Found a Centris 650 online. I read on wikipedia that the base model does not come with fpu. But I want to run A/UX on it, how would I go about asking a seller about that? Or are there any good tells from the exterior that it has an fpu? Pictures in this ad here:

Hello,

Based on the photos in the listing, it says 16MB of RAM, and one RAM slot is populated. I don't know if a 12MB SIMM exists, so I assume it is an 8MB SIMM in the socket and 8MB on the motherboard... leading me to believe the FPU is integrated. There are 2 SIMMs in the Video RAM slots, so that is upgraded as well.

** O D D D U C K ** - In the photos, it appears there are 5 RAMS slots, all other references only show 4. So this is either a prototype, or somebody has done some work to it. Most references show the 4 RAM slots and an unpopulated pad area next to them where this 5th one appears. It appears that this 5th slot is a ROM socket. Being this listing is from the EU, maybe someone added that socket and did some experimentation on it. While this reference says "Most Centris/Quadra 650's have an unpopulated ROM SIMM socket (but not all, some early models have a socketed ROM)", which may mean it is an early model, I still believe it has 8MB on the motherboard and has an integrated FPU.

References:
Apple Centris 650 Specs (4 RAM slots, 2 Video RAM slots).
EveryMac Centris 650 (Says "**The entry-level M1207LL/A configuration has an 68LC040 processor, which does not have an integrated FPU.") I assume that "entry level" means the motherboard with 4MB of RAM built in.
Low End Mac Centris 650 (shows the motherboard with 4 RAM slots).
Apple-History listing 4 RAM slots.
Wikipedia listing the 5 different models.
Another motherboard photo showing the empty 5th RAM slot pads.

Thanks. I agree. The 230MB original Apple drive points in the same direction. It was for the higher spec models.

The ROM-socket is a funny one, since it is obviously running with it unpopulated it uses soldered on ROM. I wonder if I can just populate the socket with a ROM and it runs it from there. It can be a funny machine then....

This unit has the on-board Ethernet (connector to the right of the SCSI port), which I'm 99% sure means it has the full 040 with FPU, rather than the LC040 without.

>> zoo said:

The ROM-socket is a funny one, since it is obviously running with it unpopulated it uses soldered on ROM. I wonder if I can just populate the socket with a ROM and it runs it from there. It can be a funny machine then.... Click to expand...

Some earlier models had the socket populated even though there were also onboard ROMs. My guess is they were trying to future-proof just in case there was a nasty ROM bug, and they stopped populating it after they were confident the ROM was good. Just a guess though.

Apple designed it so that installing a ROM SIMM will automatically disable the onboard ROM chips, although a lot of hobbyist ROM SIMM designs don't do it properly because they were designed based on the SE/30 and II series circuit, and Apple slightly changed how it worked with the Quadras. More info here.

I won the auction, no other bidders. And it works fine, A/UX and all. Thanks for your feedback all, without it I wouldn't have dared to buy it.

I got the 8mb CayMac ROM, Rominator and installed the Q650 ROM that I found here LC475_LC575_Q650_8MB_WITH_SYSTEM.ROM. But it is a no go, either I get a sad tune (or what it is called on the Quadras) or it is hanging on early in the system boot. Resetting is not doing anything, instead I need to power cycle it in order to get a new startup chime.

Is there a secret handshake I'm missing out on ot how would I go about troubleshooting. Or is the Quadra ROM just not good enough?

One suggestion I have for troubleshooting is to start out by flashing a stock Q650 ROM to the programmable SIMM just to rule out any hardware issues and prove that the SIMM and socket are both working fine. I'm guessing you won't run into any problems with it, but it might be a quick way to rule out problems.

With that custom ROM flashed, you might also try holding down the R key at boot to see if it can boot from the built-in ROM disk image.

I just tried that image in my LC 475 and it works great, including holding down R to boot from the ROM disk. Unfortunately I don't have a 650 to test with. @Jockelill might have some ideas though, I think he has been experimenting with these ROMs lately.

Thanks. Flashed on a stock Q650 ROM, passing checksum. Seems to work great.

Then flashed on this one https://github.com/CayMac-Team/Univ...in/LC475_LC575_Q650/LC475_LC575_Q650_BASE.ROM and it was a no go.

That would suggest that the modified ROM does not like my Centris 650. Or the other way around :)

>> zoo said:

Thanks. Flashed on a stock Q650 ROM, passing checksum. Seems to work great. Then flashed on this one https://github.com/CayMac-Team/Univ...in/LC475_LC575_Q650/LC475_LC575_Q650_BASE.ROM and it was a no go. That would suggest that the modified ROM does not like my Centris 650. Or the other way around Click to expand...

Does it chime with the hacked ROM? Are you getting picture? I've been running this ROM in my Q650 for a long time, but not sure if the Centris has like a different gestalt ID or so.

Can you also take a picture of the ROM in the machine with the power on? There are a few LED that give a lot of information.

>> Jockelill said:

Does it chime with the hacked ROM? Are you getting picture? I've been running this ROM in my Q650 for a long time, but not sure if the Centris has like a different gestalt ID or so. Click to expand...

It does. Think it's 30, instead of 36 for the Q650 and Q800.

>> Jockelill said:

Does it chime with the hacked ROM? Are you getting picture? I've been running this ROM in my Q650 for a long time, but not sure if the Centris has like a different gestalt ID not Click to expand...

>> Volvo242GT said:

It does. Think it's 30, instead of 36 for the Q650 and Q800. Click to expand...

30 seems about right

The chiming is intermittent, sometimes with a tune after and sometimes without. It sometimes tries to boot to desktop but hangs.

But now suddenly a nights sleep and after the n:th pram reset it boots to desktop. And behaves normally, then I had to run so I couldn't test further...

I'll flash in a ROM with a boot image later and see what happens.

So, I found the issue. It's more of a "feature" than actual bug. I had added support for large (128MB) simms by doing:
The change to the RAM sizing code is:

A0168 223C 0000 0100 Move.L #$100, D1
Changing that to: A0168 223C 0000 0000 Move.L #0, D1

As per Rob Brauns instructions: http://synack.net/~bbraun/djmemcrom.html

The issue is that then the machine (650) does not like all "normal" simms and sometimes does not boot. So I've created a new ROM by simple reverting this to normal (it still supports 128MB in the 475 Machine)

I'll upload it to github also.

We have been troubleshooting quite a fair bit outside the forum and we have not really figured it out. Either there is something seriously wrong with my machine or the ROM is really picky when it is a C650. I have however dug up the appropriate tools to change the gestalt ID to Q650. I'll get back to you when I have a. bricked the machine or b changed the gestalt ID and tested the ROM that works in Jockelills Q650.

Meanwhile, this is our most recent barn find. That 2400/4800 modem is a beauty..

[Image: IMG_2940 2.jpeg]

[Image: IMG_2938.jpeg]

So, I got inspiration tonight and started the mod from Centris to Quadra following this thread https://tinkerdifferent.com/threads/marchintosh-2024-overclocking-a-centris-650.3387/

When it was time to install the 1.2kohm resistor it turns out that I didnt have any. I found a 1kohm resistor and thought "close enough"... Turns out that 1kohm gets you gestalt ID 51... :)