For the first time we publish details of ‘80-BUS’, which for legal reasons
can’t be called Nasbus, but is for all intents and purposes Nasbus spec. issue 4. Most
importantly it includes system timing, which has never been published before. This
includes some minor changes which improve the bus whilst not changing its compatibilty
with existing Nasbus. (The changes are discussed later.) This is the bus standard that
the new Gemini computer uses, and it is hoped that it will be made ‘public domain’ so
that anything that is designed to use it will remain compatible with both Nascom and
It is my personal hope that Nascom and Gemini will get together to provide a
single bus specification under the guidance of a technical committee independant of
any manufacturer. That way it will be under firm control from day one and the chaos
that developed around the S-100 bus can be avoided. It took six years before the IEEE
stepped in and said, “Look lads, you’ve got a standard, don’t keep ‘bending’ it to
suit yourselves. We’re going to take it over, publish it as a standard, and if you
don’t conform, then you can’t call it S-100.” Wishful thinking? I think not, if common
sense prevails at this stage, we could even end up with a European standard Z80 bus to
rival S-100, to the benefit of all end users. Mind you, manufacturers might not be too
happy about it. (Manufacturers don’t like being told what they can and can not do by
As to the steering committee, I hesitate to volunteer ‘us’, although we have
enough technical expertese available, (including access to the originators of Nasbus).
It really requires someone with more weight. Some organisation like the BSI or IEE
would be more suitable (and acceptable to the manufacturers), although they may
consider ‘some poofling little computer bus’ as being beneath them.
80-BUS, a functional description
by Gemini Microcomputers Ltd.
The Nasbus, unlike many other bus systems, has had a very ordered development.
However, when we started developing a new range of cards it became apparent that a new
revision was urgently required. After a great deal of careful thought and many hours
of deliberation the following document was drawn up. It expands on the third issue of
the Nasbus functional specification and also attempts to anticipate some of the
possible future developments of the bus.
The original Nasbus specification made provision for the extra address and
data lines of 16 bit processors. Careful consideration reveals that the bus would not
be suitable for this, and so a number of new signals have been defined for the lines
made free. The importance of good ground signals can not be overemphasised, and so
extra ground lines have also been added.
When defining this bus a great deal of thought went into deciding whether or
not to maintain the NAS MEM, NAS IO, and DBDR signals. These signals are particular to
Nascom 1 (and NAS IO also to Nascom 2) and are unlikely to be required by any future
cards. They therefore constitute a ‘nuisance’. However, for the sake of compatibility,
to avoid the S100 situation, and with pressure from INMC80 it was decided that 80-BUS
would maintain support for these signals.
Because of the above considerations 80-BUS remains fully Nascom 1 and 2
compatible. We therefore hope that Lucas will also adapt this specification. 80-BUS
will probably not be registered by Gemini, and consequently will become public domain.
It is our wish that an independant committee be set up to ‘guard’ the specification of
the bus, and to allow any manufacturer who produces a card that fully complies with it
to advertise accordingly. The 80-BUS is a Z80 bus and no attempt has been made to make
it compatible with any other processor.
One final point is that all cards, including the bus master, should provide a
means for being switched out of the memory map under software control. This may be by
means of implementing the Page Mode structure, or by some alternative method. This
condition also applies to any I/O card that is memory mapped.
Many thanks to David Lewis for the many hours of assistance in drawing up this