The Commodore 1551 (originally introduced as the SFS 481) is a

floppy disk A floppy disk or floppy diskette (casually referred to as a floppy, or a diskette) is an obsolescent type of disk storage composed of a thin and flexible disk of a magnetic storage medium in a square or nearly square plastic enclosure lined ...

drive for the

Commodore Plus/4 The Commodore Plus/4 is a home computer released by Commodore International in 1984. The "Plus/4" name refers to the four-application ROM resident office suite (word processor, spreadsheet, database, and graphing); it was billed as "the produc ...

home computer. It resembles a charcoal-colored

Commodore 1541 The Commodore 1541 (also known as the CBM 1541 and VIC-1541) is a floppy disk drive which was made by Commodore International for the Commodore 64 (C64), Commodore's most popular home computer. The best-known floppy disk drive for the C64, the ...

and plugs into the cartridge port, providing faster access than the

C64 The Commodore 64, also known as the C64, is an 8-bit home computer introduced in January 1982 by Commodore International (first shown at the Consumer Electronics Show, January 7–10, 1982, in Las Vegas). It has been listed in the Guinness Wo ...

/1541 combination. Commodore reportedly planned an interface to allow use of the 1551 with the C64, but it was never released. Aside from faster access, the drive is very similar to the 1541. Like the 1541, it is a single-sided 170-

kilobyte The kilobyte is a multiple of the unit byte for digital information. The International System of Units (SI) defines the prefix '' kilo'' as 1000 (103); per this definition, one kilobyte is 1000 bytes.International Standard IEC 80000-13 Quanti ...

drive for 5¼" disks, with each disk split into 664 256-

byte The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable unit ...

blocks available for user data plus 19 blocks for DOS data and directory; the

file system In computing, file system or filesystem (often abbreviated to fs) is a method and data structure that the operating system uses to control how data is stored and retrieved. Without a file system, data placed in a storage medium would be one lar ...

makes each block its own cluster.

Hardware

The disk drive uses group coded recording and contains an MOS Technology 6510T processor as a

disk controller {{unreferenced, date=May 2010 The disk controller is the controller circuit which enables the CPU to communicate with a hard disk, floppy disk or other kind of disk drive. It also provides an interface between the disk drive and the bus connectin ...

. The 6510T is a specialized version of the 6510 processor used in the C64, and it is only used in the 1551. The DOS limits the number of files per disk to 144 regardless of the number of free blocks on the disk because the directory is of a fixed size, and the file system does not allow for subdirectories. Its DOS is compatible with the 1541, so disks written by one drive can be utilized in the other. The 1551 has no dip switches to change the drive's device number. If a user added more than one drive to a system, they had to open the case and cut a trace in the circuit board to permanently change the drive's number, or hand-wire an external switch to allow it to be changed externally. It is possible to connect a maximum of two 1551s to one computer.

Popularity

The 1551 is less common than other Commodore disk drives. The 1541 was more readily available as it is compatible with the popular

and

VIC-20 The VIC-20 (known as the VC-20 in Germany and the VIC-1001 in Japan) is an 8-bit home computer that was sold by Commodore Business Machines. The VIC-20 was announced in 1980, roughly three years after Commodore's first personal computer, the ...

, so many people opted to use 1541s with the Plus/4. Since the 1551 is compatible only with the Plus/4, which was a poor seller in the

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., federal district, five ma ...

, few were made. In Europe, the Plus/4 was much more successful, but because tape drives were the most popular storage device in Europe in the 1980s, the 1551 was not very popular in Europe either. {{Commodore disk drives CBM floppy disk drives