U3 on the SEthernet/30 board is an ATF1502 Complex Programmable Logic Devices that provides address decoding and glue logic.
The SEthernet/30 board uses a Nubus 'pseudo-slot' design, occupying an address corresponding to a NuBus slot, and appearing to the Slot Manager just like a NuBus card would on a workstation Mac. To allow for multiple expansion cards, jumpers JP1 and JP2 can be used to configure the card to act as though it is installed in slots 9, A, B, or E.
Each slot has an interrupt line dedicated to it on the PDS connector, U3 routes the ENC624J600's interrupt to the appropriate line based on the selection made on JP1 and JP2.
Within the card's address space, minimal decoding is performed on address line A16, so that the card's declaration ROM shows up at the top of the address space (as expected by the Slot Manager), with the ENC624J600's address space immediately below it.
Note that the Slot E configuration is not compatible with the SE/30 or IIsi. This option exists to allow the card to be used in the PDS slot of a Macintosh IIfx. This configuration has been tested and works, but the IIfx case does not provide a place to mount the breakout board.
While the ENC624J600 in PSP Mode 4 maps neatly onto the 68000's control lines, glue logic is required to interface with the 68030. Page 7-26 of the M68030 User's Manual describes logic to interface the 68030 to peripherals using 68000-style byte-select lines, a 16-bit subset of which is implemented in U4.
Additionally, U4 generates the /DSACKx signals for the 68030 bus handshaking
scheme. To accommodate the timing characteristics of the ENC624J600 (75 ns from
selection to data valid on read), a counter is used to delay assertion of
/DSACKx for 1 or 2 clock cycles after the chip is selected.