Commodore Amiga A500 Technical Reference Manual page 39

By the way, don't worry about two slaves colliding on the upstream
of the backplane; that backplane has a collision detect circuit of its
own.
Thus, each of the seven product terms indicates that a collision is not
happening at this time. Only one of them needs t o be true t o know
that a collision is not happening at this time.
Bus Arbitration Circuit The bus arbitration circuit's main job
is
t o determine which PIC will
receive BG* active (Bus Grant) when the 68000 asserts BC*. The cir-
cuit we recommend does this based on priority, where the closest
PIC t o the 68000 is the highest priority. You could implement some-
thing fancier as long as only one PIC owns the bus at a time.
PlCs are only allowed t o assert BR* off the rising edge of
7M.
This
allows the bus arbitration circuit t o operate synchronously, clocked
by the rising edge of
7M.
The output of the bus arbitration circuit only changes when the
68000
changes the state of BG*. If the
68000
is asserting BG*, the
arbitration circuit passes BG* active t o the highest priority active re-
quester. When the 68000 disasserts BG*, the arbitration disasserts
BC* also. Therefore no PIC has a grant.
RES* and RESB*
CONFIGIN*
CONFIGOUT* Daisy
Chain
Note that there are two reset lines going t o every Plc, RES* on pin
53 and RESB* on pin 94. The RESB* line is intended t o be the nor-
mal reset input t o the Plc. All normal PlCs will use this line as an in-
put, so it is buffered.
RES* is intended only t o be used by those PlCs which are designed t o
have the capability of resetting the system. Normal PlCs will not
drive nor load this line. Note that because RES*
is
not buffered, it can
reset the Amiga, as well as resetting all PlCs (via RESB*).
The CONFIG-IN* signal will be passed t o CONFIG-OUT* a t the appro-
priate time if there is a PIC plugged in the slot. On this backplane, we
have used 74LS32s t o pass CONFIG-OUT* t o the next slot if there is
no Plc. The pull down resistor allows the CONFIG-IN* signal t o pass
directly through the gate t o CONFIG-IN* of the next slot if there is
no PIC installed, thus bypassing the empty slot. If a PIC is installed,
the Plc's CONFIG-OUT* driver overrides the pull down resistor.
Another method that would work is t o use special pins on the con-
nector a t pins
1 1
and 12, such that 1 1 and 1 2 short t o each other
when there is no PIC inserted in the connector. This would eliminate
the need for the 74LS32 gates.