Commodore 1551
Encyclopedia
The Commodore 1551 was a floppy disk
drive for the Commodore Plus/4
home computer
. It resembled a charcoal-colored Commodore 1541
and plugged into the cartridge port, providing faster access than the C64
/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 was very similar to the 1541. Like the 1541, it was a single-sided 170 kilobyte
drive for 5¼" disks, with each disk split into 664 256-byte
'blocks' available for user data plus 19 blocks for DOS data and directory; the file system
made each block its own cluster. As with the other Commodore single-sided diskette drives 1540 and 1541, both sides of double-sided media could be used only after punching a second write-protection marker hole in the diskette's envelope and inserting the medium upside down into the drive. This became common practice, even more so since single-sided media became rarely available with prices matching those of double-sided disks.
The disk drive used Group Code Recording
and contained a MOS Technology 6510T processor
as a disk controller
. The 6510T was a specialized version of the 6510 processor used in the C64, and it was only used in the 1551. The DOS limited the number of files per disk to 144 regardless of the number of free blocks on the disk because the directory was of a fixed size, and the file system did not allow for subdirectories. The 1551's DOS was compatible with the 1541, so disks written by one drive could be utilized in the other.
The 1551 is less common than other Commodore disk drives. The 1541 was more readily available as it was compatible with the popular C64
and VIC-20
, so many people opted to use 1541s with the Plus/4. Since the 1551 was compatible only with the Plus/4, which was a poor seller in the United States
, 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.
Floppy disk
A floppy disk is a disk storage medium composed of a disk of thin and flexible magnetic storage medium, sealed in a rectangular plastic carrier lined with fabric that removes dust particles...
drive for the Commodore Plus/4
Commodore Plus/4
The Commodore Plus/4 was a home computer released by Commodore International in 1984. The "Plus/4" name refers to the four-application ROM resident office suite ; it was billed as "the productivity computer with software built-in"...
home computer
Home computer
Home computers were a class of microcomputers entering the market in 1977, and becoming increasingly common during the 1980s. They were marketed to consumers as affordable and accessible computers that, for the first time, were intended for the use of a single nontechnical user...
. It resembled a charcoal-colored Commodore 1541
Commodore 1541
The Commodore 1541 , made by Commodore International, was the best-known floppy disk drive for the Commodore 64 home computer. The 1541 was a single-sided 170 kilobyte drive for 5¼" disks...
and plugged into the cartridge port, providing faster access than the C64
Commodore 64
The Commodore 64 is an 8-bit home computer introduced by Commodore International in January 1982.Volume production started in the spring of 1982, with machines being released on to the market in August at a price of US$595...
/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 was very similar to the 1541. Like the 1541, it was a single-sided 170 kilobyte
Kilobyte
The kilobyte is a multiple of the unit byte for digital information. Although the prefix kilo- means 1000, the term kilobyte and symbol KB have historically been used to refer to either 1024 bytes or 1000 bytes, dependent upon context, in the fields of computer science and information...
drive for 5¼" disks, with each disk split into 664 256-byte
Byte
The byte is a unit of digital information in computing and telecommunications that most commonly consists of eight bits. Historically, a byte was the number of bits used to encode a single character of text in a computer and for this reason it is the basic addressable element in many computer...
'blocks' available for user data plus 19 blocks for DOS data and directory; the file system
File system
A file system is a means to organize data expected to be retained after a program terminates by providing procedures to store, retrieve and update data, as well as manage the available space on the device which contain it. A file system organizes data in an efficient manner and is tuned to the...
made each block its own cluster. As with the other Commodore single-sided diskette drives 1540 and 1541, both sides of double-sided media could be used only after punching a second write-protection marker hole in the diskette's envelope and inserting the medium upside down into the drive. This became common practice, even more so since single-sided media became rarely available with prices matching those of double-sided disks.
The disk drive used Group Code Recording
Group Code Recording
In computer science, group code recording refers to several distinct but related encoding methods for magnetic media. The first, used in 6250 cpi magnetic tape, is an error-correcting code combined with a run length limited encoding scheme...
and contained a MOS Technology 6510T processor
MOS Technology 6510
thumb|300px|Image of the internals of a [[Commodore 64]] showing the 6510 CPU . The chip on the right is the [[MOS Technology SID|6581 SID]]...
as a disk controller
Disk controller
The disk controller is the circuit which enables the CPU to communicate with a hard disk, floppy disk or other kind of disk drive.Early disk controllers were identified by their storage methods and data encoding. They were typically implemented on a separate controller card...
. The 6510T was a specialized version of the 6510 processor used in the C64, and it was only used in the 1551. The DOS limited the number of files per disk to 144 regardless of the number of free blocks on the disk because the directory was of a fixed size, and the file system did not allow for subdirectories. The 1551's DOS was compatible with the 1541, so disks written by one drive could be utilized in the other.
The 1551 is less common than other Commodore disk drives. The 1541 was more readily available as it was compatible with the popular C64
Commodore 64
The Commodore 64 is an 8-bit home computer introduced by Commodore International in January 1982.Volume production started in the spring of 1982, with machines being released on to the market in August at a price of US$595...
and VIC-20
Commodore VIC-20
The VIC-20 is an 8-bit home computer which was sold by Commodore Business Machines. The VIC-20 was announced in 1980, roughly three years after Commodore's first personal computer, the PET...
, so many people opted to use 1541s with the Plus/4. Since the 1551 was compatible only with the Plus/4, which was a poor seller in the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
, 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.