Editing TRS-80 (section)

=== Floppy disk drives ===
Radio Shack introduced floppy drives in July 1978, about six months after the Model I went on sale. The Model I [[disk operating system]] [[TRSDOS]] was written by Randy Cook under license from Radio Shack; Randy claimed to have been paid $3000 for it. The first version released to the public was a buggy v2.0.  This was quickly replaced by v2.1.<ref>{{cite web|title=Radio Shack Computer Catalog RSC-2, page 15|url=https://www.radioshackcatalogs.com/flipbook/c1978_rsc-02.html?fb3d-page=15|website=Radio Shack Catalogs dot com|publisher=Tandy/Radio Shack|access-date=December 2, 2022}}</ref> 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 FD1771|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.<ref>{{cite web|title=Radio Shack Computer Catalog RSC-2, page 14|url=https://www.radioshackcatalogs.com/flipbook/c1978_rsc-02.html?fb3d-page=14|website=Radio Shack Catalogs dot com|publisher=Tandy/Radio Shack|access-date=December 1, 2022}}</ref>

Demand for Model I drives greatly exceeded supply at first.{{r|green198001}} The drive is unreliable, partly because the interface lacked an external data separator (buffer).{{r|hoganm319810831}} The early versions of TRSDOS were also buggy, and not helped by the Western Digital FD1771 chip which 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 was fairly reliable.

In 1981, [[Steve Ciarcia]] published in ''[[Byte (magazine)|Byte]]'' the design for a homemade, improved expansion interface with more RAM and a disk controller for the TRS-80.<ref name="ciarcia198102">{{cite magazine |last=Ciarcia |first=Steve |author-link=Steve Ciarcia |date=February 1982 |url=https://archive.org/stream/byte-magazine-1981-03/1981_03_BYTE_06-03_Programming_Methods#page/n37/mode/2up |title=Build the Disk-80: Memory Expansion and Floppy-Disk Control |magazine=[[Byte (magazine)|Byte]] |volume=6 |issue=3| access-date=October 18, 2013 |pages=36}}</ref>

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 {{convert|5+1/4|in|adj=on}} diskette drives with up to 720 KB of storage, and also the older {{convert|8|in|adj=on}} 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 {{Fraction|5|1|4}}-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 purchase factory-made "flippies". Some software publishers formatted one side for Apple systems and the other for the TRS-80.

The usual method of connecting floppy drives involves setting the drive letter via jumper blocks on the drive controller board, but Tandy opted for a slightly more user-friendly technique where all four select pins on the drives are jumpered and the ribbon cable is missing the Drive Select line. Thus, the user does not need to worry about moving jumpers around depending on which position on the chain a drive was in.

A standard flat floppy ribbon cable is usable on the Model I, in which case the drives is jumpered to their number on the chain, or even an IBM PC "twist" cable, which requires setting each drive number to 1, but only permits two drives on the chain.

Although third-party DOSes allow the user to define virtually any floppy format wanted, the "lowest common denominator" format for TRS-80s is the baseline single-density, single-sided, 35-40 track format of the Model I.

Third-party vendors like Aerocomp made available double-sided and 80 track {{Fraction|5|1|4}}-inch and later {{Fraction|3|1|2}}-inch floppy drives with up to 720 KB of storage each. These new drives are half-height and therefore require different or modified drive housings.