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Commodore PET Projects - petSD+

petSD+ - IEC Support

Getting from this to this
Photo : Wikipedia    



As its name indicates, petSD+ was designed to allow Commodore PET/CBM computers to use SD media for removable storage; like PET floppy disk drives, petSD+ connects to the PET's IEEE-488 interface. The PET IEEE-488 interface is based on the original IEEE-488/GPIB interface and uses an 8-bit parallel data bus. Later Commodore computers, such as the C64, used an 8-bit serial bus, using a communications protocol based on IEEE-488.

Devices based on the sd2iec firmware are available that provide an SD card storage solution for C64 computers, including the SD2IEC hardware device, or non sd2iec based devices such as Ultimate II or the UK1541.

Given the availability of existing SD card solutions for the C64, there was no intention to include C64 compatibility with petSD+ until a specific request was made from one buyer who wanted to use petSD+ with a C64.

When Nils was asked about the possibility of using petSD+ with a C64, he felt that "it should be rather easy to build an adapter cable to use the petSD+ with a custom firmware with the C64. With a little more effort, it should be possible to switch menu driven between the serial IEC bus and the parallel IEEE-488 bus at runtime."

Nils has now built and tested a suitable cable and has integrated IEC support into the petSD+ firmware.

IEC to petSD+ Cable 

The IEEE-488/GPIB signal lines consist of eight data lines, three handshake lines, and five management lines.

Connector pin assignments are standardized as shown. 1

The Commodore 64 uses a serial version of the bus
Pin 1 /SRQ-IN Serial Request-in (not used by C64)
Pin 2 GND Ground
Pin 3 ATN Attention
Pin 4 CLK Clock
Pin 5 DATA Data
Pin 6 /RESET Reset

Cable configuration for petSD+ to Commodore Serial Port

(Serial port pin out as viewed from the rear of the plug)
Directory listing from my C64 using BASIC V2.

BASIC 2 doesn't have the disk handling features of later versions, so legacy commands are needed to access the disk.
e.g. LOAD"$",8
followed by LIST
to display the disk directory
And, by way of example . . . .

The Hobbit, running on my C64, loaded from petSD+


Toggle IEEE-488 / IEC Mode

The petSD+ menu is used to toggle the operating mode between IEEE-488 and IEC modes
From the main menu, scroll down to the second page of the menu until you see the
    "Select IEC/IEEE-488" option
Press Sel to select that menu entry

A submenu with the two entries IEC / IEEE-488 will appear.
Position the cursor on the required option and press Sel

    "IEC" for serial interfaces, e.g., the Commodore 64, or
    "IEEE-488" for parallel interfaces, i.e., Commodore PET
The Save Settings dialogue will be displayed
Position the cursor on the required option and press Sel
Selecting yes will save the setting to Flash and will persist through reset or power cycles

The first release of NODISKEMU to support IEC bus communications is dated 15/03/2016

At first release, Fast Loaders are not supported, only the slower standard kernal routines work (400 bytes/sec).

"All fastloader support is broken. JiffyDOS seems to work, but it doesn't. Though listing the directory contents with F1 works, the loaded files aren't usable. The fastloader code was hardcoded for a controller running at 8 MHz, but the petSD+'s controller runs at 18.432 MHz, so I'll have to fix and test the timings in order to make them work again.  

Making JiffyDOS work again has top priority because I use it myself but I have little to none interest of fixing all the others. If there is demand, I could have a look at it if I have a testcase for it (which I don't have for most of them, besides the FINAL CARTRIDGE III)."

However, implementation of Fast Loaders has proved to be problematic and support for them is unlikely to be included in the NODISKEMU firmware for the foreseeable future, if ever.


There are very few known users of petSD+ IEC mode, if you are using it, please let me know so that I can update this table

User Country User Country User Country User Country
DS uk PD de MU de -- --
-- -- -- -- -- -- -- --
-- -- -- -- -- -- -- --

References :

1 Basic Knowledge and Glossary for GPIB Communication

2 Github The Commodore IEC Bus


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