v2.0. This was quickly replaced by v2.1. Floppy disk operation requires buying the Expansion Interface, which included a single-density floppy disk interface (with a formatted capacity of 85K) based on the Western Digital 1771 single-density floppy disk controller chip. The industry standard Shugart Associates SA-400 minifloppy disk drive was used. Four floppy drives can be daisy-chained to the Model I. The last drive in the chain is supposed to have a termination resistor installed but often it is not needed as it is integrated into later cables. Demand for Model I drives greatly exceeded supply at first. The drive is unreliable, partly since the interface lacked an external data separator (buffer). The early version(s) of TRSDOS are also buggy, and not helped by the Western Digital FD1771 chip that cannot reliably report its status for several instruction cycles after it receives a command. A common method of handling the delay was to issue a command to the 1771, perform several "NOP" instructions, then query the 1771 for the result. Early TRSDOS neglects the required yet undocumented wait period, and thus false status often returns to the OS, generating random errors and crashes. Once the 1771 delay was implemented, it is fairly reliable. In 1981, Steve Ciarcia published in Byte the design for a homemade, improved expansion interface with more RAM and a disk controller for the TRS-80. A data separator and a double-density disk controller (based on the WD 1791 chip) were made by Percom (a Texas peripheral vendor), LNW, Tandy, and others. The Percom Doubler adds the ability to boot and use double density floppies using a Percom-modified TRSDOS called DoubleDOS. The LNDoubler adds the ability to read and write diskette drives with up to 720 KB of storage, and also the older diskettes with up to 1,155 KB. Near the end of the Model I's lifespan in 1982, upgrades were offered to replace its original controller with a double-density one. The first disk drives offered on the Model I were Shugart SA-400s which supported 35 tracks and was the sole -inch drive on the market in 1977–78. By 1979, other manufacturers began offering drives. Models 3/4/4P uses Tandon TM-100 40-track drives. The combination of 40 tracks and double density gives a capacity of 180 kilobytes per single-sided floppy disk. The use of index-sync means that a "flippy disk" requires a second index hole and write-enable notch. One could