For most people who used 8-bit home computers in the early 1980s, using cassette tapes to store programs was a lasting memory. Only very wealthy people can afford disk drives, so if you don’t like the idea of waiting for the code to load forever, then you are out of luck. However, if you own Sinclair Spectrum, then by 1983, you have another option, the unique Sinclair ZX Microdrive.
This is a format developed internally by Sinclair Research. It is essentially a miniaturized version of an endless loop tape cart. It has appeared in the form of an 8-track Hi-Fi cassette over the past ten years and promises lightning-fast loading times. Seconds and a relatively huge storage capacity exceeding 80 kB. Sinclair owners can keep up with the big boys in the home computer world, and they can do so without breaking the bank too much.
As a traveler returning from a hacker camp on the mainland, due to the pandemic, the British government required me to be quarantined for two weeks. I did it as a guest of Claire. Claire is my friend and he happens to be a source of knowledge. Prolific 8-bit Sinclair hardware and software collector. While chatting about Microdrive, she not only bought some examples of drives and software, but also the interface system and the original boxed Microdrive kit. This gave me the opportunity to inspect and dismantle the system and provide readers with fascinating insights into this most unusual peripheral device.
Take the Microdrive. It is a unit measuring approximately 80 mm x 90 mm x 50 mm and weighs less than 200 grams. It follows the same Rich Dickinson styling cues as the original rubber key Spectrum. On the front is an opening of approximately 32 mm x 7 mm for installing Microdrive tape cartridges, and on each side of the back there is a 14-way PCB edge connector for connecting to Spectrum and daisy-chaining through a custom serial bus Another Microdrive provides ribbon cables and connectors. Up to eight drives can be connected in this way.
In terms of prices in the early 1980s, Spectrum was a terrific machine, but the price of its implementation was that it paid very little for the built-in hardware interface beyond its video and cassette tape ports. Behind it is an edge connector, which basically exposes the various buses of the Z80, leaving any further interfaces connected through the expansion module. A typical Spectrum owner might own a Kempston joystick adapter in this way, the most obvious example. Spectrum is definitely not equipped with a Microdrive connector, so Microdrive has its own interface. Sinclair ZX Interface 1 is a wedge-shaped unit that engages with the edge connector on the Spectrum and screwed to the bottom of the computer. It provides a Microdrive interface, an RS-232 serial port, a simple LAN interface connector using a 3.5 mm jack, and Replica of Sinclair edge connector with more interfaces inserted. This interface contains a ROM that maps itself to Spectrum’s internal ROM, as we pointed out when the prototype Spectrum appeared at the Cambridge Computing History Center, as we all know, it has not been completed and some of its expected functions have not been implemented.
It is interesting to talk about hardware, but of course, this is Hackaday. You don’t just want to see it, you want to see how it works. Now it’s time to disassemble, we will first open the Microdrive unit itself. Just like Spectrum, the top of the device is covered by a black aluminum plate with the iconic Spectrum logo, which must be carefully separated from the remaining force of the 1980s adhesive to expose the two screw cases that secure the upper part. Like Spectrum, it is difficult to do this without bending the aluminum, so some skills are required.
Lift the upper part and release the driver LED, the mechanical device and circuit board appear in the field of vision. Experienced readers will immediately notice the similarities between it and the larger 8-track audio cassette. Although this is not a derivative of the system, it works in a very similar way. The mechanism itself is very simple. On the right side is a micro switch that senses when the tape removes the write protection label, and on the left side is a motor shaft with a capstan roller. On the business end of the tape is a tape head, which looks very similar to what you might find in a cassette recorder, but has a narrower tape guide.
There are two PCBs. On the back of the tape head is a 24-pin custom ULA (Uncommitted Logic Array, actually the predecessor of CPLD and FPGA in the 1970s) for selecting and operating drives. The other is connected To the bottom half of the housing that houses the two interface connectors and the motor switch electronics.
The tape is 43 mm x 7 mm x 30 mm and contains a continuous loop self-lubricating tape with a length of 5 meters and a length of 1.9 mm. I don’t blame Claire for not letting me pry open one of her old-fashioned cartridges, but fortunately, Wikipedia provided us with a picture of the cartridge with the top closed. The similarities with 8-track tape become immediately apparent. The capstan may be on one side, but the same tape loop is fed back to the center of a single reel.
The ZX microdrive manual optimistically claims that each cassette can hold 100 kB of data, but the reality is that once some extensions are used, they can hold about 85 kB and increase to more than 90 kB. It is fair to say that they are not the most reliable media, and tapes eventually stretched to the point where they could no longer be read. Even the Sinclair Manual recommends backing up commonly used tapes.
The last component of the system to be disassembled is interface 1 itself. Unlike the Sinclair product, it does not have any screws hidden under the rubber feet, so in addition to the subtle operation of separating the top of the housing from the Spectrum edge connector, it is also easy to disassemble. Inside are three chips, a Texas Instruments ROM, a universal instrument ULA instead of the Ferranti project used by Spectrum itself, and a little 74 logic. ULA includes all circuits except the discrete devices used to drive RS-232, Microdrive, and network serial buses. Sinclair ULA is notorious for overheating and self-cooking, which is the most vulnerable type. The interface here cannot be used too much, because it does not have a ULA radiator installed, and there is no heat mark on or around the shell.
