Editing TRS-80 (section)

== Peripherals ==
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=== Cassette tape drive ===
User data was originally stored on [[cassette tape]]. Radio Shack's model CTR-41 cassette recorder was included with the US$599 package.<ref name="rscat1977"/>{{rp|3–4}} Leininger described the format as similar to one published by [[Hal Chamberlin]]; Tandy did not use the [[Tarbell Cassette Interface]] because of its much greater hardware cost, Leninger said.{{r|leininger19770917}}

The software-based{{r|swaine19810831}} cassette tape interface is slow and erratic;{{r|hoganm319810831}} Green described it as "crummy ... drives users up the wall", and the first issue of ''[[80 Micro]]'' has three articles on how to improve cassette performance.{{r|green198001}} It is sensitive to audio volume,{{r|fylstra197804}} and the computer gives only a crude indication as to whether the correct volume was set, via a blinking character on screen while data is loaded. To find the correct volume at first use, the load is started and the volume is adjusted until the TRS-80 picked up the data. Then it is halted to rewind the tape and restart the load. Users were instructed to save multiple copies of a software program file, especially if audio tape cassettes instead of certified data tape was used. Automatic gain control or indicator circuits can be constructed to improve the loading process (the owner's manual provides complete circuit diagrams for the whole machine, including the peripheral interfaces, with notes on operation).

An alternative to using tape was data transmissions from the [[BBC]]'s ''Chip Shop'' programme in the UK, which broadcast software for several different microcomputers over the radio. A special program was loaded using the conventional tape interface. Then the radio broadcast was connected to the cassette tape interface. Tandy eventually replaced the CTR-41 unit with the CTR-80 which had built-in AGC circuitry (and no volume control). This helped the situation, but tape operation is still unreliable.

TRS-80 Model I computers with Level I BASIC read and write tapes at 250 baud (about 30 bytes per second); Level II BASIC doubles this to 500 baud (about 60 bytes per second). Some programmers wrote machine-language programs that increase the speed to up to 2,000 bits per second without a loss of reliability on their tape recorders.  With the Model III and improved electronics in the cassette interface, the standard speed increased to 1,500 baud which works reliably on most tape recorders.

For loading and storing data from tape, the CPU creates the sound by switching the output voltage between three states, creating crude sine wave audio.

The first version of the Model I also has a hardware problem that complicated loading programs from cassette recorders. Tandy offered a small board which was installed at a service center to correct the issue. The ROMs in later models were modified to correct this.

=== Model I Expansion Interface ===
Only the Model I uses an Expansion Interface; all later models have everything integrated in the same housing.

The TRS-80 does not use the [[S-100 bus]] like other early 8080 and Z80-based computers,{{r|thomas1977}} but according to Leininger, its I/O can easily adapt to S-100 or other buses.{{r|leininger19770917}} A Tandy-proprietary Expansion Interface (E/I) box, which fits under the video monitor and serves as its base, was offered instead. Standard features of the E/I are a floppy disk controller, Centronics [[parallel port]] for a printer, and an added cassette connector. Optionally, an extra 16 or 32 KB of RAM can be installed and a daughterboard with an RS-232 port.<ref name="rsc-02">{{cite book |year=1978 |publisher=Radio Shack |title=1978 Radio Shack Tandy TRS-80 Computer Catalog RSC-02}}</ref> The 40-conductor expansion connector passes through to a card edge connector, which permits the addition of external peripherals such as an outboard hard disk drive, a [[Speech synthesis|voice synthesizer]], or a VOXBOX voice recognition unit.<ref>{{cite web|title=Radio Shack Computer Catalog RSC-3, page 10|url=https://www.radioshackcatalogs.com/flipbook/c1979_rsc-03.html?fb3d-page=10|website=Radio Shack Catalogs dot com|publisher=Tandy/Radio Shack|access-date=December 1, 2022}}</ref><ref>{{cite web|title=Radio Shack Computer Catalog RSC-2. page 13|url=https://www.radioshackcatalogs.com/flipbook/c1978_rsc-02.html?fb3d-page=13|website=Radio Shack Catalogs dot com|publisher=Tandy/Radio Shack|access-date=December 1, 2022}}</ref> Leininger predicted in 1977 that Radio Shack or another company would release an expansion interface compatible with S-100.{{r|leininger19770917}}

Originally, printing with the Model I required the Expansion Interface, but later Tandy made an alternative parallel printer interface available.

