Although this originally had been planned when Commodore first switched from the parallel IEEE-488 interface to a custom serial interface, hardware bugs in the VIC-20's 6522 VIA shift register prevented it from working properly.
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Commodore announced a dual-drive version of the 1571, to be called the 1572, but quickly cancelled it, reportedly due to technical difficulties with the 1572 DOS.
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Like the 1541 and 1571, the 1581 has an on board MOS Technology 6502 CPU with its own ROM and RAM, and uses a serial version of the IEEE-488 interface.