Hi, all

I'm still working on my Supermicro P54VL-PCI motherboard. It's working now, but most of the jumpers for this motherboard are not documented, and the Supermicro website never held any additional info for this board as it was not sold in retail.

So, this is a Pentium VLB motherboard. It uses an Opti 82C596/82C597 chipset aka "Cobra" aka Opti Premium — apparently the second Pentium chipset, released after the original Intel 430LX. It can only take Pentium 90 and Pentium 100, so VLB runs asynchronously at 33 MHz.

However, other Opti Premium-based motherboards have an option to change VLB speed.

Here are jumpers for Shuttle HOT-543, and as you can see, it can either get LCLK from a separate chip (U77), or directly from the bus, using a clock ratio.

Another example is TMC PAT54PV. Here there are just jumpers to set VLB speed at 33, 40 and 50 MHz.

In both cases you also need to change the clock for ISA, because it comes from LCLK as well.

Unfortunately, there are no good images of the above two motherboards, so I can't see what kind of IC stays in U77 on the Shuttle motherboard, or whether the TMC board has any kind of chip for that.

So, obviously I would like to decipher the jumpers for the LCLK on my Supermicro board. The plan looks like this:

1) since you only need to change a single jumper to switch between Pentium 90 and Pentium 100 (so 60 and 66 MHz), the VLB likely gets the clock from a separate chip. So I need to identify the chip that generates 33 MHz and disable it so that the motherboard gets the clock from the FSB;
2) unless this motherboard has a direct switch to choose between 33, 40 and 50 MHz, it likely uses a clock ratio for VLB. That means that a somewhat safer configuration would be to switch the CPU to Pentium 90 (FSB at 60MHz), find a jumper to set the ratio to 2/3 and produce 40MHz for the VLB. With Pentium 100 the VLB is going to run at 44Mhz, which is kind of an uncharted territory for VLB (might work, might not, especially since I use two VLB cards with this motherboard);
3) I probably need to find the jumpers to change the clock ratio for ISA to keep it at 8MHz. Interestingly, the board also has a jumper to switch between ISA and VLB videocard (not VLB and PCI). Maybe that also has something to do with clock? I don't remember ever seeing anything like this on any 486 VLB motherboard.

Here is a photo of my board:

I'll try to make better close-up photos when daylight comes back, but for now here are some additional photos made by somebody else. Here is one made by luckybob.

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Now, looking at the pinout of VLB I can see the B56 pin responsible for LCLK, but I don't understand how exactly I should test it for continuity because there are actually four rows of pins behind the VLB connector, not two. I have a multimeter, but I've only recently started learning how to use it.

Any tips on how I should proceed with this quest? 😀