Bus Request (/BR, /CBR)
All instances of Local Expansion Ports have a Bus Request to
68000
of some kind. In the A2000, as in the A500 and A1
000,
this is direct-
ly connected to the 68000's /BR input, which
is
considered a wired-
OR input; all devices driving this input must technically drive
it
with
an open collector or equivalent driver. In actuality, the A500 and
A1
000
don't use this at all internally, so a standard driver may be
used if necessary. The
A2000's /BR input
is
shared by the /BR output
of the DMA arbitration logic, so this will be necessary on an A2000
Coprocessor Slot device. The B2000 has in place of the
68000's
/BR
line a special bus request all
its
own, ICBR. In both cases, the signal
is
an input to the
68000
used to request mastership of the Local Bus.
The signal is found on pin 60.
Bus Grant (/BG, /CBG)
All
instances of Local Expansion Ports have a Bus Grant of some kind
from the 68000. In the
A2000, as in the A500 and A1
000,
this
is
di-
rectly connected to the
68000's
/BG output. In the B2000, a Co-
processor specific Bus Grant signal,
ICBG, is in its place. In either
case, the signal
is
asserted by the
68000
in response to a Bus Re-
quest. This indicates to the device in the Coprocessor slot that the
68000
will fully relinquish the bus at the end of this cycle. A /BG re-
ceived on the Coprocessor Slot in an A2000 could be a Grant given in
response to an Expansion Bus DMA request as well as one in re-
sponse to the Coprocessor Slot DMA request. On the
B2000, ICBG
will only be asserted if the Coprocessor Slot
is
granted the bus. This
signal is found on pin 64.
Bus Grant
(BGACK)
Acknowledge
This is the 68000's IBGACK, or Bus Grant Acknowledge, signal. Any
device that receives a bus grant from the
68000
should assert this
signal as long as the DMA continues, releasing
it
once the DMA re-
quest is finished. This signal should never be asserted until the spe-
cific Bus Grant has been received,
/AS
is
negated, IDTACK is negated,
and
IBGACK itself is negated, indicating that all other potential bus
masters have relinquished the bus. This output is driven as a wired-
OR output, so all devices driving
it
must drive
it
with an open collec-
tor or equivalent device. Pin 62.
CO~~OC~SSO~
Grant
This signal exists only on the B2000, on pin 20. That pin is unused
Acknowledge
(/BOSS)
on both the A2000 and the A500 Originally. this pin was called IPA-
LOPE on the A1
000,
and was part of the planned ROM expansion
method. This
is
currently obsolete; the method of ROM expansion
was changed to work without the need for such a signal. On the
B2000, the /BOSS signal is driven by a Coprocessor instead of
IBGACK when the Coprocessor wishes the DMA access granted
it
to
be a true Coprocessor access, not a simple DMA. This is all explained
in the following section on the B2000 coprocessor interface.
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