It probably drives things like the ISA bus and legacy hardware such as the timer.
Using a faster crystal will make things faster, but can also result in undefined behaviour.
486 systems tend to give you control over clock dividers for the ISA/VLB buses anyway, so you can overclock the bus in a more reliable way.

That frequency sounds to me very familiar. On other computers it drives the TV video output, so I guess some CGA cards could take their video clocks from there.

Besides driving the ISA buses crazy, you may throw CGA video modes out of sync (I guess modern video cards have their own clocks for everything, but I don't know about older (original?) CGA cards).

Zup wrote:

That frequency sounds to me very familiar. On other computers it drives the TV video output, so I guess some CGA cards could take their video clocks from there.

Yup, most 8-bit systems are designed so the whole system runs in sync with the video output.
Which is why you have these crazy CPU frequencies (and generally different speeds for PAL and NTSC systems).
The PC is no different: 4.77 MHz is 14.318/3. 14.318 is a standard crystal for NTSC systems.
Indeed this is done so that the CGA card can just sync to the clock signals on the ISA bus to generate NTSC-compatible signals. There is actually an adjustable capacitor on the motherboard to fine-tune the clock for the TV output.
MDA/Hercules run at 50 Hz instead, and have their own crystal, they run asynchronously from the system.
I'm not sure about EGA, but all VGA cards run on their own clock.

I have tried this before. I even socketed my 14.318 MHz crystal. You can only go about 0.5-0.7 MHz faster before the timer gets messed up and the floppy disk controller won't function properly.

feipoa wrote:

I have tried this before. I even socketed my 14.318 MHz crystal. You can only go about 0.5-0.7 MHz faster before the timer gets messed up and the floppy disk controller won't function properly.

Overclock everything 😉 🤣

Odds are your 486 is using another oscillator as reference for the FSB, and this one if for the timer only. You should not mess with it.

alexanrs wrote:

Odds are your 486 is using another oscillator as reference for the FSB, and this one if for the timer only. You should not mess with it.

Pretty sure it is, because 486 FSB is 25, 33, 40 or 50 MHz, nothing directly related to 14.318 😀

If I recall correctly, the PLL on my 486 board uses the 14.318 MHz crystal as the basis for its FSB generation. It is connected to 2 pins on the PLL. Increasing the 14.318 MHz cyrstal to 14.8 MHz increased the FSB from 33.3 MHz to 34.5 MHz. 14.8 or 14.7 MHz was the highest I use before the diskette controller would no longer function properly.

hey thank you all for the replys, my only concern for changing the oscillator is to increase its ISA onboard IDE controller, because its one of the main reasons(lack of cache is one too) why my pc is sooo damn slow, im sure it uses the standard 4.77 or 7.13 Mhz bus clock untead something better, which for a pc from 1995 it should be faster or atleast jumpers to change this.. also i've seen on forums that some dude increased its bus clock speed but in BIOS for his ethernet adaptor changing the clock divider value, and noticed a big increase... however mine hasnt any settings to do that kind of job.

I would try one day and see what happens, but as feipoa said some important controllers will be really screwed, floppy, serial ports, dma controllers, keyboard, etc...

I've seen in veeery old posts from late 1999 to 2002 people talked of a tool named TurboPLL which could increase some specific freqencies replacing the ref xtal and soldering some pins.. its the same idea but much more tecnical.

Well, here comes another question 😀, its possible to find out the ISA clock input pin (B20 CLK Pin, if my memory doesnt fail) and directly insert a higher frequency from a external crystal oscillator circuit soldered close? (maybe it needs to be buffered, dunno...)

http://www.overclockers.com/do-it-yourself-pll/

What mobo is that? If it is a late 486 with an onboard IDE controller I have my doubts that controller would be sitting in the ISA BUS, and if it is (and the mobo is ISA only) then there is only so much you can do - you can't really OC the ISA bus that much anyway.

alexanrs wrote:

What mobo is that? If it is a late 486 with an onboard IDE controller I have my doubts that controller would be sitting in the ISA BUS, and if it is (and the mobo is ISA only) then there is only so much you can do - you can't really OC the ISA bus that much anyway.

The model is an Acer A1G+ OEM Mobo, heres the link : http://www.uktsupport.co.uk/acer/mb/aca1g4.htm

its practically the same to the A1G4 but with no Local IDE Support.

the mobo has 4 ISA buses on riser, no pci or vlb... and a onboard VLB Cirrus Logic GD-5424 1 MB. Yep, the mobo controls the IDE thru isa, the connector itself its named ISA IDE 1, and its left is the local bus ide connecter that was removed on factory, 😒

bloodbath2you wrote on 2015-10-13, 17:09:

The model is an Acer A1G+ OEM Mobo, heres the link : http://www.uktsupport.co.uk/acer/mb/aca1g4.htm […]
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alexanrs wrote:

What mobo is that? If it is a late 486 with an onboard IDE controller I have my doubts that controller would be sitting in the ISA BUS, and if it is (and the mobo is ISA only) then there is only so much you can do - you can't really OC the ISA bus that much anyway.

The model is an Acer A1G+ OEM Mobo, heres the link : http://www.uktsupport.co.uk/acer/mb/aca1g4.htm

its practically the same to the A1G4 but with no Local IDE Support.

the mobo has 4 ISA buses on riser, no pci or vlb... and a onboard VLB Cirrus Logic GD-5424 1 MB. Yep, the mobo controls the IDE thru isa, the connector itself its named ISA IDE 1, and its left is the local bus ide connecter that was removed on factory, 😒

This is a late board indeed, as it supports 3,3v CPU 😀
If it feels slow, did you check if the cache ram is fitted? Or maybe disabled in BIOS? That stuff can cripple the best 486 (and pretty much every system).
Your board also supports setting the fsb with jumpers. Maybe that hard-wires to an ISA divider was well, otherwise it really should be adjustable in BIOS. Or maybe the BIOS won't let you adjust it and chooses the divider itself as it measures the fsb speed.