The Sinclair Microdrive and Interface 1


A lot of hardware was developed for the ZX Spectrum, both by Sinclair themselves and any number of other third party manufacturers; you could find almost any add-on imaginable.

Possibly the most innovative add-on from Sinclair themselves, was the Sinclair ZX Microdrive.

Needing an Interface 1 to work with the Spectrum, the two together cost £79.95 at launch.

Microdrives were small mass-storage devices using tiny "stringy floppy" cartridges capable of holding 85k - 90K of data.

Although not the most reliable of mass storage devices, but it was a good and cheap alternative to far more expensive disk drives at the time. These Microdrives, slightly modified, were later used with the Sinclair QL. 

The cartridges were compatible although formatted differently, so you couldn’t use the same cartridge in both.

The drives are tape spoolers relying on the "stringy floppy" principle, driving a 6 meter loop of endless 1.9mm magnetic tape on a single spool past a cassette type tape head. The tape feeds by pulling from the middle of the spool and winding back to the outside. This type of handling put a lot of stress on the tape so the more robust magnetic tape used in video cassettes was used which probably explains why these cartridges still work very well 30+ years later.

Problems and Issues

The tape can tighten up in the cartridges causing irregular running and formatting problems, but if you format the same cartridge 2 or 3 times in succession then that’s all that is usually needed to revive it. Other than that, keep them in their cases. They’re also not partial to cups of tea, small dogs or anything magnetic.

For the Microdrive units themselves, other than cleaning dirty heads and replacing the internal roller if performance starts to deteriorate (see my servicing guide here), there’s not a lot that can be done these days to bring a dead Microdrive back to life - the important thing to know is if it’s really dead.

If the Microdrive spins with the CAT 1 command but seemingly won’t stop it is usually because you are trying to catalogue a cartridge which has been formatted on a Sinclair QL. Don’t throw them away thinking they’re duff, just use the FORMAT “m”;1;”blank” to prepare them for a standard microdrive.

The Interface 1
To use the microdrive you first needed the controller interface which was the Interface 1.

This also added networking capabilities and an RS232 port (non standard of course)! However, its main use was without doubt with the microdrives and up to 8 of them could be chained together and controlled by the one Interface via a small edge connector on the left hand side.

A flat ribbon cable connected the first drive, then small connector blocks fit between the other drives in the chain.

The early Interface 1’s had bugs in the ROM which was quickly changed. However, there were at least 2 other updates over time which resulted in incompatibilities between the different issues of Interface 1’s and some Microdrive models.

It is a common problem to buy an Interface 1 and Microdrive and believe one or the other to be non-working (usually when bought from separate sources). If you use the CAT 1 command with a working cartridge in the drive and get the error message “microdrive not present” this can be an indication of incompatibility.

Unfortunately though it is not always that clear - if you have what appear to be incompatibilities I will be happy to test them for you - it will only cost you the postage. A quick test of the I/F ROM version you have is to enter the following line after switch-on:


This will give a "0" for the first ROM or "80" for later versions. Version 2 of the ROM fixed bugs (including the famous low RAMTOP bug which crashed a lot of programs) and also improved the speed of formatting. Version 3 (if it was ever released) improved compatibility with the Sinclair QL.

ROM Issues
This subject is covered above in the Microdrive section, but another problem came along when trying to load a very large program written on a Spectrum without a Microdrive. The Interface 1 moves the start of the BASIC program up by about half a K to make room for its workspace this leaving less room for the program. The result - no “memory full” error message or anything remotely user friendly - just a complete Spectrum system crash!

ULA Problems
The “microdrive not present” message is often the result of a blown ULA in the Interface 1. This was a common problem caused usually by the Interface moving around, overheating or suddenly being disconnected.  This was why Sinclair fitted the loose screws in the case of the Interface 1 which passed through and replaced two of the case screws in the base of the Spectrum allowing the the two units to be bolted together.

A Spectrum failing to initialise when connected to an Interface 1 is also usually indicative of the same fault.

Sometimes the ULA will overheat spectacularly making a crater in the plastic above it - a more obvious sign that something is terminally wrong!

Keep an eye out for photographs of Interface 1's on offer on eBay which show this tell tale crater - usually accompanied by the words "untested, but was working when packed away 25 years ago!

The Interface 1 and Microdrive system will work with all Spectrum models up to and including the first version of the Spectrum +2 (not the +2A).


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