I've always been fascinated in 50MHz bus 486 systems too. It can be done, but it's usually not worth it in the end.

At one time I actually did have a VL/EISA board based on an OPTi chipset, and I ran it on a 50MHz bus with am5x86. Stability at 50MHz will depend on the design and quality of the motherboard, the VLB cards you are using and the BIOS settings. I went with a VL/EISA board because they are more likely to be designed specifically for 50MHz operation. Some consumer level boards usually had the 50MHz setting but weren't always reliable at that speed.

Most VLB cards will not work reliably at 50MHz, but there are a few that explicitly state in their manuals 50MHz operation is okay. A few others will also work providing you introduce a VLB wait state. I found the Tseng ET4000W32P based cards work reliably at 50MHz 0WS, but they tend to cause compatibility issues with other components...not sure why that is. VLB is FAST at 50MHz, but if you have to add a wait state you'll end up running about the same as a 33MHz system.

By far the most challenging aspect of 50MHz operation are SRAM and DRAM. You just about always need to introduce more wait states, and it just about always saps away all of the performance gain. In some cases I found 33MHz with tight timings to produce better results than 50MHz with loose ones. Basically you're going to need at least 15ns SRAM (12ns tag) and 60ns DRAM (50ns is even better if you can find it).

So basically to run reliable at 50MHz and still get the performance benefits you're going to have to make sure all of your components are top notch.

I personally find 40MHz to be an all around better solution.

It is because a good VLB card is always bottlenecked in a 486 by the CPU. Raw VGA output is better at 50 Mhz but in a real-world scenario (ie games), the winner is the 4x40 Mhz (most of the time). Not by far, of course. An Opti chipset Pentium VLB board with a Cyrix processor might drive the Tseng , S3 and Ati Mach cards even further.

I haven't been able to get Tseng based VLB cards going in either of the two OPTi Pentium systems I tried. I just get a garbled screen.

So, it is a dead end then with the Tseng. Perhaps other brands...?

I tried two different ET4000W32P cards on two different OPTi Pentium chipsets, and they both gave the same garbled screen. I think this combination just wasn't meant to be. I am pretty disappointed by this as I really had big plans for my VL/EISA dual Socket4. However, I was able to get two cards based on the ARK1000VL chipset to work in this system. ARK1000 is supposed to be roughly equal to the ET4000W32P in terms of DOS performance...but the quality of Windows graphics leaves a lot to be desired.

Interesting. Thanks.