The last sentence of the disassembly should be the manual, which is a typical well-written thin volume that can provide an in-depth understanding of the system and how it is integrated into the BASIC interpreter. The networking capability is particularly fascinating because it is rarely used. It relies on each Spectrum in the network to issue a command to assign itself a number when it starts, because there is no Flash or similar memory onboard. This was originally intended to position the school market as a competitor to Acorn’s Econet, so it is not surprising that BBC Micro won a government-backed school contract instead of the Sinclair machine.
Starting in 2020, look back at this forgotten computing technology and look at a world in which a 100 kB storage medium is loaded in about 8 seconds instead of a few minutes of tape loading. What is confusing is that Interface 1 does not include a parallel printer interface, because looking at the complete Spectrum system, it is not difficult to see that it has become a sufficient home office productivity computer today, including of course its price. Sinclair does sell their own thermal printers, but even the most star-studded Sinclair enthusiasts can hardly call the ZX printer a novelty printer.
The truth is that, like all Sinclairs, it was the victim of Sir Clive’s legendary cost reduction and the ingenious ability to create impossible ingenuity from unexpected components. Microdrive was developed entirely in-house by Sinclair, but maybe it was too little, too unreliable, and too late. The first Apple Macintosh equipped with a floppy drive came out in early 1984 as a contemporaneous product of ZX Microdrive. Although these small tapes entered Sinclair’s ill-fated 16-bit machine QL, it turned out to be a commercial failure. Once they bought Sinclair’s assets, Amstrad would launch Spectrum with a 3-inch floppy disk, but at that time Sinclair microcomputers were only sold as game consoles. This is an interesting dismantling, but maybe it’s best to leave with the happy memories of 1984.
I am very grateful to Claire for using the hardware here. In case you are wondering, the photo above shows a variety of different components, including working and non-functional components, especially the completely disassembled Microdrive unit is a failed unit. We don’t want to harm the reverse computing hardware unnecessarily on Hackaday.
I have used Sinclair QL for more than seven years, and I have to say that their microdrives are not as fragile as people say. I often use them for school homework, etc., and never miss any documents. But there are indeed some “modern” devices that are much more reliable than the original ones.
Regarding Interface I, it is very strange in electrical design. The serial port is just a level adapter, and the RS-232 protocol is implemented by software. This causes problems when receiving data, because the machine only has time for the stop bit to do whatever it needs to do with the data.
In addition, reading from tape is interesting: you have an IO port, but if you read from it, interface I will stop the processor until a full byte has been read from the tape (which means that if you forget Turn on the tape motor and the computer will hang). This allows easy synchronization of the processor and the tape, which is necessary because of the access to the second 16K memory block (the first has ROM, the third and fourth have additional memory of 48K models), and because of the microdrive buffer It happens to be in that area, so it is impossible to use only timed loops. If Sinclair uses an access method like the one used in Inves Spectrum (which allows both the video circuit and the processor to access the video RAM with impunity, just like the][ in Apple, then the interface circuit could have been simple Much.
Spectrum has as much time as possible to process the received bytes, provided that the device at the other end correctly implements hardware flow control (for some (all?) motherboard “SuperIO” chips *not* the situation. I wasted A few days of debugging before realizing this and switching to the old prolific USB serial adapter, I was surprised that Just Worked worked for the first time)
About RS232. I got 115k error correction and 57k reliable bit bumping without error correction protocol. The secret is to continue to accept up to 16 bytes after discarding the CTS. The original ROM code did not do this, nor can it communicate with the “modern” UART.
Wikipedia says 120 kbit/sec. Regarding the specific protocol, I don’t know, but I know it uses a stereo tape head, and the bit storage is “unaligned”. I don’t know how to explain it in English… the bits in one track start in the middle of the bits in the other track.
But a quick search I found this page, where the user connects the oscilloscope to the data signal, and it seems to be FM modulation. But it is QL and is not compatible with Spectrum.
Yes, but please remember that the link talks about Sinclair QL microdrives: although they are physically the same, they use incompatible formats, so QL cannot read Spectrum format tapes, and vice versa.
Bit aligned. The bytes are interleaved between track 1 and track 2. It is bi-phase encoding. A fm commonly found on credit cards. The interface reassembles the bytes in the hardware, and the computer only reads the bytes. The original data rate is 80kbps per track or 160kbps for both. The performance is similar to floppy disks of that era.
I don’t know, but there were several articles about saturated recording at the time. In order to use an existing cassette recorder, audio tones are required. But if you modify a direct access tape head, you can directly feed them with DC power and directly connect a Schmitt trigger for playback. So it just feeds the serial signal of the tape head. You can get faster speeds without worrying about the playback level.
It is definitely used in the “mainframe” world. I always think it is used in some small computer programs, such as “floppy disks”, but I don’t know.
