What is added to the MAX3421E when operating as a USB peripheral that makes it different than the MAX3420E?

The MAX3421E adds four general-purpose input pins, and four general-purpose output pins. This brings the GPIO total to eight in, eight out. The MAX3421E adds interrupt requests for the eight general-purpose input pins. Each pin has an interrupt enable bit and edge polarity bit associated with it. The MAX3421E has a bit called SEPIRQ. This bit allows the GPX pin to serve as a second interrupt output pin (in addition to INT), which is active only for the GPIN interrupts. The MAX3421E can program the time at which the INT pin deasserts and reasserts in edge interrupt mode when one interrupt is cleared and another is pending. The MAX3420E uses a fixed time of 10.6µs for this interval.