AlessandroB wrote on 2021-06-23, 23:10:

i have found a mainboard for 386 class pc with 386dx40 solder onboard and 386 and 387 socket. Can i add a 386DX chip in the empty socket? event if the 386dx40 is present? tnks

If it already has a 386dx40, then that is the fastest processor for the board you can get (other than some TI/Cyrix 486tlc upgrade chips)

If you want to make the 386dx40 run slower, simply change the clock crystal (would likely be one labeled 80mhz) to a slower one (66mhz = 33mhz CPU, 50mhz = 25mhz CPU)

If you want to change to one of the fore-mentioned Cyrix chips, there are likely to be some jumpers on the motherboard to change to the empty socket (this is assuming the motherboard has a 386 socket already installed). If there is no empty socket, but pads for one, it is likely there are additional modifications to the board required as well as soldering in a 386 socket to get it working.

Regarding the 387, you shouldn't have an issue plopping one in. There will likely be a jumper on the motherboard that you will need to change to get the system to work correctly with the FPU installed.

Socket is already present, not only the pad but ready socket. If i insert simething like intel 386sx16 on the converter PQFP->PGA (if exist) it work? how to deactivate the 386DX40 already present on the board? And if insert a faster 486 upgrade on the same empty socket?

Basicly i want to slow down the 386 to try different 386 speed, from sx16 to dx40 and 486upgrade. What is the suggested chipset i must search? I don't have much knowledge of the 386 period

You won't be able to install an SX on the same board as a DX - they have different pinouts and data bus interfaces (16bit vs 32bit).

You will have to find if that board has a jumper to disable the soldered-on DX CPU.

I've got a mainboard with both the DX-40 AMD 386 soldered and the two sockets for the cpu and npu for the PGA ceramic packages. But ...after many researches online it seems it's difficult to expect a jumper to simply select which cpu will be used cause most probably the original manufacturer idea was to have a version with the soldered 386 OR a version with the CPU socket only without the soldered one installed. Some thinks (and I'd agree) that these mixed versions might probably be there for industrial production "errors" not to be used with the CPU PGA socket (the NPU on socket in my mainboard works with the soldered 386) because both cpus would work on the same lines and powered at the same times and who knows depending on the PCB design what might happens.
I've read people actually desoldering the original soldered cpu or a specific pin that might "disable it" to try to use the socket one, but it's difficult to know what might happens, maybe it would not work anyway (maybe missing components.. bios version.. who knows). In my mainboard I decided to just use it with the soldered cpu and the npu @ same freqs and leave it that way without risking. I'd NOT risk a mainboard that old for some electric problems on the PCB lines.

AlessandroB wrote on 2021-06-24, 05:41:

Socket is already present, not only the pad but ready socket. If i insert simething like intel 386sx16 on the converter PQFP->PGA (if exist) it work? how to deactivate the 386DX40 already present on the board? And if insert a faster 486 upgrade on the same empty socket?

Basicly i want to slow down the 386 to try different 386 speed, from sx16 to dx40 and 486upgrade. What is the suggested chipset i must search? I don't have much knowledge of the 386 period

386SX won't work on that board, different system buses. (386SX = 16bit data bus, 24bit memory bus. 386DX = 32bit data and memory bus)

What you can do (like I said before) is change the crystal chip. If you drop in a 33mhz crystal, the 386DX-40 would run at 16mhz speeds. You may need to disable the onboard cache as well if it has one to more closely match a typical 386DX-16.

You should note that in any case, you would need to change the clock crystal to operate the motherboard at a different frequency, simply changing the CPU on these old systems isn't enough.

canthearu wrote on 2021-06-24, 10:46:

386SX won't work on that board, different system buses. (386SX = 16bit data bus, 24bit memory bus. 386DX = 32bit data and memory […]
Show full quote

AlessandroB wrote on 2021-06-24, 05:41:

Socket is already present, not only the pad but ready socket. If i insert simething like intel 386sx16 on the converter PQFP->PGA (if exist) it work? how to deactivate the 386DX40 already present on the board? And if insert a faster 486 upgrade on the same empty socket?

