VAX 11/750 rescued, alas...

On 10/6/07, J Blaser <oldcpu2 at rogerwilco.org> wrote: That is unfortunate. That looks to me (from appearance and the part number) to be the 11/750-specific SI-9900 interface. I have one in an 11/750 in my quonset hut. SI made a number of host-specific interfaces (Qbus and 11/750, for certain, others likely)... this one fits in the same kind of slot as a DEC RH-750 MASSBUS interface, but is not, itself, specifically a MASSBUS card (there is at least one other board for that slot include, an additional UNIBUS interface). For the case of what you have there, you should also have a pair of 40-pin cables going off the backplane for that slot, out to an I/O bulkhead header. There would then be a pair of (possibly shielded) 40-pin cables running off to a 5.25"-tall rackmount box marked "SI 9900". It should have at least one host board to receive the pair of 40-pin cables, then on the other side of the box, one or more disk interfaces (there should be room for two or three of those). We had a pair of SMD interfaces in ours, attached to 160MB and 450MB Fuji disks, emulating a pair of RM03s and a "custom-sized" RM05, respectively. The 450MB disk always caused us heartburn because we had to patch the, IIRC, DBDRIVER.EXE to change the RM05 geometry, every time we upgraded VMS. The 160MB drive emulated a pair of RM03s/9762s, with no tweaking. I did notice you have a cable hanging down for a UDA50... it's likely that they had both SI disk _and_ Unibus-attachd SMD disks. That was a common arrangement for a variety of reasons. You will also want to be on the lookout for a DU boot ROM. The 11/750 didn't use the TU58 for boot files as the 11/730 and 11/725 did. You just twisted the ROM selector to the right device, then pressed reset/boot. the D ROM was typically the TU58 boot (DDA0?) so you could boot tape-based diagnostics. If you don't find anything, the board looks to me like a papertape punch/reader interface. Try throwing that into your searches. Good luck on finding boards. They are out there, but I doubt they'd be inexpensive from a dealer. If you do find boards, you will have to match up the memory to your memory controller. There are three flavors. The oldest only can talk to 256KB boards (the same ones that work in an 11/730 or 11/70). You can stuff a max of 2MB in there - enough for ancient UNIX or VMS, but I wouldn't want to run VMS 4.0 or newer on that. The most common version will support eight 1MB boards (same as used with an 11/730 or 11/725). The newest version supports a pair of 4MB boards and a six 1MB boards (14MB). I have never worked with that one, but it would be something to look for. The other thing to do, and I don't have the docs handy for this, is to check your backplane in the memory area, to confirm what it's wired to accept. I have 11/750 S/N BT0000354. It shipped with 512KB, then I upgraded it to 8MB by adding the extra refresh/mux line and changing the SID dipswitch (rather than paying DEC to do it). I presume an additional wire is needed for the 14MB mod. I think you should be able to put a lower-rev memory controller in a backplane designed for a newer one, but you might need to update the SID switch to match what the OS is expecting. Don't let this worry you until you get enough boards to throw together into a CPU... just get some memory - 1MB boards _should_ be safe - then worry about which memory controller you find next. 256KB boards shouldn't be expensive, I wouldn't think - since nobody wanted them much after 1990. Everyone wanted 1MB boards for several varieties of VAXen, so lots of those were produced, too. We even had a couple 3rd-party boards with a handful of 256Kbit DRAMs, but the DEC boards (packed solid with 4164s) were the easiest to locate at surplus dealers. Have them keep an eye out for that SI disk box - it's the other half of the board you already have. I have no spares, sorry. Just a couple of machines and lots of experience crawling around inside them (at Software Results, I was our own primary hardware maintenance resource from about 1990-1994). -ethan P.S. - don't forget to pick up a short length of 1/2 ID Tygon tubing to rehab the TU58 roller. If you _do_ eventually have to scrap out the machine, save the backplanes, PSUs and TU58 for other 11/750 owners. Best of luck on finding a boardset, though. The 11/750 is a really neat beast. It would be good to see you getting it going.