Just a heads-up that the 1979 edition of the "pdp11 bus handbook" has a very serious editing error in it, in the description of UNIBUS arbitration. On page 38, immediately after step 13 of the NPR Arbitration Sequence ("13. .... SACK must be negated before BBSY may be negated."), it says "A bus master may issue an interrupt command to the interrupt fielding processor." Despite its location in the text, this does __NOT__ apply to the "NPR arbitration sequence" being discussed above. There is an editing error - this text is (or _should be_) separate from the "NPR Arbitration Sequence" section just before it; it belongs with "BR Interrupt Arbitration Sequence" - that header (on pg. 39) was put in the wrong place. The 1975 "peripherals handbook" has very similar text, but it _does_ have a section header after the NPR details (line 13 is identical), and before the start of the (very similar) BR text ("A bus master that has gained control ... through a BRn/BGn arbitration transaction may issue an interrupt command to the processor."). Noel