This definition appears very rarely
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Samples in periodicals archive:
ESCON extended distance feature (ESCON XDF -- See Note --) allows I/O devices to be placed farther from the processor.
3 miles) the maximum distance possible between Enterprise System/9000(A) (ES/9000(A)) processors and 3990 Model 2 and Model 3 controllers attached by ESCON with the Extended Distance Feature (ESCON XDF/A/).
3 miles) from the processor when attached by Enterprise Systems Connection Architecture(A) with the Extended Distance Feature (ESCON XDF(A)).