The Machine Behind the Diskette
About this particular machine.
October 28th, 2020 a listing on local classifieds surfaced for a TRS-80 Model II Microcomputer and Disk Expansion Unit, with photos showing two floppy drives, additional software, and documentation. The photo above shows the system tucked away in a back room with the computer sitting on the floor on top of the disk unit.
Scattered around the system are the printer cable, serial cable, and the paperwork, along with a RadioShack diskette organizer. In the organizer were various disks for WordStar and TRSDOS 2.0a, along with a few data disks and one particular floppy that instantly caught my eye.
Some of the photos showed the computer running, but the Disk System Unit is non-functional at testing. There is no power-up, and all three fuses are intact. These all indicate the power supply inside will need service.
The system only contains one drive, single-sided, double-density. There is no way to tell if it works, but when a machine is found in such a clean state as this one, there is a higher expectation that the drive is okay but will still need to have its checks for common failures. One of the positives is no rust; these expansion units are all metal except the front.
The other featured point, besides the classic machine is a backup copy of the Adventure Games demo. When I first glanced at this diskette, I thought to myself; this cannot be the game; there weren't many folks gaming on these. Then I thought, meh, it is likely not going to read even if it is. I honestly did not know how wrong I was.
With this diskette, we needed to build a disk station and capture this image for backup. The eight-inch floppy and drives are not very common today but can still be found. With the right technology and knowledge, reading and writing this obsolete media can still be done.
What is the Model II
New Machine, New Features
New Machine, New Features
New Machine, New Features
The TRS-80 Model II was one of Tandy RadioShack's business-targeted computers. This system wasn't designed as a replacement for the Original TRS-80, now defined as the Model I, but was supposed to succeed the previous model's limits and compete with other business systems in the market like the IBM 5110 and CP/M based computers.
The Model II now provided users with a sharper screen with dedicated graphics hardware combined with upper and lowercase alpha-numeric character sets with special business characters. Tandy RadioShack incorporated the system into a rugged chassis supporting all of the components. The users now had more storage, power, and capability compared to other microcomputers of its time. Add the support of the Nationwide Service locations provided through the RadioShack retail locations, with technical assistance and expertise close by; the TRS-80 Model II was a comparable choice.
Powerful Hardware
New Machine, New Features
New Machine, New Features
The system sported 4 cards by default:
- The processor/serial card utilizing a Z80A at 4MHz and supporting two RS232 asynchronous high-speed serial ports
- The floppy disk controller and printer card based on the Western Digital 1791 diskette controller and supporting a Centronics printer port
- The memory card held 32K or 64K of memory and additional boards could be added to support up to 256K for use with other operating systems
- The last of the four cards combined the display and keyboard controller (Optionally supporting a fifth tandem connected graphics expansion card that gave the system additional bitmap graphics capabilities.)
Expansion Options
Expansion Options
Expansion Options
The Model II featured four additional slots allowing the system to be expanded with options like a hard disk controller to support up to four external hard disk drives, bitmap graphics cards, and the early versions of the 68000 Motorola processor and Memory cards. Using the Motorola 68K processor technically made the Model II a 16-bit dual-processor system, sharply increasing the investment value and capability of the machine.
More cards for other options like a Bernoulli 10/10 diskette drive and the ARCNET network adapters were made for the system but are even rarer to see or find these days.
Storage
Expansion Options
Expansion Options
The previous systems used either cassette or limited 5.25" diskette storage depending on the modifications made. Users could later add double density and double-sided support to the TRS-80 Model I by modifying their systems with third-party products and later official upgrades.
The Model II now provided operators with one to four half-megabyte storage options. Tandy RadioShack included the Shugart eight-inch single-sided drives and offered the optional expansion bay for the additional disks. More details are below on the diskette system as we repair and set up our first image station.
Service and Repair
The only maintenance needed for the primary system was to replace the old filtering capacitors on the main power supply pictured here. They were an involved change as removing the power supply for service meant removing the entire chassis from the plastic case. The power supply shown here is an early version. If the machine is to be upgraded with additional features, this machine will require the later-revision Aztec power supply. They support the higher loads demanded by the hard disk controller and Motorola processor cards.
