Commodore Amiga A500 Technical Reference Manual page 95

Processor Status
(FC0-FC2)
Bus Error (/BERR)
System Reset (/RST)
System Halt (/HLT)
These signals are the 68000 Processor Status outputs, which can be
used by bus devices to determine the internal state of the 68000
any time /AS is asserted. When a coprocessor is in charge, it must
drive these pins in a way compatible with how the 68000 does it.
The different 68000 status codes can be found in any 68000 spec
sheet. Pins 31. 33, 35.
This is an input that goes directly to the 68000. Its 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
/BERR when that bus error condition is detected. At other times, the
card must monitor /BERR and be prepared to tri-state all of its on-
bus output buffers whenever this signal is asserted. The
Coprocessor card won't have to tri-state on /BERR. but it must note
it and provide some way of handling the occurrence {the 68000
under normal Amiga OS control merely signals a Guru Error based
on the Bus Error Exception). Since any number of devices may
assert /BERR. and nearly everything in the system must monitor it.
any device that drives /BERR must drive with an open collector or
similar device capable of sinking at least 12ma. and any device that
monitors /BERR should place as little load on it as possible (1 "F"
type load or less, per board, is suggested). This signal is connected
to a low valued onboard pullup resistor, and shouldn't need any
more pulling up. Pin 46.
Pin 53 of the bus contains the /RST signal which is in common with
the original 68000 reset signal. The /RST signal is bidirectional, and
the 68000 tri-states it when the coprocessor takes over. It is only
necessary for the processor to output this signal if it needs to reset
the system under program control. The /RST signal is connected to a
medium valued on-board pullup resistor and shouldn't need any
more pulling up. The coprocessor must monitor this signal and re-
spond to it appropriately; this may mean a complete reset, but it
doesn't have to. The Coprocessor can also assert this line if a system
reset is desired.
This is the 68000's processor halt signal, tied directly to the 68000.
It is connected to a medium valued on-board pullup resistor and
shouldn't need any more pulling up. This signal, when asserted, will
halt and tri-state the 68000 at the end of the current bus cycle. If
driven by the 68000. it indicates detection of a double bus fault. For
a complete system reset, the 68000 looks for both the /RST and
/HLT lines to be asserted. The Coprocessor should handle this signal
in a similar fashion. Pin 55.
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