"When this control input, denoted PE, is in the high
state, the bus outputs operate in the high-speed totem-pole mode. When PE is in the low state, the bus outputs operate as open collector outputs which are necessary for parallel polling."
This is from a DS75160 data sheet. The reverse logic of GPIB and the OR gate will automatically convert the 75160 to open collector when a parallel poll is occurring (ATN and EOI logically asserted). You should probably follow the NAT9914 manual since it handles the parallel poll case correctly.