ZX81 floppy disk systems

Discussions about Sinclair ZX80 and ZX81 Hardware
David G
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ZX81 floppy disk systems

Post by David G »

Recently reading the "10 Year Sinclair Retrospective" piqued my interest in floppy disc systems for the ZX81. And it occurred to me that I knew virtually nothing about them. Here's a summary of the rabbit hole i've gone down

When the ZX81 came out in 1981, home computers used tape recorders to save and load programs. Of course a floppy disk is more convenient:
* more reliable/consistent
* faster in loading
* Faster in finding a particular program (no fast-forwarding/rewinding needed)
* better organizing & storing. Multiple programs stored on a diskette can be shown using the DIRECTORY command, and can be loaded on demand

Imagephoto: George Chernilevsky, Public domain, via Wikimedia Commons

In 1981, the 5.25 inch floppy disk was the choice of hobbyists -- the smaller 3.5 "hard-case" floppy had not been introduced yet. With the rise of CP/M, floppy drive prices were dropping (e.g. a raw drive was $399 in 1979, down to $179 in 1982 and $79 in 1986) And with the recent introduction of the IBM PC, the 5.25" disk drive and media would soon become even more affordable

In 1982, the first floppy disk system for the ZX81 appeared

November 1982 seems to be the first ad for the Aerco controller.
Floppy Disc Interface $179
5.25" double density floppy disc drive $189
Power and cable for two drives $69

That's a total of $437 for a single-floppy setup. In 2022 dollars it would be $1,346

In 1982 I was in my last year of high school, in the Computer class we had a TRS-80 with floppy disk drive (I still have a couple of disks from it). I don't know what the class was officially titled, it wasn't really a computer literacy class, and there was no fixed curriculum. It was the first year at our school for such a class. The mathematics teacher was assigned the class and he just let us do whatever. The only requirement was to complete a "project", which was self-selected. I chose to write a music playback program in machine code. It played twinkle twinkle little star, and could be heard over an AM radio. The teacher said he didn't understand how it worked, and gave me a passing grade for the class. But mostly we tried out the BASIC examples from the manual, played games (especially Scott Adams adventures) and hacked about. For example, we changed the bootloader on floppy disks to display custom messages. Of all the students in the class only one other guy besides me showed this much interest. Later we both ended up working at Microsoft. The little 8-bit machines started many people down a path in the industry

This other student got an Apple II with floppy drives. The cost was more than a year's home rental for me. So with that background, I never even considered get a floppy drive setup for the ZX81

And it seems that many felt the same. It's been said that while USA home computer enthusiasts favored floppy disks, Europeans favored tapes

ZX81 ALTERNATIVES to Floppy Disks
* tape recorder: cheap, ubiquitous but slow
* digital tape player: TZXDuino/MaxDuino/CASDuino/SVI-CAS
* SD-card system: ZXpand+ multi-purpose interface
* USB stick system: Vdrive
* Floppy Stringy: CAI/ESF Stringy Floppy by Extatron/A&J
* Hard drive: IDE controller (pre-SATA technology) see IdeDos
* Online fileserver: ZX-WESPI

FDC setups for ZX81
* Aerco FD-ZX a.k.a. "Aerco 1000 Disc System"
* Larken "ZX81 Disk System" and LDOS
* Mageco Electronic "Interface floppy ZX81"
* Macronics "F.I.Z. Floppy Disk Interface"/Compusa "Floppy Disk Controller"
* Compusa "FDC-100 Floppy Disk System"
* Analogue Interface Systems
* Audio Vision "Floppy Disc Controller"
* Sinclair Place "Floppy Disk Interace"
And the new one:
* ZX1541 interface to use Commodore 1541 disk drive with ZX81
David G
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Review: Aerco FD-ZX Floppy Disk

Post by David G »

I wonder how easy it would be to use something like this in 2022. Surely there must be some old IBM PCs sitting in closets here and there with 5.25 disk drives. I had twenty or so myself in 2013 and they still worked