There are probably a ton of technical bulletins that need to be addressed in this system; RadioShack and Tandy Service Technicians used Technical Bulletins to track and fix issues, just like we get software updates today for modern supported operating systems. In some cases, if a part were predetermined to fail, customers would get a replacement at no cost the next time the owner brought in the machine for service. Since this machine still had its seal, it likely did not see many updates in its time.
With the primary system working, voltages at safe, quiet levels, and passing diagnostics, the attention turns to the diskette unit. The expansion chassis contains an internal power supply for the three additional drives it supports.
The system does not power on, and after inspection, the old filtering capacitors show they have cracked and shorted the power supply. Otherwise, the assessment doesn't find anything else physically or visually out of place.
With the power supply removed, the older filtering capacitors are visible in the upper left; they are gold or amber in color and resemble a rectangular box. They are installed in just about every computer, and power supply made up to the mid-1980s. They are essentially oil-in-paper, and after the 20-25yr mark, the oil evaporates, and the combined heat and electrical stresses cause it to crack and smoke. These have to be replaced with modern components and keep noise and other electrical interference minimized.
Note: Others state to leave these off, but our experience has shown that these also prevent noise on the AC line from affecting the system, so we always replace them with modern components designed to ensure the system operates as intended.
Now that the system powers up, the focus turns to the disk drive. The drive uses 5v, -12v, and 24v voltages. The 5v and -12v connect to the logic of the disk drive; 24v is used to power the stepper motor driving the heads.
This drive did not want to function once it was powered up. If power is connected to the drive logic board and turned on it causes the PSU to shut down, and after pulling out the meter and checking a few components, We found a couple of shorted capacitors. It is typical for the capacitors on these drives to fail shorted; a previous drive also suffered from the identical shorted capacitors.
With the capacitors replaced on the disk drive controller card, the drive is reassembled and tested. Now the diskette expansion system powers up, and the 5v rail is no longer tied down. The red LED on the drive, and the drive expansion unit now powers up, and after connecting to the Model II and testing reading a disk, all seemed to be in working order.
So far, no alignment or any other maintenance on these drives is needed, the drives both format and read blank diskettes, and while one of our Tandy diskettes had errors, countless others work without issue. After running a single-sided disk head cleaner disk and swabbing the heads with denatured alcohol, it was time to build out the disk station and image the first floppy disk.
About Floppy Media
Floppies are a dying media, a day will come for all floppy diskettes when the medium will come apart, and data will be lost. It is a sad fact that we all know and understand, so the work is essential. Here are a few of the issues that come along with storing and caring for floppy drives and media:
- Temperature and Humidity
- Material Breakdown
- Bending/Damage
- Scratching/etching
- Degassing/Magnetic Shielding
Floppy drive heads can scrape or etch the metal-oxide material off the mylar or polyester due to mechanical drift, breakdown, dirt, and other foreign objects that get caught in the head's path. Mold on the disk surface can also accumulate on the drive heads and affect the drives operation and other diskettes. Even media stored ideally for years can crumble on first use due to the degradation of the binder used to hold the oxide coating to the diskette cookie.
For these reasons, it became imperative to develop a method to digitally store and recreate these invaluable software images that complete each of the computers they were designed to run on.
The RadioShack Floppy Disk Format
Tandy RadioShack based the disk format for the Model II on the IBM 3740 diskette standard. The diskette system for the Model II was designed around the Western Digital 1791 and Shugart SA800 diskette drive. Previous eight-inch disk drive designs were SD (FM) (Shugart SA400) and followed the 3740 format. This combination used mixed-density tracks where the boot track (the first cylinder), even on double-density diskettes, is single density (FM), and the rest of the diskette is double density (MFM). Tandy and RadioShack targeted businesses early using mainframes marketing the model II as either a smart-terminal or substitute for larger applications, also touting the machine as CP/M compatible. Using the 3740 formats allowed the computer to read older single-sided single-density diskettes with software written for CP/M and allowed users to access software from other machine types.
The 3740 format specifies that the track or cylinder 0, side 0 will always be formatted in FM (single density) format. Cylinder 0 is defined as the index cylinder and contains 26 sectors of data that identify the disk, error maps of the media, re-allocated data, and data set labels for the data recorded on the disk. Other than the boot track, the rest of the data is encoded in the MFM (double-density) format for double-density (MFM) media.
