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Another Attempt, Dell Optiplex SX260 Computer Won't Boot Up
SNIP
Hi Paul, Thanks for that information. I don't know a lot regarding computer hardware. I use to repair TVs, VCRs, stereos, etc., but not computers. I failed to find a schematic for this model's mother board. I wonder if there is a schematic for a "typical" mother board anywhere? Thanks in advance, John Yes, there is one. There are two reference schematics I located, one being for the Slot 1 era, the other is for S478 with the 875P dual channel Northbridge. The latter is the one I'd suggest as a reference. One problem with this schematic, is it does not use a "real" SuperI/O, so lots of valuable info is unavailable from it. The other problem, is the schematic is drawn in Mentor Graphics "stick font", which makes the Acrobat search function useless. If you're designing at a company with Mentor as your CAD system, you can change that to get "real" text fonts, and I'm surprised the Intel staff haven't fixed theirs. At my company, our "IT" type hardware manager, figured out how to fix that, all on his own (the manuals for that crap, are a couple feet thick and just the thought of wading through that gives me nausea). (875P chipset reference schematic, File is 25281202.pdf. Chip notable because of the extra (CSA) interface for a LAN chip. In effect, it's almost like the Northbridge has enough interfaces to support two Southbridges.) http://developer.intel.com/design/ch...ics/252812.htm You already have the datasheet for the 4 phase regulator on my S478 / 875P motherboard. That's this one. Table 1 on page 7, shows when the CPU is pulled from the socket, the all 1's code on VID is expected to happen. It's a 6 bit code, and you'd need anywhere from one to six jumpers stuffed into the S478 socket to set a code other than the "float" value. (I.e. Jumper a VID to logic 0 GND, if you need a zero on that signal.) http://web.archive.org/web/200403310...5ADP3180_0.pdf The logic levels on the VID, reading between the lines, seem to be 0 to 1.25V. The VCore chip will tolerate up to a 5.5V level on the pins (almost like it's TTL or CMOS compatible), but the internal pullup on the pin, is to a 1.0V internal source. So both the thresholds and pullup, are centered around much lower voltages. With the CPU pulled, all six VID lines will float to logic 1, and the VCore regulator will put out zero volts. If you use grounding jumpers on some of the VID signals, you could attempt to define a different value. On PDF page 89 of 25281202.pdf, in the lower right hand corner, you can see the VID bus is tied to VCC3 (3 volts) with 1K ohm resistors. Which means, for whatever reason, Intel thinks they're 3V levels. If that were the answer, then using your multimeter, the six VID signals, with the CPU pulled, would sit at around 3V to 3.3V or so. If the motherboard had no provision like that, the regulator chip itself has internal pullups, and on the regulator chip my board uses, they bias at 1.0V when the CPU is missing. The 1K ohm pullups would "override" the weak pullups inside the 4 phase regulator chip. When a board won't run, the "RESET" is a key thing to check. As a lot of initialization logic funnels into it. A motherboard isn't allowed to "start", until a number of subsystems report in that they're "ship-shape". If you have fans spinning, and a black screen, anything that will jam RESET is enough to prevent POST. Good luck reading the schematic :-) Paul Hi Paul, I downloaded both PDF's and I will have a look. This is very educational for me and I am learning. Fortunately I have other computers so I am not desperate to repair this SX260. Once again, I am very grateful to you for your help. Thank You, John |
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