Editing Macintosh SE/30 Troubleshooting (section)

=== !BERR ===
When the 68030 attempts to access memory or an I/O device, it uses the address and data buses to communicate with the external chips and hardware.

If something goes wrong during this communication, such as a memory access error, a parity error, or a failure to receive a data acknowledge signal, a "bus error" is generated.

This !BERR is then sent low by some other device (the one the 68030 attempted to read/write invalid address data to) and so it halts its execution and goes into an error handling state.

If you see !BERR toggling low it is a sign something isn't happy with the data/address bus and something is invalid.

On the Macintosh SE/30 the following chips connect to !BERR:

* 68030 CPU (UK8)
* PAL1644B (UI6) 
* Glue Chip (UI8)
* PDS Header (J13)

It is pulled up to 5V through the resistor pack RP7 so make sure that is connected and pulled high otherwise it would be constantly low.

This means that as the !BERR is an input into the 68030, and the PDS header is generally empty for initial boot attempts, the Glue chip or the PAL must be the ones driving the !BERR low.