Commodore Amiga A500 Technical Reference Manual page 86

Bus Error (BERR)
This is an input that goes directly t o the 68000. It is used to
indicate
the occurrence of some kind of bus error. Any expansion card capa-
ble of detecting a bus error relating directly to that card can assert
IBERR when that bus error condition
is
detected. At other times, the
card must monitor IBERR and be prepared t o tri-state all of its on-
bus output buffers whenever this signal is asserted. Since any num-
ber of devices may assert IBERR, and all bus cards must monitor it,
any device that drives IBERR must drive with an open collector o r
similar device capable of sinking at least 1 Zma, and any device that
monitors IBERR should place as little load on it as possible (1 "F"
type load or less, per board, is suggested). This signal
is
connected t o
a low valued on-board pullup resistor, and shouldn't need any more
pulling up. Pin 46.
System Reset (/RST,
Pin 53 of the bus contains the IRST signal, pin 94 contains the
/BUSRST)
IBUSRST signal. Both of these reflect system reset, however, the
IRST signal is bidirectional, unbuffered, and in common with the
original 68000 reset signal. It should only be used on boards that are
capable of resetting the system. The IBUSRST signal
is
a buffered
output-only version of the reset signal that should be used as the
normal reset input t o boards not concerned with resetting the sys-
tem on their own. The IRST signal is connected t o a medium valued
on-board pullup resistor and shouldn't need any more pulling up.
System Halt (MLT)
System Interrupts
This is the 68000's processor halt signal, tied directly t o the 68000.
It is connected t o a medium valued on-board pullup resistor and
shouldn't need any more pulling up. This signal, when driven by
a
Plc, will halt and tri-state the 68000 a t the end of the current bus
cycle. If driven by the
68000,
it indicates detection of a double bus
fault. Pin 55.
Six of the
68000
interrupts are available on the Expansion Bus, and
these are labelled as /INT2,/INT6, IEINTl, lEINT4, IEINTS, lEINT7.
The interrupt structure of the original ZORRO specification has been
slightly changed for the A2000lB2000. This change affects the avail-
ability of decoded interrupt inputs and multiplexed interrupt inputs.
Specifically, the 68000 accepts 7 levels of interrupt that are present-
ed to it as
8
possible values priority encoded into 3 multiplexed in-
puts. The original ZORRO specification called for decoded interrupt
inputs on pin 1 9 for interrupt level 2 (IINTZ), and on pin 2 2 for in-
terrupt level 6 (lINT6). These are the same interrupts used by the
Amiga internal system chips and encoded by the Paula chip. The in-
terrupts could be used by external devices by wired ORing interrupt
requests into one of these available interrupts. The original ZORRO