Ryan's SepTandy Spectacular! 4: Peripheral Vision Part #2 - The TRS-80 Expansion Interface and disk drives

Welcome back to part 2 of my fourth SepTandy Spectacular. In the last post we took a closer look at the TRS-80 monitor and tape decks, the earliest periferals for the TRS-80. In this post we'll have a look at some more substantial peripherals, starting with the TRS-80 Expansion Interface.

The TRS-80 Expansion Interface 

The Expansion Interface was important for the TRS-80 as it allowed for hooking up more periferals to the system to extend in usability. It was released in 1978, just a few months after the TRS-80 launched, and sold at a retail price of  $299.00 dollars for one with no extra RAM installed, $448.00 dollars for one with 16K of RAM installed, or $597.00 dollars for one with 32K of RAM installed

On the front you have the card edge for the cable to connect the Expansion Interface to the TRS-80 itself, a power light, and a cover for adding an optional RS232 Serial Port card.

The front of The TRS-80 Expansion Interface

On the left side you have 2 card edge connectors one for hooking up a printer, and the other serving as an extension to the Expansion Interface itself. 

The left of The TRS-80 Expansion Interface

On the back you have holes for the dual power supplies, 3 cassette jacks, and the floppy disk interface.

The back of The TRS-80 Expansion Interface

On the top side you the covers for both the power supply compartment and the expansion card compartment held in with some screws doubling as the monitor stand and the model information sticker.

The top of The TRS-80 Expansion Interface

And on the bottom you have holes for ventilation and the rubber standing feet.

The back of The TRS-80 Expansion Interface

Now the Expansion Interface came with its own external power supply that was exactly the same as the one used for the Model I. How it worked was you plugged one into a din socket on the Expansion Interfaces motherboard and you plugged the other into the computer itself and rerouted the AC power cords through the holes.

A section from the  Expansion Interface manual for setting up the power supplies

Each unit came with cover-plates protecting all of the ports, including one on the interface cable that doubled as a cover door on the computer. However, most people ended up them when they plugged in their peripherals, so finding one with the plates intact is pretty rare today.

Oddly detail even though there were three cassette plugs, only two worked at a time. Even the manual pointed this out by saying: If you have just one recorder, leave it connected directly to the CASSETTE jack on the TRS-80. There is no advantage in having a single recorder connected via the Expansion Interface. Why Tandy did this is unclear.

As for hooking up drive, you just simply hook up one drive and daisy chain more as needed, a practice that would make its way into other Tandy drives.

One of the floppy drives for the TRS-80 Model 1

In order to operate the drive, The drives came bundled with an operating system called TRSDOS, TRSDOS consisted of a command shell and a Disk BASIC with extended features. Unlike other computers at the time, the Model I was designed to support floppy drives as a future option, with The Level II ROM booting from a floppy disk if any were connected, and the ROM BASIC included special hooks to support Disk BASIC commands.

 TRSDOS (excluding Disk BASIC) was written Randy Cook as Shugart, the company which was manufacturing floppy drives for Radio Shack, recommended Cook as an expert in floppy disk interfacing. While TRSDOS worked, it had its flaws mainly due to faulty hardware, so most users quickly switched to other third party operating systems like NewDOS/80.

TRSDOS 2.3 (1979)

Each unit came with cover-plates protecting all of the ports, including one on the interface cable that doubled as a cover door on the computer. However, most people ended up them when they plugged in their peripherals, so finding one with the plates intact is pretty rare today.

Either way, The Expansion Interface proved to be a huge success. However much like the TRS-80 itself, it had some reliability issues, mainly due to the interface cable being unshielded leading to signal interference. Tandy tried three different solutions to the problems, At First, they tried buffered cables which had special circuitry inside to amplify the signals and reduce noise, Then they tried A DIN cable modification separating crucial memory signals from the main cable into another three-pin cable with DIN connectors, and finally, they redesigned the Expansion Interface itself in 1980. the redesign reduced electrical noise and placed buffers directly on the Expansion Interface board making the Interface much more reliable. They also changed it so that only 1 RAS memory signal was sent to the Expansion Interface through the cable with the other 2 memory signals were derived from RAS. This did lead to compatibility issues with some Model I speed-up kits that required all three signals, but these could be remedied with some modification.

However the problems many users experienced with the Radio Shack Expansion Interface led to several third party companies, including LNW and Exatron, to design and sell their own compatible interfaces. The first of these was the The LNW System Expansion, released in 1979, and retailed for $69.95 as a kit with a bare printed circuit board and manual, and later $399.95 for a fully assembled one in a metal case known as the LNW System Expansion II.

An ad for the LNW System Expansion (1980)

The reason why it was sold so cheap was because it laced the electronic components required to build it. By buying the components separately and using a modular design, A user could buy and build only the things they wanted, thus depending on the price they paid for the components they could save over $100 dollars over the price of the Radio Shack Expansion Interface. it had all the same features as the Expansion Interface, but was far more reliable, making it popular with many TRS-80 users. LNW would later incorporate these features into the LNW80 clone which we talked about previously.

Another option was The Lobo LX-80 Expansion Interface by by Lobo Drives International, originally announced in the Fall of 1979, but problems with the supplied operating system lead it to be released in 1981 for a retail price of $799.00 dollars.