Anyways, the Aerco system from 1982 was one of the first to market for the ZX81, if not the first

advertisement from SYNC magazine v2.6 (November 1982) page 7
Aerco_advert_sync_v2_n6_p7.jpg
* Maximum 16 programs per disk
* Disk operating system (DOS) program supplied on disk
* USR command to load DOS, then menu for SAVE, LOAD, COPY, RENAME, FORMAT and others
* USR commands for SAVE and LOAD

Syntax/Timex Sinclair news Vol. 5 No. 7 (July 1984) pages 5-6
HARDWARE REVIEW

Product: AERCO FD-ZX Floppy Disk
Machines: ZX/TS

Price:
Interface $179.00
Disk Drive $189.00
Cable & PS $79.00+$4 P&H

From:
AERCO
POB 18093
Austin, TX 78760
512/331-0719

If you value your time and would like to eliminate waiting 6 minutes for a program to LOAD, a floppy disk drive may answer your needs. AERCO's FD-ZX, while not inexpensive, satisfies my needs for fast mass storage.

Floppy diskettes come in various sizes. They are thin mylar disks impregnated with iron oxide, just like the recording medium on cassette tapes. Magnetic heads in the disk drive record and play back data, like cassette players. But in a floppy disk drive, the diskette rotates at a constant 300 RPM and the recording head can position itself (under computer control) to many different tracks on the diskette. Each track is circular and concentric with all the other tracks. An entire diskette can be searched for a program in a few seconds.

AERCO's disk system consists of three major components:

1. Interface board. This professionally designed circuit board contains a 2716 EPROM chip containing the disk operating system, a 1797-02 floppy disk controller IC, and a number of smaller interfacing chips. It mounts on a special ribbon connector to the expansion bus of the ZX/TS. It provides a 34 pin edge-card connector for connecting the disk drive(s). Up to 4 drives may be run from this board.

2. Disk drive. I purchased a Pertec FD-250 drive, which handles 35 tracks per side. It can store almost 1/4 megabyte per side. Pertec no longer manufactures disk
drives, so you may want to get a different brand.

3. Power supply. Supplies 12V at 2 Amps and 5V at 1 Amp. AERCO supplies cables for the drive with the power supply. You could make up your own cable from parts if you already have a power supply to use.

AERCO provides a somewhat sketchy 10 page operating manual, but it suffices to get you up and running. After a brief discussion of disk theory it presents a step-by-step installation procedure. It says how to power up, insert the DOS disk and execute a USR call to bring the system up. In a scant 2 seconds you'll get AERCO's 16K or 64K DOS Menu.

This will allow you to format blank disks (you have to do this before storing programs or data on them), LOAD one of sixteen 16K programs into memory, rename programs on disk, copy disk to disk (if you have two drives), and a few miscellaneous utilities. To SAVE a program, you must break the program (or add an extra line) and execute a USR call to the "page" on which you want to SAVE the program. I leave DOS on page 1 of all my disks and store 15 other programs on the disk. Any of these can be called from DOS by selecting the page number from sub menu, or they may be LOADed directly with a USR call.

You can save up to six 64K programs on one disk, select single or double density recording (single more reliable; double more data). In some cases, the buffer used for double density recording (which is twice as long as the buffer for single density) may intrude into the variables area of your program.

Despite its advantages, the AERCO system has some disadvantages you should know about. You must use USR calls to store and retrieve programs. SAVE and LOAD would have been easier to work with. It's too easy to use the wrong USR routine and write over the disk when you meant to read.

Addresses of some of the EPROM disk routines appear in the documentation, but not elaborated upon enough for the average user. The section on 64K programs seemed to indicate that they can be written to disk, but only 16K might be retrieved. AERCO says up to 52K will be retrieved, all the way up to E_LINE.

You can only store 16 programs (up to 16K each) on a disk, regardless of program length. Even a 1K program takes up 16K disk space.

This unit is an excellent value for those of us committed to our ZX systems. It's fast, reliable, and reasonably easy to use. Better documentation and a more versatile operating system would have been a plus, but even as is, the FD-ZX fits the bill. [Editors Note: AERCO no longer offers the Pertec drive, and instead now sells an equivalent Panasonic drive for $210 + $6 P&H.]

