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  September 24, 2018 UPDATE
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I've been sent a 3Com EtherLink II (3c503) ethernet adapter to check out with the working Mach386 distribution.

I've successfully installed the hardware and, after setting the card jumpers and reconfiguring the BIOS to accommodate this legacy ISA card, it has proven to work well.

The configuration quirks of the 3c503 and Mach386 currently allow for one of the following card configurations to be utilized:

1. Address 0x2e0, IRQ 5, BNC Connector
2. Address 0x2a0, IRQ 2, AUI Connector
3. Address 0x280, IRQ 2, BNC Connector
4. Address 0x300, IRQ 2, AUI Connector

The memory must be set for 0xDC000 or 0xD8000.  The selection will be detected automatically.

As with the WD/SMD 80x3 ethernet cards, the motherboard BIOS settings may need to be changed.   The changes required would be to reserve the IRC to Legacy ISA assignment, and to reserve the memory block to prevent it from being used for RAM addressing.

The address, IRQ and Connector settings are hardcoded, and could be changed by modifying the driver and rebuilding the kernel.