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486 CPU upgrade

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Reply 20 of 32, by Anonymous Coward

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Four tag chips does seem awfully excessive. The most I've ever seen is three.
My guess is you need two for 128kb and four for 256kb...but it's best to consult the manual if you can find it.

"Will the highways on the internets become more few?" -Gee Dubya
V'Ger XT|Upgraded AT|Ultimate 386|Super VL/EISA 486|SMP VL/EISA Pentium

Reply 21 of 32, by LewisRaz

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Unfortunately I cannot find anything online about this motherboard.
The board will post how it is currently but hangs as soon as it has moved from POST to go and boot.

Can anyone suggest some chips that would work in those dip22 sockets?
the only ones I can find are CY7C164-15PC

I am guessing even 128kb cache will be a nice boost over 0kb cache 😀

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Reply 22 of 32, by Anonymous Coward

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Micron made 16kx4 cache chips. They were pretty commonly used as tag RAMs during the 486 era. MT5C1604 I think was the model number.
The Cyprus chips you found most likely work too.

Last edited by Anonymous Coward on 2020-03-11, 11:13. Edited 1 time in total.

"Will the highways on the internets become more few?" -Gee Dubya
V'Ger XT|Upgraded AT|Ultimate 386|Super VL/EISA 486|SMP VL/EISA Pentium

Reply 23 of 32, by Intel486dx33

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derSammler wrote on 2020-03-07, 09:17:
https://upload.wikimedia.org/wikipedia/commons/thumb/3/35/KL_Intel_Overdrive_DX40DPR100.jpg/800px-KL_Intel_Overdrive_DX40DPR100. […]
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LewisRaz wrote on 2020-03-06, 23:20:

but the socket says it can accept overdrive chips. Something I have never seen in person before.

800px-KL_Intel_Overdrive_DX40DPR100.jpg

That's a 5V DX4-100 Overdrive CPU. They shouldn't be that hard to find on ebay and the like.

This should work but you need to change the motherboard jumpers for a 486dx-33 CPU. You need the 33mhz bus.
if you keep it at the 25mhz bus speed the Overdrive CPU will run at 75mhz.

Reply 24 of 32, by LewisRaz

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Anonymous Coward wrote on 2020-03-11, 02:13:

Micron made 16kx4 cache chips. They were pretty commonly used as tag RAMs during the 486 era. MT5C1604 I think was the model number.
The Cyprus chips you found most likely work too.

With your help I had adjusted my search terms and now have 4 chips on the way for a touch over £15 which is not too much from the hobby funds 😀

International delivery so now I shall have to wait a few weeks.

thanks for all of the help so far.

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Reply 25 of 32, by DNSDies

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The absolute best processor you can get in there would be an AMD P75 Overdrive.
I recently bought one from ebay and it works very well on my Alaris Cougar.

I would add a small fan to it, though. It does tend to get a little spicy...

Last edited by Stiletto on 2020-03-11, 19:52. Edited 1 time in total.

Reply 26 of 32, by LewisRaz

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DNSDies wrote on 2020-03-11, 16:35:

The absolute best processor you can get in there would be an AMD P75 Overdrive.
I recently bought one from ebay and it works very well on my Alaris Cougar.

I would add a small fan to it, though. It does tend to get a little spicy...

Ah the trouble is my board has no voltage regulation so I am stuck with 5v cpu only.

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Reply 27 of 32, by DNSDies

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Apparently I can't post ebay links? Sorry.
Anyways, a AM486DX5-133W16BHC will work on a 5v CPU socket. There are some on ebay right now with a heatsink attached.
See this thread:
info about "Am5x86-P75 AMD AM486DX5-133W16BHC Chip"

Reply 28 of 32, by derSammler

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DNSDies wrote on 2020-03-12, 16:46:

Anyways, a AM486DX5-133W16BHC will work on a 5v CPU socket.

No, they won't!

The ones you link to all have an additional adapter attached that does voltage conversion and other things. You can not just put any AM486DX5-133W16BHC into a 5V CPU socket.

Reply 29 of 32, by DNSDies

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The AM486DX5-133W16BHC is only available in a SQFP package. It's not a standard PGA CPU. It's only normally available on those overdrive boards.
If you search on ebay for "AM486DX5-133W16BHC" you will see them.

The model number AM486DX5-133W16BHC, the H in the BHC designates is as being SQFP.
if it were the PGA, it would be BGC.

Reply 30 of 32, by LewisRaz

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Well this is certainly interesting!
Once I have the cache figured out I will see if the seller is willing to ship to the UK for a reasonable price.
I have ordered 4 TAG chips so will be able to see how many are in fact needed for 128kb and 256kb.

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Reply 31 of 32, by MN_Moody

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Pardon the necro, but I stumbled upon this thread and wanted to direct the OP to the Wikipedia article regarding both the socket in his mainboard and the 486 overdrive CPU's, which came in two flavors:

From the 486 Overdrive Wikipedia article linked below: "Each 486 Overdrive typically came in 2 versions, ODP and ODPR variants. The ODPR chips had 168 pins and functioned as complete swap-out replacements for existing chips, whereas the ODP chips had an extra 169th pin, and were used for inserting into a special 'Overdrive' (Socket 1) socket on some 486 boards, which would disable the existing CPU without needing to remove it (in case that the existing CPU is surface mounted). ODP chips will not work in Pre-Socket 1 486 boards due to the extra pin. The ODP and ODPR labeling can be found in the CPU's model number(i.e.: DX2ODPR66)."

If you look closely at the socket in the original photo of the OP's motherboard, there is an extra pin hole inside the 168 pins (lower left, inside corner) meaning it's a 169 pin Socket 1 board and designed to accept the 169 pin, DX4ODP100 model Overdrive processor, which includes the 5v to 3.3 v adapter and a heatsink connected right to the CPU. This model includes the 3x multiplier and upgraded write-back cache which gives a little performance bump beyond that provided by the bump in clockspeed alone over a typical 486/DX2 - 66. I believe his board would also work with the ODPR variant of the Overdrive CPU (you can put a 168 pin socket in a 169 pin socket, but not the other way around) though they tend to be more expensive than the 169 pin version with no other direct benefit.

The bigger question is, does the OP's mainboard support a selectable or auto-detected 33 mhz FSB option? Not all 486 boards do, including a number of early IBM PS/1's and other OEM examples I've run into.

https://en.wikipedia.org/wiki/Intel_80486_OverDrive

https://en.wikipedia.org/wiki/Intel_DX4

Reply 32 of 32, by mpe

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The socket doesn't matter. The extra ODP pin (D4) doesn't matter either. If you have a ODP version, you can clip the pin and it would work just as well in normal socket as it has no electrical function. It just prevents non-technical people from installing it upside down which apparently happened a lot. Or at least makes it harder as some people would use hammer to get it in...

I don't judge anyone I also installed a 486 CPU wrong way which destroyed it (and the CPU socket).

ODPR on the left, ODP on the right:

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ODP matters only in case you need to disable the soldered-in CPU. There is another pin called UP, which is one of regular pins (C11) in the inner section of the pins. This pin is defined as output on ODP CPU and input on all other newer 486 CPUs.

When you put ODP to the socket the signal on this pin is detected by the original CPU and it disables itself.

However many upgradeable systems have additional jumper so that the ODP CPU is not needed. And if the original CPU is in the socket you can always remove it.

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