Modern mass storage solution for Commodore PETs


petSD+ front view

petSD overview

A petSD is a modern mass storage solution for Commodore PETs with an IEEE-488-bus. It can be used instead of and alongside vintage floppy disk drives. The petSD makes file sharing between the PET and the PC easy by using standard SD cards for storage.

Since the first petSD from 2011 (now obsolete), a small family of successors emerged, varying in price and capabilities. Dave Stevenson offers a nice comparison:

Dave Curran distributes his pet microSD: a small and affordable device in a form-factor comparable to the PETdisk but with a more powerful controller running the NODISKEMU firmware (formerly sd2iec), offering real plug'n'play.

The petSD-duo will eventually sit at the upper end of the range. I'm occasionally sharing pictures of the development on flickr.

Between these in terms of price and capabilities lies the petSD+:

petSD+ Hardware

The hardware is based on an Atmel AVR ATmega 1284P microcontroller with 128 KB Flash and 16 KB RAM which connects to the IEEE-488-bus using dedicated bus drivers (75160/ 75161) to ensure proper operation when multiple devices are connected.

A green LED indicates activity, a red one flashes on errors.

As an option, a battery-backed real-time-clock (PCF8583) is available, accessible from the CBM and allowing the usage of a current time stamp when writing files to the SD-card.

The PCB offers multiple assembly variants:

Internal usage

Here, the petSD+ is mounted inside the Commodore and attaches to the IEEE-488-bus with a ribbon cable plugged into pin headers. The device number is set by two DIP switches in a range from 8-11, as seen on the 1541-II.

External usage

The petSD+ offers the option to use a "proper" IEEE-488-connector to fix the cable with screws. A text LCD display (4 x 20 characters), having the same dimensions as the petSD+ PCB, can stack on top for a sandwich construction. A menu system is controlled by three buttons (prev, next and select) to operate on settings such as the device number.


NODISKEMU is a fork of sd2iec by Ingo Korb (et al) with some modifications and extensions by Nils Eilers.

The IEEE-488 bus routines were re-written from scratch for improved reliability and higher speed.

Binaries are available as nightlies. Release-versions currently don't exist.

In case you'd like to show Ingo Korb your gratitude, you might want to have a look on his wish lists at, or .

100% Open Source

Firmware, Hardware and all necessary tools to work on them are free software.

The firmware NODISKEMU is licensed under GPL 2 only.

Schematics, the PCB layout (made with KiCad) and the further documents required to build the hardware are licensed under CC BY-SA 4.0.


petSD+ rev. 1

Alternatives you might want to consider

  • pet microSD — plug'n'play petSD without LCD display
  • Flyer Internet Modem — internet modem and disk drive emulator
  • PETdisk — cheap and simple microSD card storage (status unknown)
  • ZoomFloppy — attach vintage floppy disk drives to your PC over USB


Thankfully Dave Stevenson is willing to offer them in one form or another, he even made a huge website about the petSD/petSD+ including an Ordering Page.