Commodore Amiga A500 Technical Reference Manual page 93

Coprocessor Expansion and
86
Pin Signals
INTRODUCTION
This section details the signals found on the various types of
86
pin
expansion connectors on different Amiga computers, especially the
signals found on the
B2000
computer's
86
pin Coprocessor Slot, and
how these differ from the similar signals found on
A2000
computers
and those of the original
A1 000
computers. This paper also explains
the Coprocessor Slot's autoconfiguration and DMA protocols and
how they fix the problems introduced in the
A2000
Coprocessor
Slot.
Changes from Previous
We've kept the
86
pin specification on the
B2000
as similar t o those
Documents
available on the
A2000, A1 000
and
A500,
wherever possible. How-
ever, some major changes were absolutely required. With the design
of the
A2000,
the function of the
86
pin slot had shifted from a gen-
eral expansion connector to expansion specifically intended for co-
processors and similar devices. Thus, while the
A500's
and
A1 000's
86
pin connectors have t o support both some kind of coprocessor
expansion and the normal ZORRO expansion, the
A2000
machines
can optimize each slot for its purpose
if
required (or if necessary,
which is more the case).
The 86 pin connector on the A500 and A1 000 becomes something
of an advantage, because of the fact that all expansion must be done
externally. When a coprocessor device, something that needs t o com-
pletely replace the
68000
in all forms of bus access and operation
(like a
68020
accelerator card) is added, it can physically sit between
the computer motherboard and the
100
pin expansion box, thus al-
lowing the device t o completely replace the action of the mother-
board's processor from the point of view of the expansion box. A
machine with both slots on the motherboard must provide some fa-
cility to logically insert the
86
pintslot in front of the
100
pin slot for
certain applications.
In the
A2000,
the Coprocessor Slot signals that control
DMA
can be
used t o insert the coprocessor in the place of the normal
68000
via
the standard
68000 DMA
request protocol. This, however, isn't a to-
tally transparent replacement; the action of the coprocessor taking
control over the local bus from the
68000,
in the
A2000,
can block
other
DMA
events coming over from the
100
Expansion Bus. For to-
tal control of the Expansion Bus on the
A2000,
the
68000
could be
physically removed from the motherboard, but that would result in
the "coprocessor" being a complete "replacement" processor, with
no swapping between the two permissible. The
B2000
solves these
problems with a higher-level
DMA
protocol between the main and
coprocessor
d ~ v i r ~ ~ .