The Microdrive and Interface 1 finally arrive

SIXTEEN MONTHS after they were announced the first Microdrives are just being delivered.

The price is still £50 for a drive which takes 85K tape cartridges instead of the 100K originally promised, but you can find and load a program in a few seconds. You will also need the Interface I before you can plug in up to eight Microdrives, but this interface gives you RS-232 and local area networks.

The Microdrive is housed in a box 90 by 85 by 40mm in size with an aperture at the front for loading the cartridge and printed circuit board edge connections on both sides towards the rear. The right hand connector plugs via a flexible cable to the interface unit, if it is Microdrive 1, or plugs directly through a double ended socket to the next Microdrive. The drives are secured to each other by a plate underneath the housings. This should prevent the dreaded wobbles.

There are two small printed circuit boards inside which are without modification. The lower board provides the two edge connections, while the central vertical board houses a ROM and the tape head, which is used for both record and erase functions. The mechanism is uncomplicated. The drive is generated by a rubber roller mounted directly on the end of the motor spindle. Pressure is applied by a leaf spring on the opposite side of the cartridge. The spring also maintains the cartridge position relative to the head, a very simple and highly efficient set up. My only worry would be head wear due to the abrasive action of the tape.

When the Microdrive is running, an LED indicator is lit. A warning is given not to remove the cartridge while this light is on. The cartridge is claimed to be able to handle up to 50 files and have a minimum storage capacity of 85K. The two spare cartridges supplied for the review both had 90K of storage space available when formatted. Typical access was very fast in comparison with cassettes but, human nature being what it is, you tend to notice when it is slower than usual. It can take longer to type in the load command than to find and load a short file.

The storage media is an endless loop of tape. The cartridge is very small 45 by 35 by 7mm., including cover and makes the opposition's 75mm. floppies seem huge in comparison. The cartridge can be write-protected in a similar manner to cassettes by removing a plastic tab, a piece of sticky tape will re-enable write. I assume that before long we will be able to purchase S100 and S200's - Sinclair 200K - cartridges. In some applications it is quantity of storage rather than speed of access that is important. It will also be necessary to store the cartridges in a holder similar to those available for cassettes.

The Sinclair hardware add-on industry is in business again. Sinclair only supplies one blank cartridge per Microdrive, further cartridges are supplied singly with an order form for repeat purchases. Bearing in mind Sinclair's advice on the need for back up copies, it just is not sufficient. It is possible to use strings for filenames in the Microdrive commands and variables for the Microdrive number. Three points emerged from our tests:

• Each file appears to occupy a minimum block of 512 bytes.
• As the limit of 50 files is exceeded, file names disappear from the catalogue in an apparently random fashion, but they still occupy space on the cartridge and are still loadable.
• The average time taken to Save a short file in this manner is 10 seconds.

An attached Microdrive has no effect on the vast majority of commercial software. Only in one case where a program used interrupt control was a program found not to work, and I am not sure why not. If you open a stream to the Microdrive or Net then problems will arise, but if you do not try to use the Microdrive or try to Network a program then it should make no difference whatsoever. The user is warned not to remove a cartridge while the Microdrive's LED is lit and not to switch on or off with a cartridge in the drive. This I continually forgot with no apparent mishaps to the stored data. I think the user would be wise not to leave a cartridge in the drive for long periods of storage, the pressure roller may take a permanent set and upset the tape transport. I would have liked a protective flap over the cartridge entrace - not all drives will be used in a clean environment.

The ZX Interface 1 was also announced in April 1982. It was simply called the RS232/Network interface board and was expected to sell for £20. Since then it has been expanded and now serves three functions:

• Microdrive controller
• RS-232 interface
• Local-area network

This little box of goodies is something special. There are flaws but it is still a very powerful addition to the Sinclair range of computer peripherals. Surprisingly the two printed circuit boards in the Microdrive show no signs of modification whereas the printed circuit board in the interface has been altered. The board contains two integrated circuits, a prototype Ferranti LSI and an EPROM. A Sinclair spokesperson has stated that the equipment delivered to customers will, in fact, contain proper production devices.

The interface plugs into the Spectrum rear connector and is attached through two existing screw holes to the underside of the Spectrum, the original screws may be discarded. A duplicate rear connector port is available on the interface as are the two 3.5mm. network sockets and the nine-pin d-shell connector for the RS-232 printer/peripheral interface.

The Microdrive plugs into the exposed printed circuit board edge connector at the side through a flexible cable. The whole assembly is rigid and now the keyboard has been tilted, feels much easier to use.

The interface unit acting as the Microdrive controller, expands Sinclair Basic to include elementary file handling and communications commands. These are listed in Table 2.

The commands are created by paging the new ROM which creates an additional set of system variables. It looks from an cursory glance, that it is possible to re-vector the inputoutput routines used in both the Microdrive and Network channels. The commands become a bit of a chore to handle after a while and you are soon driven to looking at producing your own simple operating systems.

The RS232 interface, which operates over a range of 50 to 19.2K baud with no parity, 8-bit character and two stop bits, should cover just about all but split-speed operation. There are two modes of operation.

The 't' channel is normally used for listings. the control codes are not sent except for 13 (carriage return) and the token codes are expanded. There is no graphic capability. These characters are replaced by code 63 (7). The 'b' channel sends the full 8-bit code and is used to send control codes to printers etc.

I cannot recall ever having said that something from Sinclair was expensive, but £15 for an interface lead is a bit steep.

The ZX Net - local area network - promises to be a very powerful tool. The network is set up by simply joining two Spectrum interfaces together with the 10ft. long net lead. The manual reveals a network operating at 100K baud with handshaking between two attached devices. Up to 64 Spectrums can go on the net.

There is also a broadcast facility to download information to anybody listening on the net. This would permit a teacher to broadcast a program to the whole class at once. Other uses include the ability to set up a print station to service all the Spectrums on the net. Perhaps when Sinclair has managed to clone a few interfaces these properties can be evaluated.

The manual falls short of the usual high Sinclair standard due, in part, to the fact that it is not a derivative of an earlier work. Everything appears to be there but I think some of the text needs presenting in a much simplier form. The demo tape contains a print server program, untested, and a poor man's Horizons tape - a database program which provides the meaning of the Microdrive commands from a database, at what can only be described as a slow speed. I was not impressed.