Commodore Amiga A500 Technical Reference Manual page 101

Bus Request (/BR, /CBR)
All instances of Local Expansion Ports have a Bus Request t o 68000
of some kind. In the A2000, as in the A500 and A1 000, this is direct-
ly connected t o 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 t o the 68000 used t o 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 t o a Bus Re-
quest. This indicates t o the device in the Coprocessor slot that the
68000 will fully relinquish the bus a t 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 t o 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.
C O ~ ~ O C ~ S S O ~ Grant
This signal exists only on the B2000, on pin 20. That pin is unused
Acknowledge
(/BOSS)
on both the A2000 and the A 5 0 0 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 t o 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 t o
be a true Coprocessor access, not a simple DMA. This is all explained
in the following section on the B2000 coprocessor interface.