An Ad for the Lobo LX-80 (1981)

The LX 80 had a few neat features up it sleeve, for one thing, it had a built in disk controller that could handle many different disk formats, including 8 Inch disks and Hard Drives, It also had a switchable ROM that could replace the Model I ROM.

The LX 80 was also notable for coming with its own Operating System called LDOS (Or Lobo Disk Operating System), Which was derived from VTOS and the initial work was done by Randy Cook, the author of VTOS and the Model I TRSDOS. Because LDOS wasn’t specific to the LX 80, Many users used it without the LX 80 as well. LDOS became so popular that Lobos soon spun off into a separate company called Logical Systems, and soon LDOS became one of the major TRS-80 operating systems.

LDOS 5.3.1 (1991)

The next interface was the Micromint Disk-80 Expansion Interface by Micromint Inc, Also released in 1981, for a retail price of $48.00 dollars for a bare printed circuit board for $48.00, $275.00 dollars for a kit (with printer port), or $380.00 dollars for a fully assembled and tested one with a printer port.

An ad for The Micromint Disk-80 Expansion Interface (1981)

Although it was completely compatible with the Radio Shack Expansion Interface, the Disk-80 was smaller than other Expansion Interface replacements, coming in at 2½″ by 9¼″ by 8½″ and weighed only 3 lbs. The Disk-80 offered the same features as most other Expansion Interface alternatives, but unlike most other Expansion Interfaces, the Disk-80 didn’t offer a second cassette port or a RS-232 port. Micromint did make The Comm-80, which offered a RS-232 option. The Disk-80 was also very reliabile and featured a number of other extras like a fully buffered address and data buses, A CMOS chips to help reduce power usage to eight watts, a built-in data separator to ensure reliable floppy transfers, a floppy cabling system that required no termination packs, and a small sized board to help reduce electrical noise.

And finally there was The Holmes Expansion Mainframe By Holmes Engineering, released in 1982, for a retail price of either $150.00 Dollars for the the MF-1 which accepted up to four plug-in modules, or

 $99.50 dollars for the the MF-2, which accepted up to two plug-in modules. .

An ad for The Holmes Expansion Mainframe (1982)

It worked by plugging in modules to add hook ups for different peripherals, with the 5 available: the DX-1S (a single density disk controller with parallel port) for $148.50, the DX-2D, (a double-density disk controller with parallel port) for $219.50, the VX-1 VID-80 (an 80-column video system) for $279.50, the RX-232 (an RS-232 interface) for $99.50, and the the RX-232M (an RS-232 interface with 32K of RAM) for  $179.50.

While were on the topic of disk drive controllers, those are also something that changed with the TRS-80 over the years. While Floppy disks were a great improvement over cassettes, many people reported problems with reliability, particularly when reading or writing lower disk tracks. CRC errors and locked out tracks occurred with disturbing regularity. The solution that most people used was the The Separator by Percom Data Corporation, released in 1980, for $29.95.

An ad for the PERCOM Seporator (1980)

It fixed all of those problems by providing a data separator with far higher resolution. By plugging the Percom Separator into the socket that housed the Expansion Interface’s Western Digital 1771 controller chip, it could boost it up to 16 MHz, far faster than the separator used by the Expansion Interface at 1 MHz, which was necessary for isolating the clock and data pulses that made up a disk track. The Separator was a popular item and Percom continued to sell it for many years.

However users had another problem, The Expansion Interface could read and write to single-density disk, making file sizes limitetd to up to 87K could be stored on a 35-track single-sided disk or 100K on a 40-track single-sided disk. Most people at the time thought it would be impossible to support double-density on the Model I without Either increasing the speed, or adding an interrupt driven disk controller. Percom would also remedy this by with the Percom Doubler, also launched introduced in 1980, at a retail price of $219.95 dollars. 

An ad for the PERCOM Doubler (1980)

Radio Shack also sold their own Double Density kit. It was released in 1982, and sold for $149.95 dollars.

An listing for the Double Density kit  in the 1982 Radio Shack Catalog

While it opperated similarly, it was incompatible with the Doubler, making it less apealing to users. it still managed to get support in TRSDOS 2.7, and was sold until 1984.

Other operating system mods were availible too such as the Omnikron Mapper, Released in 1979, for a retail price of $169.00.

An ad for the Omnikron Mapper (1981)

This plugged into the computers CPU Socket, and gave you the option to read standard CP/M 2.2 formated disks, making it an apealing option for buisnesses.

The last add on well look at is MicroMerlin (Also known as the µMerlin) by Micro Projects Engineering, released in 1982, with a retailprice of $1,195.00.

An ad for the MicroMerlin/µMerlin (1982)

This plugged into the expansion bus and allowed the TRS-80 to run MS-DOS software. It was actually quite compatible with the IBM PC, and was even more compatible than the Tandy 2000. Impressive!

No matter what the users chose, The Expansion Interface and drives were an important step for the TRS-80 to go from Hobby Machine to mainstram device and helping it stay in production for many more years. This concludes part 2 of my fourth Septandy Spectacular, join me next time as we move forward to 1978, and take a closer look at the TRS-80's printers, Modems and alternatives.