Although nowhere near as profuse as Spectrum software, various add-ons have now appeared to bridge the gaps in the micro's specification. Tim Langdell reviews.

It is almost a truism to say the Spectrum has been pushed to its limits. A great deal of time has been devoted by software houses to creating programs that put more sophisticated machines with less well-thought-out software to shame. Nevertheless, there is scope for peripherals which even up some of the Spectrum's primary hardware inadequacies.

UNDER CONTROL

Following up its success with a controller - that is, a device which allows you to switch between Loading and Saving, disconnecting the computer when saving and vice versa - Abacus has introduced one for the Spectrum. This means more reliable Saving and Loading. Unlike the ZX-81 version, the Spectrum controller also includes an amplifier, a particularly useful feature in view of the Spectrum's own weakness in amplification. Setting up the controller simply requires plugging the Spectrum's power supply into the side of the Abacus box and the power lead from the box into the back of the Spectrum. The Mic and Amp leads are used to connect the Spectrum to the tape recorder.

Kempston's joystick is a standard design with fire button and control stick working on a switching rather than analogue base.

This means that it can be used to simulate the detection of keypresses. The joystick is mapped to Port 31 of the Spectrum, and can be accessed with the IN command.

The value returned by detecting Port 31 in this manner can be between 0 and 26 with the eight directions from forward through NE to E, SE, S and so on being values 1 to 10 - omitting 3 and 7 - and the fire button returning 16. The joystick can be read in machine code too for a faster response and arcade-like action.

The joystick's connector is a small black plastic box with two matt-black TTL chips visible on the outside which plugs straight in to the Spectrum's printed-circuit board. The joystick itself is very responsive. My only quibble is that a spring-back-to-centre mechanism would make the central off position easier to reach when playing fast arcade games. The joystick costs £19.50.

This company has also brought out one of the first 24-line I/O ports for the Spectrum. Simply constructed - a single TTL chip supports the operation of the 8255 PPI chip - the board comes with clear documentation which indicates how one can use the three 16-pin dual in-line sockets or a 24-way edge-connector attached to the back of the printed circuit board. The 8255 chip has several modes of operation: the three I/O ports can be configured in the following ways - setting up one bank as inputs, another as outputs, with the option of "handshaking" on port C.

Using the In and Out commands the port can be accessed from both Basic and machine code. Apart from the back-to-back edge connector the port can be limited to the Spectrum with a stackable edge-connector, or a motherboard. The stackable edge-connector allows an extra device to be attached to the Micro whilst the PPI is attached. The motherboard will take two cards in edge-connector sockets as well as a Printer or a Microdrive. The motherboard is supplied with its own 5V regulator.

The 24-line PPI has been configured in the Spectrum's I/O map so that it will not clash with the Printer Microdrive or RS-232 interface. The PPI costs £16.50, the stackable connector is £5.50, and the two-slot motherboard is £16.95.

ADD ZX-81 HARDWARE

There are three Adam adaptors for the Spectrum; each allows you to add ZX-81 hardware on to your machine. The fundamental adaptor provides a ZX-81-compatible output port at the rear of the Spectrum, but no address conversions. The adaptor comes with a computer printout description which tells you how to attach memory-mapped devices which do not decode the lower five address lines in the I/O map. The next adaptor - the Adam II adaptor - allows the simultaneous use of two sets of peripherals on the rear of a 16K Spectrum. This adaptor allows you to attach a ZX-81 16K RAMpack to a 16K Spectrum.

The Adam II adaptor can interface with any ZX-81 peripheral which was designed to work in the 0 to 16K region of the ZX-81's memory map, by using it in the 48K to 64K region of the Spectrum's map. Thus ZX-81 equipment will require 49152 to be added to its addressing, and for this reason EPROMs designed to work on the ZX-81 will probably not work on the Spectrum.

Finally, there is the more adventurous Eve adaptor, for those with a 48K Spectrum. With a 48K Spectrum all 64K of the address lines on the Z-80A CPU are already accounted for - 16K ROM and 48K RAM - hence adding any device mapped in the RAM area seems impossible; especially as Sinclair did not provide a RAMCS line at the rear which would have allowed us to turn off some, or all, of the internal RAM. By adding this adaptor and making a few soldering connections inside your Spectrum you can turn off 8K of the upper 32K of RAM and map devices into this area. Of course, making the alteration could invalidate your guarantee.

The RZ-1 Tape Controller is another more recent device from Stephen Adams. It provides both Spectrum and ZX-81 owners with a programmable tape control unit for stopping and starting the cassette motor under software control, or disconnecting the load lead when Saving and vice versa. The RZ-1 plugs in via an edge-connector. The rear connector of the machine is duplicated behind the RZ-1, so a peripheral unit can still be attached. The RZ-1 can control two tape recorders and provide a minimum of five latched outputs for controlling external devices.

Furthermore, it does not use any memory locations, either in RAM or in the I/O map. The unit is located in ROM space between 512 and 1024. Poking various locations in this region allows you to turn on any of the tape motors, or to provide an output to one of the ports.

UPGRADE RANGE

East London Robotics provides a range of RAM upgrades for the Spectrum: the SP-48A which adds a further 32K of RAM to the 16K Spectrum Series 1 on a plug-in board; the SP48 for the current Series 2 version of the Spectrum and an SP-80 RAM add-on which gives not only 32K of extra memory but another 32K which can be accessed as well, using Out commands. It is thus possible to use a full 80K of user RAM with this board.

East London Robotics' Slowloader allows you to load ZX-81 programs into a ZX Spectrum.

The Slowloader adapts itself upon loading to whichever type of Spectrum is in use.

CONCLUSIONS

Perhaps if Sinclair had put a little more thought into primary design, devices like the Abacus controller probably would not be necessary. Its boost to the Spectrum's amplification is useful, as is the capability to switch it off when the keyboard bleeper gets on your nerves.

Once Kempston's joystick is fitted, nothing else can be added to the connector. But what sort of peripheral would you use in conjunction with a joystick? By putting a stackable connector between the Spectrum and the stick's connector one could test the joystick with the supplied demonstration program.

The Adam adaptors are extremely useful, but consider whether or not you really want to make hardware alterations.

The Adams tape controller is effectively transparent to the Spectrum or ZX-81, since it does not use any memory location either in RAM or in the I/O map. The relays involved can cope with 1A at 28V dc or 100V ac. They are sufficiently heavy-duty for many purposes like controlling model train layouts.