The TDD-2 (Tandy Portable Disk Drive - version 2) is currently the only disk drive still available for use with the Tandy line of portable computers, the Model 100/102 and Tandy 200. This is a captured message thread discussing some aspects of it's technical capabilities. Fm: Craig Lovell 73030,425 To: sysop (X) I am thinking about purchasing a TDD2 for my 32K Tandy 102, but I haven't been able to find the information I need about the drive. I have browsed many of the files in the Review library, but haven't been able to find what I need to know. So, I have several questions: 1: Is there any special reason that the TDD2 will only hold 200K? The 3 1/2" drives normally format to at least 720K. 2: What about the 40 file per bank limit? Is that a hardware or soft- ware limitation? 3: Has anyone been able to exceed these limitations? I plan to use the drive as a more reliable, faster alternative to cassette archive storage; at least for now. Any help/information you can give me would be greatly appreciated. Thanks, Craig Lovell Fm: Tony Anderson 76703,4062 To: Craig Lovell 73030,425 The amount of storage space on a disk/disk-drive combination is determined by many technical factors. In the case of the TDD's it is based on the following: 1. The drive is single-sided; it uses only one side of the disk. 2. The hardware limits the disk to 40 "tracks". 3. In the original design, the software formats each track into two "sectors", each sector capable of storing 1280 bytes of data. 4. 40 x 2 x 1280 = 102,400 bytes, what is called a 100K disk; the TDD-1. 5. 1 sector is reserved for a disk directory. 102,400 - 1280 = 101,120 bytes actually available to the user. 6. In the TDD-2, the data is written in "double density", twice as many sectors are written in the 40 tracks. 40 x 2 x 1280 x 2 = 204,800 bytes. 7. The disk directory stores a file's name, it's storage location, size, and some other data. The TDD-1's directory stores data for 40 files; the data for 40 files, in one sector. To accomodate more files, the TDD-2 uses a second sector as a second file directory, pointing towards another 40 files. In order to maintain disk read compatibility, the TDD-2 writes the directories in single density, and uses double density on the rest of the disk for data storage. This allows the TDD-2 to read the directories of the TDD-1 disks. 204,800 - 2 sectors (2 x 1280) = 202,240 bytes available to the user. If the disk were double-sided, this would be (204,800 + 202,240) 407,040, or a 400K storage capacity per disk. Not too dissimilar from what is available with several other machines with a 3.5 inch disk. The difference is that they have double-sided capability, the TDD does not - no second head, and no software instruction set. 720K requires that the disk and drive be capable of double- sided, quad density data storage, or the ability to write more tracks on the disk, which would mean smaller components, closer tolerances, and less overall ruggedness in portable applications. - You wouldn't be able to throw it in your briefcase and carry it to the Sahara desert, and expect it to work. Why not? Well, to begin with, the TDD was originally designed as a data storage device for an electronic typewriter. It is/was made by Brother Industries in Japan, and it's design was very simple - receive, store, and retrieve data sent to it by an electronic typewriter. Very simple demands. Being an "intelligent" storage device, with the actual operating system on a ROM in the drive itself, it could be command driven from any device that could connect to it through an RS-232 port - thus making it usable with any of the Brother line of typewriters with an appropriate interface. So in order to make it into a disk drive for the "hot" Model 100 line of computers, all Tandy/Radio Shack had to do was get a program that would reside in the computer, which would send the proper commands to the drive to make it do it's thing... after all, the computer already HAD that interface - through the RS-232 port. The disk drive itself, at least the early versions, cost about $40 each in the quantity Tandy was prepared to buy them in. Tandy grossly underestimated the market, and in fact, the first order of 50,000 units was a sell-out almost as soon as it hit the stores. They immediately placed a second order, but had to wait several months for them to arrive. Given the pricing structure, and the original design, it was not possible to make a "compatible with anything else" disk drive. They were not going for compatibility - they relied on adapting the existing Brother design; and in fact, only viewed the disk drive as a disk- based replacement for cassette use, which would allow faster access to files, and be more reliable. They never envisioned its use as a fully functional, fully capable disk drive, and did nothing to develop or implement a real DOS for the drive. - Another example of Tandy's failure to realize the capabilities of what they had available. On your second question, why the 40 file limit; each filename, along with the location and size of the file is stored in 32 bytes in the directory sector. 