To adequately capture and write the diskettes, particular hardware is required. Floppy drive controllers have long since been removed or reduced to limited media support for only higher densities in modern systems. To build something that would work required some special hardware.
Hardware
The first part needed to capture floppies is the floppy drive. For this project and the final disk station, there will be three drives: the Shugart SA801, a TM848-2, and an additional MPI eight-inch DSDD drive. For now, the single-sided drive from the TRS-80 Model II is acceptable for the disk image that we want to capture.
We will use a spare Disk Expansion Unit from another Model II with a rusted case for the final eight-inch diskette system to hold and provide the proper power and mounting.
The next component of the mix will be the Adaptec AHA®1522A ISA-to-SCSI with onboard BIOS and Floppy Disk Controller. This card adds SCSI 2 support and features an onboard disk controller and header. The card uses the DP8473 as the FDC from National Semiconductor. It features Software selectable PC data rates of 250/300 and 500kb/s, an Intelligent read algorithm, and mixed density support, critical for reading and writing these diskettes and others for different machines.
The last part of the formula physically connects the drives to the PC floppy bus. The 50 pin Shugart floppy disk protocols for eight-inch media used a particular signal on pin 2 of the floppy cable. The TG43 (Track Greater than 43) would tell the drive to switch to a reduced write current when writing the inner tracks from 43 to 77 on the floppy disk.
The reason for the reduced write current is because as the heads get closer to the inside of the floppy disk, the magnetic fields can alter bits written on either side of the current track. The space between bits on the disk become closer together as you reach the center. Early hard and floppy disk drives would depend on signals from the controller to tell it when to switch the write current (reduce).
TG43 was only a concern when writing disks. Still, since We would be creating diskettes, it would be required since the TPI drive did not yet have the logic built-in and the PC controller no longer supplied this signal as newer drives incorporated it into their logic. Luckily the FDADAP floppy disk adapter takes care of the TG43 and adapts the signals between the buses. You can find more information about it here.
Building a Disk Station
We installed the Adaptec controller into an AST Advantage Adventure 486-DX2 66MHz PC running DOS 5.02 and Windows 95. The system also contains a 3COM EtherLink III 10Mb Ethernet network card connected to the modern LAN. The tower includes the original 250MB hard disk drive still functional and the internal 1.44MB and 1.2MB floppy disk drives that shipped with the system.
The internal FDC was disabled in favor of the Adaptec controller and thankfully was accepted by the system BIOS without issue. The FDC connector has been routed to the back of the system to connect various external floppy drives without opening the case. The external drives outside the system feature the standard 360K and a quad-density 720K 5.25” floppy drives in a single expansion chassis with PSU and another expansion chassis containing the three eight-inch diskette drives seen above.
Capture the disk Image
After getting everything set up and ready, it was now time to try and capture and see if we encountered any more issues. Since there is likely the chance we will get only one or two reads, and the diskette drive could damage the floppy, I wanted to test the setup with a diskette I wouldn’t be too upset about destroying. Since we have made multiple purchases with copies of the TRSDOS 2.0 disk, we had plenty to spare. We started the process and attempted the first image with success!
The system indicated it had read all tracks successfully without any errors or bad sectors. Successfully reading the diskette was great news for both the floppy diskette and the drive. Reading without issue meant both were in good working shape, and everything was working in order enough to capture and create images.
Since the system was ready, we started capturing the Adventure disk image. After waiting on pins and needles, the system fully ingested the diskette. We grabbed a new-old-stock blank and began writing a new diskette to try in the actual Model II. If the diskette read in a genuine Model II, it would tell us if the drives were still in a good alignment.
As you can see here, the new disk reads excellently. A directory listing shows the different adventure game data files, and reading here, we can see 12 separate game files, 0-9 and A-C.
The disk is a data diskette and will not boot. The TRS-80 Model II will need two drives to play the game. The first drive (disk 0) should have the operating system, in this case, TRSDOS 2.0a. The second drive (disk 1) should have this data diskette. At the system prompt, typing “ADVENTUR” loads the game and presents the selection screen for choosing each game.