The Macintosh Quadra computers support all sizes of block transfers when the main logic board is accessed as a slave device. (The NuBus specification states that if a slave accepts any block transfer, it must accept all block transfers.) As a NuBus master, the Quadra doesn't initiate block transfers. NuBus masters don't have to support all sizes of block transfers.
The data rate a NuBus master will see when doing block transfers to the main logic board is 8 to 10 MB/sec, depending on main memory refresh cycles and synchronization delays. This is a maximum rate that can be achieved only if the NuBus master does nothing but block transfers (that is, it moves data continuously without doing any other operations). A more realistic maximum rate is 4 to 5 MB/sec.
When the main logic board is the bus master, the transfer rate depends greatly on the speed of the NuBus slave device. A zero wait state NuBus slave (none of which exist as far as we know) could give peak transfer rates approaching 20 MB/sec. Real NuBus slave devices will probably have transfer rates under 10 MB/sec (more like 2 to 4 MB/sec). The only way to really determine how fast a Quadra can transfer data to a NuBus slave is to run real code and time the bus transfers.
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Article 24493: "Apple Tech Notes: What They Are, Where To Find Them"