Commodore Amiga A500 Technical Reference Manual page 41

EXAMPLE Plc DESIGN
This section is a description of the schematic for a small 1 6 kilobyte
RAM board that we designed as our first test PIC for the expansion
architecture. The schematic for this board is Figure A.2, in Appendix A.
It is valuable as an example because it implements all of the basic fea-
tures of a slave Plc.
The PIC at System
Startup
Reading the ID
Locations
The heart of auto-config is in U1 (address register), U2 (address
comparator), and U 3 (ID PAL and control PAL).
When the board comes out of Reset, CONFIG-OUT* is inactive, and
does not pass the config token on t o the next Plc. CONFIG-IN* may
or may not be active at first. If it is not active, the board will not re-
spond t o any bus cycles. For instance, we can see a t U 1 1 that SLAVE*
is disabled when CONFIG-IN* is inactive (high), because this does not
allow BOARD-SEL* t o go active.
In turn. BOARD-SEL* is an input t o U 3 , the control PAL. Without
BOARD-SEL*, all ten of the PAL outputs are held inactive (see PAL
equations for test ram).
Eventually, during execution of the auto config code. CONFIG-IN*
will be asserted t o this PIC between bus cycles (AS* inactive). Notice
that the address latch is tri-stated off so that the pull-up and pull-
down resistors are inputing a pattern of E8 t o the address compara-
tor. When the backplane addresses E8xxxx, this board will now re-
spond because CONFIG-IN* is active but CONFIG-OUT* is not yet
active. In other words, CONFIG-IN* is enabling board select, and
CONFIG-OUT* has not yet allowed the address latch t o move the
board t o a different address space.
Notice that whenever BOARD-SEL* goes active, SLAVE* will go ac-
tive unless SHUT-UP-FOREVER is latched active. SHUT-UP-FOR-
EVER* is a feedback latch in the PAL. It is only set by the software if
the board cannot be configured into the system (for instance, if the
user has plugged in too many large address space PlCs and there is
no room left for this one).
If you analyze the PAL equations for BD1
5
through BD1 2, you will
see that their data drivers turn on for all reads ANDed with BOARD-
-SEL active, until CONFIG-OUT* is set active (or some exception hap-
pens such as reset, bus error, or shutup).
By the way, if you're not used t o PALS, it's normal old Boolean:
*
means AND,
1
is negation,
+
is OR, IF(term) means "If the term eval-
uates t o TRUE then turn on the tri-state driver".