The answer is almost certainly in here? http://bitsavers.trailing-edge.com/pdf/dec/vax/640/EK-179A-MG-001_KA640_Oct88.pdf I don't know, I kind of assumed you checked the manual... but the Yeti's might have eaten it ;) -- Mark Benson http://DECtec.info Twitter: @DECtecInfo HECnet: STAR69::MARK Online Resource & Mailing List for DEC Enthusiasts.