Odd memory error in PDP-11/04

Yup. Oooh, good point. That's a better explanation of the symptoms than mine, since it answers the thing that was confusing me ("why it can be either set or reset with a write, but freezes in one state for reads"). A fully-populated 64KB MS11-J card has 4 rows of 16Kx1 chips, so if the machine ever runs again, the first thing to check is to see if that bit at 040000 (or 0100000, if it's a larger than 32KB card) is tied to that bit at 0; if not, it's the addressing circuitry in the chip. (Looks like E75, but it might be E72). That I don't think is a memory chip issue, since it causes the entire word to repeat, and on that card, each bit of the word is in a separate chip. Good point. So it's probably not something in the CPU that's repeating every 020 locations. Also, IIRC, that ODT code doesn't use memory, it runs entirely out of the registers. There's some good reason for that (probably to avoid messing with the contents of memory), but maybe also so that it runs without working memory - ISTR that we discussed this at one point here, but I'm too lazy to look for the discussion. So that's why ODT runs even if the RAM isn't working. Anyway, if the 020 problem is in the memory too, it's probably the A04 bus receiver (E55), although it might be the address latch (E88, a 7475) or the RAS/CAS mux (E99, 74S153). Step a would be to put a scope probe on the output of the bus receiver (pin 2) and see if it's hopping up and down - if that, that chip is OK, and the problem is further down the line. Yes. The ROM for the ODT is stored in the M9301 card (at least, if it's an 11/04, it's probably an M9301 - could be an M9312, too). The RAM is in another card, an M7847 (MS11-J). Which indicates that the UNIBUS is probably OK; the console serial is on yet another card, the M7856 (DL11-W); the CPU, RAM, bootstrap and serial line all talk to one another over the UNIBUS. Noel