On 2019-Feb-06, at 5:29 PM, Paul Koning wrote: >> On Feb 6, 2019, at 8:25 PM, Brent Hilpert via cctalk <cctalk@classiccmp.org> >> wrote: >> On 2019-Feb-06, at 5:11 PM, Fritz Mueller via cctalk wrote: >>>>> On Feb 6, 2019, at 2:24 PM, Brent Hilpert via cctalk >>>>> <cctalk@classiccmp.org> wrote: >>>>> >>>>> Is the schematic available for the memory board at-issue? >>>>> Curious myself to see what approach for refresh DEC used. >>>> >>>> Yes, here: >>>> http://bitsavers.trailing-edge.com/pdf/dec/pdp11/memory/MP00672_MS11L_engDrw.pdf >>> >>> For completeness, from the technical manual: >>> >>> "The refresh logic, shown in sheet 6 of the print set, generates REF CLK H >>> and the refresh address. Sig- nal REF CLK H is derived from a 555 timer >>> (E5) which is set up as a free running oscillator, powered by the + IS V / >>> + 12 V module input (V-555). The REF CLK H signal oscillates with a period >>> of 14.5us and has a positive pulse width of 6us during each period." >> >> So I could have saved myself some fun if I had read the manual rather than >> just looking at the schematic. >> Not that they're way out of whack, but the mild disparity between the >> manual's 14.5uS and my calculated 11.7uS is curious >> (the calculation being based on the schematic RC values and the 555 >> equations). > > Perhaps the period was changed in a schematic rev or ECO, and the manual > wasn't updated to reflect it. It would be interesting to check the data > sheet for the RAM chip to see what it likes for refresh cycle. And given > that this is an RC oscillator your theory about out of tolerance timing > definitely deserves checking.
Checking further.. 4116 datasheet specs 2mS, my calcs give a refresh period of 1.5mS, the 14.5uS from the manual would give 1.86 mS, 7% shy of 2. The schematic specs 1% resistors, and the parts list does appear to spec a high-tolerance "1%200PPM" cap. Although there are the internal voltage divider Rs in the 555 which are also critical for the timing and everything is 40+ years old. Idle speculation at my distance, we'll see what Fritz observes. Could be other problems in the refresh circuitry too, like failed outputs from the row counter, etc.