I have a QL with 2 micro-drives, which is true, at least QL is more reliable than people say. I have a ZX Spectrum, but no microdrives (although I do want them). The most recent thing I got is to do some cross-development. I use QL as a text editor and transfer files to the Spectrum which assembles files via serial (I am writing a printer driver for the ZX Spectrum PCB Designer program, which will upgrade and Insert pixels to a resolution of 216ppi so that the track does not appear jagged).
I like my QL and its bundled software, but I have to hate its microdrive. I often receive “BAD OR CHANGED MEDIUM” errors after get off work. Frustrating and unreliable.
I wrote my computer science BSc paper on my 128Kb QL. Quill can only store about 4 pages. I never dared to overflow the ram because it would start shaking the micro drive and the error would pop up soon.
I have been so worried about the reliability of Microdrive that I cannot back up every editing session on two Microdrive tapes. However, after writing for a whole day, I accidentally saved my new chapter under the name of the old chapter, thus overwriting my work the day before.
“I think it’s okay, at least I have a backup!”; After changing the tape, I remembered that today’s work should be saved on the backup and overwrite the previous day’s work in time!
I still have my QL, about a year ago, I actually successfully used a 30-35 year old mini drive cartridge to save and load it 
I used the floppy drive of the ibm pc, it is an adapter on the back of the spectrum, it is very fast and fun (compare it with tape day and night)
This brings me back. At that time I hacked everything. It took me a week to install Elite on Microdrive and let LensLok always be the role AA. Elite loading time is 9 seconds. Spent more than a minute on Amiga! It is basically a memory dump. I used an interrupt routine to monitor int 31(?) for a Kempston joystick fire. LensLok uses interrupts for keyboard input, so I just need to squeeze in the code to make it automatically disabled. Elite only left about 200 bytes unused. When I saved it with *”m”,1, the shadow map of interface 1 swallowed my interrupt! Wow. 36 years ago.
I cheated a bit… I have a Discovery Opus 1 3.5-inch floppy disk on my Speccy. I found that thanks to a happy accident on the day when Elite crashed while loading, I can save Elite to the floppy disk… and it is 128 version, no lens lock! result!
It is interesting that about 40 years later, the floppy disk is dead and the tape still exists:) PS: I use a tape library, each with 18 drives, each drive can provide 350 MB/s speed;)
I want to know if you disassemble the cassette adapter, can you use the magnetic head to load data into the computer via the microdrive?
The heads are very similar, if not the same (but an “eraser head” should be integrated in the schematic), but the tape in the microdrive is narrower, so you must build a new tape guide.
“Only very wealthy people can afford disk drives.” Maybe in the UK, but almost everyone in the US has them.
I remember the cost of a PlusD + disk drive + power adapter, in 1990, was about 33.900 pesetas (about 203 euros). With inflation, it is now 433 Euros (512 USD). This is roughly the same as the cost of a complete computer.
I remember that in 1984, the price of the C64 was US$200, while the price of 1541 was US$230 (actually higher than the computer, but considering it has its own 6502, this is not surprising). These two plus a cheap TV are still less than a quarter of the price of the Apple II. A box of 10 floppy disks sells for $15, but the price has declined over the years.
Before I retired, I used an excellent mechanical design and manufacturing company in the north of Cambridge (UK), which manufactured all the machines used to manufacture Microdrives cartridges.
I think in the early 1980s, the lack of a parallel port compatible with centronics was not a big deal, and serial printers were still common. Besides, Uncle Clive wants to sell you ZX FireHazard…well printer. The endless hum and the smell of ozone as it moves down the silver-plated paper.
Micro drives, my luck was very bad, I was full of desire for them when they came out, but it was not until a few years later that I started to pick up some hardware cheaply from second-hand goods, and I didn’t get any hardware. I ended up with 2 ports 1, 6 micro-drives, some randomly used carts, and a box of 30 brand new 3rd square carts, if I can make any of them in any 2×6 combination I am very annoyed when I work in one place. Mainly, they don’t seem to be formatted. Never thought about it, even if I got help from newsgroups when I went online in the early 90s. However, now that I have “real” computers, I did get the serial ports to work, so I saved things to them via a null modem cable and ran some dumb terminals.
Has anyone written a program to “pre-stretch” tapes by running them in a loop before attempting to format them?
I don’t have a micro drive, but I remember reading it in ZX Magazine (Spain). When I read it, it surprised me! 
I seem to remember that the printer is electrostatic, not thermal… I may be wrong. The person I worked on developing embedded software in the late 80s plugged one of the tape drives into Speccy and plugged the EPROM programmer into the back port. To say that this is a bastard use would be an understatement.
Neither. The paper is coated with a thin layer of metal, and the printer drags the metal stylus across. A high voltage pulse is generated to ablate the metal coating wherever black pixels are needed.
When you were a teenager, ZX interface 1 with RS-232 interface made you feel like the “king of the world”.
In fact, Microdrives completely exceeded my (minimum) budget. Before I met this guy who sold pirated games LOL, no one I knew. In hindsight, I should buy Interface 1 and some ROM games. As rare as a hen’s teeth.
Post time: Jun-15-2021