Commodore Amiga A500 Technical Reference Manual page 37

It is fairly difficult t o avoid tri-state fights on the data buffers. In or-
der to get data out to dynamic RAM PlCs a t an early enough time, we
do not use the data strobes to enable the data drivers, because these
strobes can go active very late in a write cycle.
On a read cycle we use the data strobes, so that in case the cycle
turns out t o be a Read-Modify-Write cycle, the drivers will be turned
off (to avoid tri-state fight) while the
RNV
line is changing state.
Refer to the PAL equation for DBOE in the STEERING PAL appendix.
The term (AS * RD*) turns on the drivers for all write cycles, includ-
ing the write portion of Read-Modify-Write cycles. Note that since
AS turns off the data drivers, the data hold time is not guaranteed
beyond AS going inactive, so it is poor design practice t o try t o use
the rising edge of AS*, UDS*, or LDS* to latch data.
The terms (UDS
*
RD
*
ASQ) and (LDS
*
RD
*
ASQ) turn on the driv-
ers for all read cycles. The UDS and LDS turn off the drivers in the
middle of a Read-Modify-Write cycle.
The ASQ (ASDELAYED equivalent) keeps the data buffers from turn-
ing on until after there has been enough time for the collision detect
circuit to assert BERR* low and thus disable the.data drivers before
they fight (see collision detection).
Generaing D-TCPROC* The inverse of the D-TO-PROC* signal is called D2P in the PAL equa-
tion.
Collision Detection
Each backplane or device that passes the bus or allows more than
one slave device must have a collision detect circuit. This circuit will
usually be implemented in a PAL. This circuit must detect any in-
stance of two slaves responding to the same bus cycle and assert
BERR* immediately upon detecting such an error.
The collision circuit has an input (see schematic) SLAVEIN* which
is passed from the upstream backplane or device (if any is present).
If no upstream device is present, the pull-up resistor will hold
SLAVEIN* inactive (high). SLAVEIN* tells the circuit whether or not
an upstream PIC is responding to the current bws' cycle as a slave.
The circuit also has one input for each slot on this backplane. If any
PIC on this backplane is responding as a slave, the corresponding
SLAVEn* will be active.