Chapter 2
13
1. The device supplements the Spectrum’s BASIC ROM by becoming active when a
BASIC error occurs or when particular BASIC commands are executed, e.g.
ZX Interface 1, floppy disk interface, Centronics printer interface.
2. The device temporarily overrides the Spectrum’s ROM when instigated by manual
intervention from the user, e.g. pressing the transfer button on a device such as
Romantic Robot’s Multiface or Datel’s Snapshot. Note that the Snapshot interface
actually contains an onboard RAM that must be initially loaded with the transfer
program from cassette. However, for the purposes of this document any interface
that operates using RAM in this manner is still deemed a ROM based device since
it operates by overriding the Spectrum’s ROM.
3. The device permanently overrides the Spectrum’s ROM, e.g. the ROM cartridges of
the ZX Interface 2.
Typically, these categories form the order in which device types should be connected,
i.e. those that supplement the Spectrum’s BASIC ROM should be connected first,
followed by those that temporarily override the Spectrum’s ROM, and finally those that
permanently override the Spectrum’s ROM. Devices that fall within the same category
may generally be connected in any order, unless the instructions for a particular device
specifies otherwise.
Note that some ROM based peripherals that provide a full rear expansion port might
not monitor for devices connected behind them requesting access to the ROM
address space. Such peripherals should be connected at the end of the chain of
attached devices, but identifying them could prove a problem since there is no
obvious visual indication (a detailed examination of their circuitry is the only way). An
example of such a device is the RAM Turbo interface (a ZX Interface 2 clone, providing
ROM cartridge and twin joystick sockets).
Another potential problem can occur with devices that provide a Kempston joystick
interface but do not fully decode it on input port 31. The RAM Turbo is one such
device, which only decodes address line A5 instead of A5, A6 and A7. This causes it to
clash with the display mode register provided by the SPECTRA interface (described in
Chapter 4). The SPECTRA interface prevents these clashes occurring if such devices
are connected behind it (see Chapter 8 for further details).
The SPECTRA interface can support an onboard ROM or a ROM cartridge socket, but
if neither of these options has been fitted then the interface can be safely connected
anywhere along the chain of devices. When one of these options has been fitted, the
SPECTRA interface can be configured to operate as a device of category 1 or 3. Its
ROM may be configured to supplement the Spectrum’s BASIC ROM, in which case it
should be connected towards the front of the chain of attached devices. Or its ROM
may be configured to permanently override the Spectrum’s ROM just like a
ZX Interface 2 ROM cartridge, in which case the SPECTRA interface should be
connected towards the end of the chain of attached devices. However, unlike the
ZX Interface 2, the SPECTRA interface does monitor for devices connected behind it
requesting access to the ROM address space. This allows the SPECTRA interface to
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