40 x 32 = 1280, thus filling the sector. To get more files listed in the directory, you need to use more sectors. Thus the TDD-2's use of two sectors to store two directories. On your third question, the TDD-1 can be formatted into different sector arrangements - the TDD-2 cannot. With the TDD-1 you can have sectors as small as 64 bytes; 1580 of them. But 64 x 1580 = 101,120, the same amount of data storage space, just broken up into many smaller bites. But the software in the ROM that writes the disk directory cannot deal with that number of sectors, so the technique is only good for externally controlled random-access software. Traveling Software released a program that took advantage of that capability soon after the TDD-1 was released. It is called TS-RANDOM. It was not overly popular, and is no longer available. My guess is that there are few users of the Tandy portables who needed random-access capabilities in a 100K disk drive for use with a computer that was limited to 28K of RAM space. - Few people needed a 24 carat gold carrying case for a garden shovel. Four enterprising programmers envisioned greater capabilities with the TDD-1, and developped alternate DOS's for use with the disk drive... after all, the real DOS was in the drive on a ROM chip - all we had to do was write a program that would send the correct command to the drive, and it would do whatever it was told to do. The very first of these programs, which we loosely call a "DOS", was a rewritten version of Tandy's file manager software, called DSKMGR. It had the same limitations as Tandy's file manager software, it was basically designed to load and save whole files to and from the disk. The first really capable DOS that actually operated like a "real" DOS, was Power-Disk, by Ed Geise of Acroatix, Inc. It has since been released into the public domain, although Ed still holds the copyright, and a version for Tandy 200 users is in our library 9. Then came Traveling Software's TS-DOS, now discontinued, and Hugo Ferraya's Disk-Power, sold through UltraSoft Innovations, originally based in Canada. About here, in the release sequence, came TS-RANDOM. These DOS's gave the user a read/write capability directly to the disk files, more like the capability of "real DOS's" for other computers, and have become more-or-less standard for any serious users of the Tandy portables. Ed Geise subsequently developped POWR-DOS, a second generation of POWER-DISK, and included many disk utilities and supporting programs which offered much greater latitude in use of the rather basic, primitive TDD. Still talking about the TDD-1 here. A whole series of utilities were developed, including many which were published on this forum, and which are still available in Library 9. POWR-DOS was subsequently released for public access, and a 100/102 version is currently available in Library 9. - Tandy 200 users continue to use Power-Disk, since no version of POWR-DOS for the 200 has ever been located. Acroatix left the portable support market shortly after releasing these DOS's for public access, and no versions specific to the TDD-2, which took advantage of the second disk directory were ever developed. Traveling Software and UltraSoft eventually developed versions of their DOS programs that could access both disk directories, and were therefore fully usable with the TDD-2. The Traveling Software version, TS-DOS 4.0 is now discontinued. So only UltraSoft has a DOS that works fully with the TDD-2, accessing both disk directories, and allowing the user to store up to 80 files in that 200K of disk space. But many users have found that the original DOS's, those written for use with the TDD-1, will still work with the TDD-2 (except for the random-access version TS-RANDOM). But being written for the TDD-1, the programs only know how to locate and read the first directory sector on the disk. Thus, such programs can only find 40 files on the disk - either to read them, or create them. But due to a good bit of luck, they can still use all 200K of the storage space on the disk -- as long as they create no more than 40 files. Those who have TDD-1 drives, can upgrade to a TDD-2, and have full access to the files, except that they cannot write to those files, or save additional files on the same disk due to the format differences between the TDD-1 and the TDD-2. But the TDD-2 can read from those TDD-1 files just fine. So it's apparent that the TDD-2 has the capability of reading either single or double- density data, but it can only write in double-density, on disks that were formatted in double density; i.e. disks that were formatted in a TDD-2. A TDD-1 cannot read or write to disks that were formatted on a TDD-2.