Commodore Floppy Drive 1541 Adapter: use the Commodore 1541 drive as PET floppy drive (1541 to IEEE488)
The drives for the Commodore PET computers are quite scarce, unreliable, bulky and expensive and use a lot of components almost impossible to find noways in case of fault.
The Commodore 1541 drive, instead, is quite common, reliable and easy to fix, if needed. However it is incompatible with the PET series, because they use a different data transfer protocol (IEC vs. IEEE).
With this simple, not destructive and totally reversible mod, you can now use a 1541 floppy disk drive with your PET.
This project is based on an old article published on a german magazine in the 1980s; it is tested, works fine and it's quite easy to build.
See the Bill of Material for the required parts (really few parts, all through-hole, easy to find on Mouser, Digi-Key and also eBay and Aliexpress). No SMD soldering is required for this project.
Instructions:
- Solder the components on the PCB. Use sockets for all the ICs. NOTE: BEFORE to install the 40 pins socket in this adapter, you need to put 2 rows of 20 pins (rounded model) header from the bottom side and solder from the top side. I suggest to put them into the 40 pins socket to keep them aligned and then solder them in the adapter board. This two 20-pins headers will serve to install the board into the 6522 socket of the 1541 drive (in place of the 6522 chip, that need to be removed from its socket and plugged in the socket of this adapter).
- Install the three small ICs (IC2, IC3, IC4) in the sockets.
- Solder a cable on S2. You don't need a 24 wires cable; since pins #12 and #18...#24 are GND, you only need a 17 wires cable, from pin #1 to pin#17 (below you can find a table with the IEEE cable pinout). You will also need a connector on the PET side. What kind of connector depends on which model of PET you have.
- Program the 2764 EPROM with the IEEE488ROM.BIN file (see the ZIP file attached in this project) using an EPROM programmer. I use a MiniPro (TL866 II) programmer, that's quite reliable, versatile and cheap.
- The 2764 EPROM isn't pin to pin compatible with the 2364 ROM, so you must to build an adapter before to install the EPROM on the 1541 disk drive; you can build it using two sockets (Google is plenty of examples) or, easier and better, you can build one of my adapters purchasing the PCB on PCBWay at: https://www.pcbway.com/project/shareproject/2364_2764_27C64_28C64_adapter_with_option_for_Commodore_disk_drive_8050_8250_e6407192.html (the version WITHOUT the small SMD chip)
- Open the 1541 drive cover removing the 4 Phillips screws from the bottom side
- Remove UC3 (6522) and UB4 (ROM). Both ICs should be on socket, but if they're soldered on the board, you need to remove them and replace with a socket. You will only need the 6522 IC, so you can keep the ROM in case you want to reverse the mod.
- Install the 6522 you removed from the 1541 in the adapter board (IC1)
- Install the board in the 6522 socket (UC3)
- Install the EPROM (using the adapter) in the ROM socket (UB4)
- Put two jumpers on J1 and J2 to set the drive as drive #8
- Connect the 1541 drive to your PET and switch them on.
- Put a floppy disk into the FDD and try to format it using the command OPEN15,8,15,"N0:DISK,01":CLOSE15 - You should hear the familiar noise of the drive formatting the diskette
Good luck, and enjoy!
The board installed in the 1541. Note the flat cable I used: it is very easy to trim very gentle the lower shell of the drive and close the cover without to cut or damage the cable. The mod is almost invisible, so if you want to revert the mod, there will be no holes in the 1541 shell.
The connector with an (rather ugly) 3D printed shell
IEEE (PET) cable pinout
PET IEEE-488 connector flavors
Commodore Floppy Drive 1541 Adapter: use the Commodore 1541 drive as PET floppy drive (1541 to IEEE488)
*PCBWay community is a shared platform and we are not responsible for any design issues.
- Comments(2)
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- Jim Jim Apr 03,2024
- erikhuybregts Feb 17,2023
- Arkadiusz Bronowicki Feb 16,2023
- Roman Klövdahl Dec 15,2022
- giobbino Nov 28,2022
- (DIY) C64iSTANBUL Nov 28,2022
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