The Model I Expansion Interface is the most troublesome part of the TRS-80 Model I system. It went through several revisions. The E/I connects to the CPU/keyboard with a 6-inch ribbon cable which is unshielded against RF interference and its card edge connector tends to oxidize due to its [[base metal]] contacts. This demands periodic cleaning with a pencil eraser in order to avoid spontaneous reboots, which contributes to its "Trash-80" sobriquet. Aftermarket connectors plated with gold solved this problem permanently. Software developers also responded by devising a recovery method which became a standard feature of many commercial programs. They accept an "asterisk parameter", an asterisk (star) character typed following the program name when the program is run from the TRSDOS Ready prompt. When used following a spontaneous reboot (or an accidental reset, program crash, or exit to TRSDOS without saving data to disk), the program loads without initializing its data area(s), preserving any program data still present from the pre-reboot session. Thus, for example, if a [[VisiCalc]] user suffers a spontaneous reboot, to recover data the user enters {{key press|V|C|SPACE|✶}} at TRSDOS Ready, and Visicalc restores the previous computing session intact.<ref group="note">The user must take care not to do anything that could cause memory to be overwritten; they should recover immediately without running any other programs. Since programs may start automatically via the TRSDOS AUTO command, during a spontaneous reboot they take the precaution of holding down {{key press|ENTER}} to prevent this from happening. It takes TRSDOS some half a dozen seconds to load from a floppy disk; the user has this long to keep an AUTO'd program from loading. This star parameter became a standard for many software applications running on the Models III and 4; for instance LS-DOS's TED and Prosoft's Allwrite.</ref>

The power button on the E/I is difficult to operate as it is recessed so as to guard against the user accidentally hitting it and turning it off while in use. A pencil eraser or similar object is used to depress the power button and the E/I has no power LED, making it difficult to determine if it is running or not.

The expansion unit requires a second power supply, identical to the base unit power supply. An interior recess holds both supplies.

The user is instructed to power on and power off all peripherals in proper order to avoid corrupting data or potentially damaging hardware components. The manuals for the TRS-80 advise turning on the monitor first, then any peripherals attached to the E/I (if multiple disk drives are attached, the last drive on the chain is to be powered on first and work down from there), the E/I, and the computer last. When powering down, the computer is to be turned off first, followed by the monitor, E/I, and peripherals. In addition, users are instructed to remove all disks from the drives during power up or down (or else leave the drive door open to disengage the read/write head from the disk). This is because a transient electrical surge from the drive's read/write head would create a magnetic pulse that could corrupt data. This was a common problem on many early floppy drives.

The E/I displays a screen full of garbage characters on power up and unless a bootable system disk is present in Drive 0, it hangs there until the user either presses {{key press|RESET}} on the back of the computer, which causes it to attempt to boot the disk again, or {{key press|Break|Reset}} was pressed, which drops the computer into BASIC. Due to the above-mentioned problems with potentially corrupting disks, it is recommended to power up to the garbage screen with the disk drives empty, insert a system disk, and then hit {{key press|RESET}}.

''InfoWorld'' compared the [[cable spaghetti]] connecting the TRS-80 Model I's various components to the snakes in ''[[Raiders of the Lost Ark]]''.{{r|hoganm319810831}} Radio Shack offered a "TRS-80 System Desk"<ref>{{cite web|title=Radio Shack Computer Catalog RSC-2, page 19|url=https://www.radioshackcatalogs.com/flipbook/c1978_rsc-02.html?fb3d-page=19|website=Radio Shack Catalogs dot com|publisher=Tandy/Radio Shack|access-date=November 30, 2022}}</ref> that concealed nearly all the cabling. It can accommodate the complete computer system plus up to four floppy drives and the Quick Printer. Since the cable connecting the Expansion Interface carries the system bus, it is short (about 6 inches). The user has no choice but to place the E/I directly behind the computer with the monitor on top. This causes problems for a non-Tandy monitor whose case did not fit the mounting holes. Also, the friction fit of the edge connector on the already short interconnect cable makes it possible to disconnect the system bus from the CPU if either unit is bumped during operation.

=== 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.

