Commodore Amiga A500 Technical Reference Manual page 35

EXAMPLE BACKPLANE We have designed a backplane as an example implementation of our
DESIGN
expansion architecture. This section is a detailed description of the
schematic of that backplane. The schematic appears as Figure A-l in
Appendix A.
Backplane Schematic
While reading this section, refer to the backplane schematics for the
o v e ~ e w
A2000 and PALS to see what is being described. The B2000 uses a
gate array to handle steering; however, this example backplane de-
sign is functionally equivalent, and should be useful in that sense.
The bus comes in on the left from the processor via J10. Note that
both the data bus and address bus are buffered through bi-direction-
al buffers. The buffers are bi-directional in order to allow external
DMA controllers.
The BUS Buffers and
This subsection describes the bus buffers, their timing and control
Their C0ntr0l Logic
logic. In this discussion, "upstream" means away from the processor,
and "downstream" means toward the processor. For instance, if you
daisy chain two devices on the bus, the further away of the two is
"upstream" from the closer (downstream) device.
Throughout this document, there are references t o signals going ac-
tive. Active is defined in the glossary for this section.
The Address and
Control Buffers
The address lines, function codes, UDS*. LDS*,
W,
and AS* are all
buffered in the same manner by 74F245s. Their buffer direction is
determined by DMAOUT. They are enabled by ADDROE* (address
output enable bar).
Generating DMAOUT
This section explains the PAL equation for DMAOUT found in the
STEERING PAL equations. (Table 3-2, later in this section).
DMAOUT active means that the current bus master is upstream of
the buffers. Since the buffers are a t the extreme downstream end of
this backplane, the master is either on this backplane or upstream
from this backplane. Thus when DMAOUT is high, the drivers drive
the address and control lines downstream (toward the Amiga).
The PAL equation for DMAOUT is very straightforward:
DMAOUT
=
DMAlN
+
OWN