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 to be true to know
that a collision
is
not happening at this time.
Bus Arbitration Circuit
The bus arbitration circuit's main job
is
to 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 to 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 to assert BR* off the rising edge of
7M.
This
allows the bus arbitration circuit to 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 to 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 to every Plc, RES* on pin
53
and RESB* on pin 94. The RESB* line
is
intended to be the nor-
mal reset input to the
Plc. All normal PlCs will use this line as an in-
put, so
it
is buffered.
RES*
is
intended only to be used by those PlCs which are designed to
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 to CONFIG-OUT* at the appro-
priate time if there is a PIC plugged in the slot. On this backplane, we
have used
74LS32s to pass CONFIG-OUT* to the next slot if there
is
no Plc. The pull down resistor allows the CONFIG-IN* signal to pass
directly through the gate to 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 to use special pins on the con-
nector at pins
1 1
and 12, such that 1 1 and 12 short to each other
when there is no PIC inserted in the connector. This would eliminate
the need for the
74LS32 gates.
To Next Page
Loading...