P. J. Donnelly, Centerport, NY
David G
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Aerco FD-ZX Floppy Disk

Post by David G »

seems like there is only one photo of the Aerco FD-ZX on the entire internet!
2007-10-01 $188.50.jpg
this one was on eBay and came with several different versions of DOS: including the original and SADOS

Another web site says it will work with Larken DOS too. Seems like the Aerco controller was fairly standard for the time

The controller chip (and its variations) was used in IBM PCs, Atari ST, Commodore etc so perhaps an interface card could be hacked for the ZX81?
David G
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Re: Aerco FD-ZX Floppy Disk

Post by David G »

oh here's another one

neither show the power supply?

Image
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1024MAK
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Re: ZX81 floppy disk systems

Post by 1024MAK »

The hardware would not be too difficult. The problems that I see are:
  • The software/firmware - is it available, if not, it needs writing from scratch…
  • What documentation exists, if not enough is available, the hardware needs recreating. If the software/firmware is not available, then it will be tricky trying to work out the hardware details (which FDC - floppy disk controller chip), which addresses etc.
  • How much ZX81 RAM is used, and what does the EPROM contain apart from a bootloader, does it contain BIOS or DOS code?
Of course, if you are happy to start the software/firmware code from scratch, then the hardware can be designed without worrying about the previous system designs…

But before anyone goes down the rabbit hole, the details of the ZX1541 are available.

Mark
ZX81 Variations
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mrtinb
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Re: ZX81 floppy disk systems

Post by mrtinb »

1024MAK wrote: Mon Sep 19, 2022 10:05 pm
  • The software/firmware - is it available, if not, it needs writing from scratch…
Well the VB81 supports the Larken disk drive, and another one as well, which I cannot remember. So for the Larken disk drive, the ROM is available.
Martin
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David G
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Re: ZX81 floppy disk systems

Post by David G »

yeah, the ZX1541 seems like the "best" solution. However, Commodore 1541 drives are spendy/desirable around here (USA) while the PC drives are basically giveaways when you find them

EightyOne also has the Larken ROM (LDOS for ZX81, LKDOS for Spectrum)

I always thought it was crazy that Commodore disk drives had their own 6502 (the first "smart" drive?), raising the cost of the drive way up. Or maybe it was the Atari 800 drive. I remember buying one Atari drive after the other as they broke. But the price of the Atari smart drive was down to $150/each in the late 1980s

Apple had the right idea by letting the software control the drive. No need for a second processor or dedicated Western Digital chip
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mrtinb
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Re: ZX81 floppy disk systems

Post by mrtinb »

David G wrote: Mon Sep 19, 2022 11:36 pm I always thought it was crazy that Commodore disk drives had their own 6502 (the first "smart" drive?), raising the cost of the drive way up. Or maybe it was the Atari 800 drive.
Yes, both of them had a built in processor. So the DOS was not in the computer, but in the drive.

Well later we had SCSI drives, and they also were “smart” and received commands on their bus.

Fun to think about that the first hard drive for the Commodores used SCSI. So they had the IEC bus between the computer and drive case, and inside the drive case the SCSI bus to the drive. However the controller chips for SCSI is specialized, and the controller for the Commodore and Atari was general CPUs. The main difference being there was RAM and ROM as well, for the firmware.
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David G
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Aerco REVIEW: Disk Drive System (SYNC magazine 1983)

Post by David G »

I have not experienced one failure in data transfer
In this review:
  • he used a surplus power supply. It powers both the disk drives and the ZX81
  • what hoops users had to jump through back in the early days of home computers
  • the advantage of disk drives over tapes
I cannot say enough good about the way I have been treated at Aerco [and] their product is good, and well-supported by them
From SYNC magazine Vol. 3 No. 6 (November/December 1983) pages 90-92
The Aerco Disk Drive System Paul B. Caley

Paul B. Caley. RD 1, Box 56. Duanesburg, NY 12056.

The Question

Disk drive or no disk drive? That is the question. Or, rather, it was the question, I looked long and hard at all the options and decided to stay with my Sinclair and to add the Aerco floppy disk interface and disk drive to my miniature computer. Many of you out there in Sinclairland are wrestling with the same questions. Let me tell you about my choice and about the happy results.

