEE-488 interface. It also provides a passthru interface for connecting other bus devices. Programs in .prg format can be loaded and saved.

Jumper settings

The PETdisk device number is selected by jumpering the pins on the 4-pin header. The device number settings are as follows:

No jumpers – device 8

Jumper on right – device 9




Jumper on left – device 10




Both jumpers – device 11




Preparing the micro-SD card

The PETdisk uses a micro-SD card for its storage. These are generally available cheaply at any electronics store in capacities of 1gig or greater. Any micro-SD card should work with the PETdisk, although I have only had the opportunity to try a few. SDHC cards are supported as well as standard SD.

To prepare a micro-SD card for use in the PETdisk, format it with a FAT32 (not FAT16) file system. In Windows, you can select the desired filesystem from the format dialog box when formatting the card. I have only tried formatting the card to FAT32 in Windows (as the only computer I have with an SD reader is a Windows machine at the moment), but I believe this is quite possible in OS X or Linux, although I don’t have the details here. A quick Google should solve this.


The IEEE-488 PETdisk board connects via the IEEE-488 connector on your PET. The board receives power through its USB connector. If you connect a standard USB cable between the cassette board and the IEEE board, the power from the cassette port will power the PETdisk. The PETdisk can also be powered through a standard USB charger.

Loading the Directory

Insert the formatted micro-SD card into the PETdisk’s uSD adapter, connect the power, and turn on the PET. Assuming that you have one jumper on the right 2 pins, setting the PETdisk to device 9, type:

LOAD “$”,9

After a moment, you should see a “loading” and then “ready” prompt on the PET. Type “LIST” and you will see a directory listing for your PETdisk. It is most likely empty at the moment, unless you copied some .prg files there.

Saving Files

Type in a simple BASIC program, something like:


20 GOTO 10

Let’s try saving this program onto the PETdisk with the command:


You should see “saving” followed after a few moments by “ready”. After this, try loading the directory again and list it. You should see the “TEST” program in your directory listing.

Note: in version 1 of the firmware, saved files are limited to filenames of 8 characters or less. This is because the current firmware does not yet support creating long file name entries in the FAT32 filesystem. Look for this feature to be added in a future firmware revision.

Loading Files

Try loading one of the files stored on the PETdisk. Let’s try loading the test program we just saved:


You should see “loading” and then shortly after, “ready”. Type LIST and you should see that the program you typed in before has been loaded into memory.

This covers the basic PETdisk operation. Please contact me if you have any questions or encounter any difficulties in using your PETdisk.

Firmware notes and PETdisk limitations

Here I will list the things to note about PETdisk operation, as they are reported to me. Many of these will be on the list for future firmware revisions.

- the * operator is not supported in load commands. Commands like ‘LOAD “*”,9′ or ‘LOAD “partialname*”,9 are not currently supported. Loads must use the exact filename.

more to come..

4 Responses to PETdisk – operation

  1. jjpou says:

    i’m very glad with my fully assembled PetDisk. IEEE488 features work well, i will test the audio feature through ‘audacity”, it seems to be cool too !

  2. Bob H says:

    Happy, happy. Built the PETdisk long ago and finally plugged it in. Works great!

  3. John Miranda says:

    I ordered an assembled PETDisk, works wonderfully, a great flash storage solution for your vintage CBM PET machine. Thank you!

  4. bitfixer says:

    Thank you, it should now be fixed. I just switched hosting providers and some of the media did not get copied over.

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