Hah! Eureka! I had a brainwave, and decided to look at my machine with the serial console board pulled out! I then get the exact same behaviour on SRUN as you're seeing - a very brief spike every so often (about every 25 usec, here). So I'd be looking pretty hard at your DLV11-E; start by making sure it is configured correctly. Past that, you'll need prints; Manx claims there aren't any online, but there are a set in the jumbo assemblage, ET-LSI-11 (MP00706); the DLV11-E prints are on pp. 31-37. All you have to do to get a nice 'scope loop is to power the machine on; the CPU will dutifully try and read the RCSR, in a fairly tight loop. Probably worth picking up a spare DLV11 of some type for fault localization via board-swapping/etc; DLV11-J's can be found on eBait for not a large sum. Noel