=== Exatron Stringy Floppy ===
An alternative to cassette tape and floppy disk storage from [[Exatron]] sold over 4,000 units by 1981. The device is a continuous loop tape drive, dubbed the [[Exatron Stringy Floppy|stringy floppy]] or ESF. It requires no Expansion Interface, plugging directly into the TRS-80's 40-pin expansion bus, is much less expensive than a floppy drive, can read and write random-access data like a floppy drive unlike a cassette tape, and it transfers data at up to 14,400 [[baud]]. Exatron tape cartridges store over 64 KB of data. The ESF can coexist with the TRS-80 data cassette drive. Exatron also made a complementary RAM expansion board that installed in the TRS-80 keyboard to increase memory to 48 KB without the EI.<ref name="lundell19810831X">{{cite magazine |url=https://books.google.com/books?id=rD0EAAAAMBAJ&q=%22stringy+floppy+from+exatron%22&pg=PA47 |title=Stringy Floppy from Exatron |magazine=[[InfoWorld]] |date=August 31, 1981 |volume=3 |number=17 |access-date=February 15, 2015 |author=Lundell, Allan |pages=47–48}}</ref>

=== Hard drive ===
Radio Shack introduced a 5 MB external hard disk for the TRS-80 Model III/4 in 1983. It is the same hard disk unit offered for the Model II line, but came with OS software for Model III/4. An adapter is required to connect it to the Model I's E/I.<ref>{{cite web|title=Radio Shack Computer Catalog RSC-8, page 52|url=https://www.radioshackcatalogs.com/flipbook/c1983_rsc-08.html?fb3d-page=52|website=Radio Shack Catalogs dot com|publisher=Tandy/Radio Shack|access-date=December 1, 2022}}</ref> The unit is about the same size as a modern desktop computer enclosure. Up to four hard disks can be daisy-chained for 20 MB of storage. The [[LDOS (operating system)|LDOS]] operating system by [[Logical Systems]] was bundled, which provides utilities for managing the storage space and flexible backup. The initial retail price for the first (primary) unit ({{US$|2495|1983|round=-2}}). Later, a 15MB hard disk was offered in a white case, which can be daisy-chained for up to 60 MB. Like most hard disks used on 8-bit machines, there is no provision for subdirectories, but the DiskDISK utility is a useful alternative that creates virtual hard disk ".DSK" files that can be mounted as another disk drive and used like a subdirectory would.  To display the directory/contents of an unmounted DiskDISK virtual disk file, a shareware DDIR "Virtual Disk Directory Utility"<ref>{{cite book |last1=McKay |first1=Bryan |title=DDIR read-me |date=Jan 2, 1986 |page=1}}</ref> program was commonly used.

=== Printers ===
The "Quick Printer" <ref>{{cite web|title=Radio Shack Computer Catalog RSC-2, page 16|url=https://www.radioshackcatalogs.com/flipbook/c1978_rsc-02.html?fb3d-page=16|website=Radio Shack Catalogs dot com|publisher=Tandy/Radio Shack|access-date=December 1, 2022}}</ref> is an electrostatic rotary printer that scans the video memory through the bus connector, and prints an image of the screen onto aluminum-coated paper in about one second. However, it is incompatible with both the final, buffered version of the Expansion Interface, and with the "heartbeat" interrupt used for the real-time clock under Disk BASIC. This can be overcome by using special cabling, and by doing a "dummy" write to the cassette port while triggering the printer.

Two third-party printers were for {{convert|57|mm|in|abbr=on}} metal-coated paper, selling for approximately DM 600 in Germany, and a [[dot-matrix printer]] built by [[Centronics]] for normal paper, costing at first DM 3000, later sold at approximately DM 1500 in some stores. It has only 7 pins, so letters with descenders such as lowercase "g" do not reach under the baseline, but are elevated within the normal line.

Radio Shack offered an extensive line of printers for the TRS-80 family, ranging from basic 9-pin dot matrix units to large wide-carriage line printers for professional use, daisy-wheel printers, inkjet, laser, and color plotters. All have a Centronics-standard interface and after the introduction of the [[TRS-80 Color Computer|Color Computer]] in 1980, many also had a connector for the CoCo's serial interface.

FP-215 is a flatbed [[plotter]].<ref>{{Cite web|url=https://github.com/human39/fp-215-plotter|title=GitHub - human39/fp-215-plotter: Radio Shack TRS-80 FP-215 Flatbed Plotter Info|date=August 11, 2019|via=GitHub}}</ref>