About a year ago when I put together my $99 handful of hardware the little mite worked. Though I did not know the difference between LIST and LET, I laboriously worked on the miracle membrane. By summer my knowledge had increased and I decided to double my investment by adding a box of Byte-Back parts which gave me a neat 64K memory. By fall I had acquired useful software: a data base (The Fast One) and a word processor (Z-Text/L-Text) from England. I had begun to use this little wonder in a small (church) office.

My part-time secretary politely balked at the toy keyboard when I showed her how to use it. The result was that the original investment increased again when I bought a $25 surplus keyboard, a few diodes, wire, and connectors to attach it to the computer. At least it worked, and it worked well.

Horizons continued to lift. Uses for the expanded Sinclair loomed into view beyond all dreams. By winter I needed a good quality printer to create labels for my mailing list of 250 names and to do

Photo 1. Complete Aerco Disk Drive with connecting cables and 16K RAM pack.
90.jpg
lots of other printing. At this point I made an important decision. I would buy the printer as long as I could use it later on with a "good" computer as well as use it now on my "Rube Goldberg," I was into this whole project to the tune of about $250 for hardware. I added double that investment by getting a Memotech parallel interface and a Prowriter printer. Even though I had now spent $750 for hardware, I knew that my $400 printer could be used later with a "real" computer. From now on I thought I should keep my eyes for the Sinclairland exit door.

New problem: My data base had grown to the point that to load a program took nearly 20 minutes; using the word processor meant a 10 minute wait. Not that one cannot wait and do other things during long LOAD and SAVE times, but what about a disk drive?


The Disk Answer

This decision required a lot more thought and comparison than my printer decision. I could buy a "suitcase" computer and have a neat, integrated unit with lots of super software. I looked, read, talked around, and came to these conclusions: 1) I do not have $1800 for a suitcase computer; 2) My Sinclair does what I need a computer to do; 3) If I buy the right disk drive, I can use it later on a "real" computer.

So I sank another $450 into that $99 Sinclair (which today can be bought for $39, and you get it assembled for the price, too!) on the Aerco floppy disk drive system.


The Aerco Disk Drive

The disk drives sold by Aerco are Pertec FD 250 units. Any Shugart type drive could be used, but at $189 each from Aerco I felt this was the best price around.

The Aerco disk drive unit transfers data at a rate of 250,000 bits per second. Disks (soft sectored, double density, double sided) accommodate 320K. This is considerably faster and a much larger volume of data than is reported for the F12-Floppy and the cost is comparable.


The Interface

The disk interface is a 4 1/2" x 6" uncased printed circuit board. A 12" ribbon cable attaches between the back of my 64K memory pack and one end of the interface which has a mating edge connector. For someone with the Sinclair memory pack which does not have a rear edge connector access, the 12" ribbon cable has a female connector midway down its length that accepts the memory pack. You then plug the computer end of the 12" cable directly into the back of the Sinclair.

A second ribbon cable (3' length) plugs into an edge connector on the opposite end of the interface. This cable has two female connectors at the far end. These connectors are about 4" apart. You plug either one of these connectors into your disk drive. The extra plug is for a second drive. Actually, you can connect up to four disk drives to this interface. All you need do is to add two more connectors onto the 3' cable.

The Aerco interface is equipped with 17 integrated circuit chips, one of which is an EPROM that is used in disk formatting. Also provided is one 5 1/4" disk containing the systems monitor.


The Power Supply

Power supply required for the interface and drive is +5 volts and +12 volts. Aerco sells a power supply for $60, but I decided to save a little at this point. I bought a 3 amp regulated, filtered surplus 5V/12V/24V unit mail order from John Meshna & Co. in Boston for $20. (This is the same place I had earlier bought my keyboard). For $7.50 I bought a power supply cable from Aerco with a plug that fit the disk drive. There is an extra plug on this cable to power a second disk drive should one ever be added. Also I dropped the 12V to 9V so that I could use this new power supply to power the computer.


