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How to connect Commodore disk drives

Connecting a single drive to a Commodore C-64, 128, or VIC-20 is pretty easy: Plug a 6-pin serial cable from the port on the back of the computer to one of the two ports on the back of the drive. It doesn’t matter which port you use. The second port is for “daisy chaining” additional peripherals, such as a printer, or multiple drives. And even though a fast load cartridge speeds up the drive, it plugs into the computer.

Older drives like the 1540, 1541, and 1571 are self-contained. Plug a power cable (which, conveniently, is no different from the power cable used on a most desktop PCs) into the back and power it on. Later 1541-IIs and 1581s use an external power brick. The two drives’ power bricks are interchangeable; however, they do differ from the power brick used by the computer itself. Fortunately, the original power bricks are labeled with the compatible devices, either on a silver sticker on top or molded into the underside.

It’s multiple-drive setups that get trickier.

How to connect Commodore disk drives

All Commodore drives have two serial ports. On this 1541-II, it’s the two circular ports on the far left. The computer has an identical port as well.

Each drive has to have a unique device number. The default number is 8. The 1541-II, 1571 and 1581 have a set of DIP switches on the back to set them to device 8, 9, 10, or 11. Some older 1541 drives have a user-installed toggle switch to change between device 8 and 9. This was a common modification among power users way back when. Also, many aftermarket 1541 clones such as the Oceanic OC-118/Excelerator Plus had DIP switches on the back or on the underside to change device numbers.

Testing an unknown drive

To test the device number on an unknown drive, connect the drive alone, then issue the disk directory command:

LOAD “$”,8

If the drive accesses, then you know it’s set to device 8. If it doesn’t, substitute the numbers 9, 10, and 11 until the drive responds.

It’s not a bad idea to temporarily mark what drive is what, using a sticker or a piece of tape.

Changing device numbers via command

If you want to connect multiple drives, but don’t want to change device numbers, you can change them in software. Connect the drives, but only power one of them up.

Now, issue the following command:

OPEN 1,8,15,”U0>”+CHR$(9):CLOSE 1

This changes the drive to device 9 until you cycle the power again. Now you can power up the other drive and use both of them. Substitute 10 or 11 to use more than two drives.

A caveat with fast load cartridges

Early fast load cartridges assume a 1541 drive and won’t work with 1581 drives. Later fast loaders worked with later drives like the 1571 in native mode and the 1581.

Further reading

If you need help with disks, here’s some advice on disks for Commodore drives. If you need some more commands, here’s a list of common Commodore disk drive commands. And if you need help hooking up your Commodore to a TV, here’s how to connect a 64 and a VIC-20.

2 thoughts on “How to connect Commodore disk drives”

  1. To permanently change the device number of an old 1541, owners had to open up their drives, dive down to the circuit board and scratch off a solder trace. There are actually two traces on the PCB: scratch one off to change the device number from 8 to 9, scratch the other to make it a 10, and both to make it an 11. This was a pretty scary operation at the time — imagine spending $200+ for a second floppy disk drive, only to have to disassemble it and scratching off solder traces before you could use it! And you’re right, some people connected toggle switches to those traces, allowing their drives to be reconfigured without having to re-disassemble them. How far we’ve come — can you imagine a modern device like an iPod or a cell phone requiring disassembly?

    I now (essentially permanently) have a u1541 as my 8 device, and a real physical 1541 as a 9, which allows me to convert real disks to virtual D64 images (for emulation purposes) and back. The only thing my current setup doesn’t make allowance for is booting multi-load original disks — those almost always reference device 8, and few will load from any other device number.

    1. Yep, that design oversight was very much a sign of Commodore not expecting the 1541 to be as successful as it became. Maybe it was a nod to dealers, so they could sell $30 worth of service when someone bought that second drive. And that might have been OK, except the majority of us bought our Commodore gear at Kmart or Target. Dual-drive Commodore setups would have been more common if they’d come with that switch from the get-go.

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