Basicly i want to slow down the 386 to try different 386 speed, from sx16 to dx40 and 486upgrade. What is the suggested chipset i must search? I don't have much knowledge of the 386 period

386SX won't work on that board, different system buses. (386SX = 16bit data bus, 24bit memory bus. 386DX = 32bit data and memory bus)

What you can do (like I said before) is change the crystal chip. If you drop in a 33mhz crystal, the 386DX-40 would run at 16mhz speeds. You may need to disable the onboard cache as well if it has one to more closely match a typical 386DX-16.

You should note that in any case, you would need to change the clock crystal to operate the motherboard at a different frequency, simply changing the CPU on these old systems isn't enough.

I think the same, though there's some little known detail:

The 386DX (simply called "i386" originally) has the ability to mimic a 386SX, ie. it can use a 16bit data bus, 24bit address bus, too.
So in theory, a 386DX motherboard might be able to fall-back to the low mode, IF the 386DX is wired accordingly / set-up for the 386SX configuration.

That feature likely was implemented to simulate a 80286 interface, for use with existing 286 motherboards and peripherals:
It's called "MINimum" mode (?) and intended for small systems - it involves the "BS16#" feature; to optain a 24-Bit I/O limit, some address pins may simply be left unconnected.

Anyway, in practice, the cheap 386SX was the one who was used for this purpose.

Advanced 386SX motherboards were sometimes using (slightly modified) 286 chipsets.
- IMHO that's what makes 386SX systems worth saving. They still have intelligent chipsets, to assist the CPU.

That being said, this was no critisism. I'm just a bit chatty when it comes to such topics, hope that's okay. 😅

Jo22 wrote on 2021-06-24, 21:06:

I think the same, though there's some little known detail: […]
Show full quote

canthearu wrote on 2021-06-24, 10:46:

386SX won't work on that board, different system buses. (386SX = 16bit data bus, 24bit memory bus. 386DX = 32bit data and memory […]
Show full quote

AlessandroB wrote on 2021-06-24, 05:41:

Socket is already present, not only the pad but ready socket. If i insert simething like intel 386sx16 on the converter PQFP->PGA (if exist) it work? how to deactivate the 386DX40 already present on the board? And if insert a faster 486 upgrade on the same empty socket?

Basicly i want to slow down the 386 to try different 386 speed, from sx16 to dx40 and 486upgrade. What is the suggested chipset i must search? I don't have much knowledge of the 386 period

386SX won't work on that board, different system buses. (386SX = 16bit data bus, 24bit memory bus. 386DX = 32bit data and memory bus)

What you can do (like I said before) is change the crystal chip. If you drop in a 33mhz crystal, the 386DX-40 would run at 16mhz speeds. You may need to disable the onboard cache as well if it has one to more closely match a typical 386DX-16.

You should note that in any case, you would need to change the clock crystal to operate the motherboard at a different frequency, simply changing the CPU on these old systems isn't enough.

I think the same, though there's some little known detail:

The 386DX (simply called "i386" originally) has the ability to mimic a 386SX, ie. it can use a 16bit data bus, 24bit address bus, too.
So in theory, a 386DX motherboard might be able to fall-back to the low mode, IF the 386DX is wired accordingly / set-up for the 386SX configuration.

That feature likely was implemented to simulate a 80286 interface, for use with existing 286 motherboards and peripherals:
It's called "MINimum" mode (?) and intended for small systems - it involves the "BS16#" feature; to optain a 24-Bit I/O limit, some address pins may simply be left unconnected.

Anyway, in practice, the cheap 386SX was the one who was used for this purpose.

Advanced 386SX motherboards were sometimes using (slightly modified) 286 chipsets.
- IMHO that's what makes 386SX systems worth saving. They still have intelligent chipsets, to assist the CPU.

That being said, this was no critisism. I'm just a bit chatty when it comes to such topics, hope that's okay. 😅

Very interesting! Tnks!