The Finishing Touch

To finish off the unit, 1 bought a disk drive cabinet for $35 from Aerco. It accommodates two drives, mounted vertically. Since I only have one drive, I plan to install the interface card in the vacant side of the cabinet.


Using the System

When I first hooked up the system, I had trouble getting it to operate. I got a "DISC ERROR" report on the monitor each time 1 tried to load the system. So I picked up the phone and put in a call to Aerco and got Jerry who walked me through a diagnosis process. I reported each thing that happened, and he was able to diagnose the problem right then and give me the fix over the phone. What happened was that a mechanical link between one of the drive motors and the arm which moves the recording heads across the face of the disk had become disconnected during shipment. All I needed to do was to rotate the motor by hand for one turn. This re-connected it. (Later on when I read the disk drive manual that came with it, naturally I found out that this was covered in the manual!)

It was really great to see this thing come to life and to be ushered into the world of a "real" computer, Sinclair and all. After the system was booted and the disk formatted, I was now ready to take programs that previously had been stored on tape and put them onto the disk.

It works this way. First, load a program into the Sinclair. Then, get into command mode and initialize the disk drive by typing RAND USR 12865, and ENTER. This process brings the disk drive to life and on-line. Next, with a formatted disk in the drive, type RAND USR 12721 and ENTER. The disk drive whirrs a few seconds, and stops. You have just written the contents of Sinclair memory onto page 1 of your disk.

Now for the acid test: press the re-set button (which hangs from a wire on your disk drive interface). When the cursor appears, type RAND USR 13303 and ENTER. Up pops a menu on the video screen with six choices. Type L for List Directory. Up pops a Directory with 16 choices (16 pages). Type 1 for page 1. Up pops your program. I like it.

Photo 2. Component side of disk interface board.
91a.jpg
Photo 3. Bottom side of disk interface card.
91b.jpg

But one problem: I had no difficulty loading my programs onto disk except the one program that took 20 minutes to load by cassette tape. This disk drive would not work with the very program for which I had bought it! Idea: call Aerco and talk to Jerry. No, first, try to figure out why it will not work. Ah, yes, each side is divided by the formatting process into 8 pages. 160 divided by 8 means that each page can only accommodate 20K. And my master data base contains nearly 40K! No wonder it would not load. Now call Jerry at Aerco!

Jerry said he would write a routine to accommodate my 40K program. I sent him an extra $10 for the service. A week later he called me back to let me know a new EPROM was on the way and that all I had to do was to switch the EPROM on the interface card. I could now format pages in two ways: disks with 16 pages at 20K each or disks with 6 pages at 53K each. Problem solved!


Conclusion

Complaints? Only the common one that all of us novices have: more documentation, please (but Jerry knows the need for this). Pete, my 15 year old son, and I (mostly Pete) were able to figure out how to do things, even if the documentation did not tell us. Otherwise, a phone call was all that was needed, and Jerry will talk to you.

I have found my dealings with Aerco to be a delight. Jerry has now become a friend. Sometimes shipments may be slow, (I did have to wait for some deliveries), but, when I decided to order the disk drive from him, he shipped it the same day I called. I had not called him till 4 p.m.! I cannot say enough good about the way I have been treated at Aerco. Besides, their product is good, and well-supported by them.

I like my Aerco-Sinclair, (or is it Sinclair-Aerco?). I like the rapid LOADs and SAVEs. I like having the contents of 16 cassette tapes now on one disk. I like the fool-proof transfer of data, I have not experienced one failure in data transfer.

So if you have long programs, do not fear staying in Sinclairland. At a total hardware cost of $1300 I have a 64K disk system with 9x9 dot matrix printer, with a used video monitor that meets my needs. If I ever opt for out, I take my monitor, printer, and disk drive with me and that is $900 of the $1300. Not a bad deal!
David G
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Aerco FD-ZX Floppy Disk System

Post by David G »

In that photo, the largest chip is marked 'SMC FDC 1797' which was the compatible chip of the Western Digital FD1797. This added internal support for double density (MFM) modulation while the original 1771 only did single density

This chip or variant thereof was used by many early home computers like Acorn computers, Commodore 1541, etc.
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