IOSB Chip Pinout Question

[Phipli]

Hey folks

The IOSB chip used on the Centris / Quadra 610 / 650 and 800s, does anyone have information on its pinout?

Specifically, I'm interested in knowing where PA1 is from VIA1, which is embedded within IOSB.

This pin is the one that defines whether a board is a 650 or a 610, for my current project I was interested to see if I can fool a 610 into thinking it is a 650. I've found the other three VIA1 Port A pins, which are as follows (it would have been nice if they were in an obvious sequence) :

 

[Phipli]

At a high level, the only differences between a 610 and a 650 are the NuBus controller, NuBus slots, two missing RAM slots and their associated bus transceivers. The RAM is sort of passive (in that missing entirely shouldn't be any different to just not being populated), so the only thing that might upset things is if the ROM takes exception to... well, the NuBus chip being missing

The incentive for doing this is that the ROM contains selectable (by resistor strap) timings for running the chipset at 20, 25, 33 and 40MHz. On the Wombat (650 / 800) all four options appear to be available, but on my 610, my computer only chimes if I select the timings for 20, 25 or 33MHz.

Note these timings are independent of the actual clock speed - I can for example select 33MHz timings while running the system at 25MHz, the most obvious outcome is that the video subsystem runs slower than it should when you do this silly thing.

My current situation is that I can run my 610 overclocked to 40MHz using 33MHz timings, but I'd really like to use the correct 40MHz timings, like I can on my Centris 650.

The following Architecture diagram uses "WLCD" to refer to the 610 while the remainder refers to the 610, 650 and 800. I'm going to guess WLCD is "Wombat Low Cost Derivative" or something similar.

 

[GRudolf94]

Remind me to look-poke at my own Q800 when I get it going.

 

[joshc]

I somehow missed this. I don't have the answer but I think this would be cool. Basically a Wombat imposter? A womposter?

 

[Phipli]

I somehow missed this. I don't have the answer but I think this would be cool. Basically a Wombat imposter? A womposter?

It's already a Wombat imposter

Just giving it a bit more of an identy crisis...

Mind you, my Centris 650 currently thinks it is a "Centris 839", but that's mainly due to a mixture fiddling with resistor straps, WishIWere and editing the system file.

 

[RandomDamage]

In case others come across this thread, I figured I’d correct the info. The picture below shows the IOSB PAx pin annotations and connected resistor locations on my Centris 610. I didn’t trace out where PA6 was connected to.

 

[dankcomputing]

So if I pull R137 (300 ohm) and add R130 (1200 ohm) on a Centris 610 motherboard, 33mhz will be guaranteed stable? I'm dropping a 33 mhz full 040 into one here (without onboard ethernet) and I want to make sure it'll be guaranteed stable.

 

[dankcomputing]

There's a 300 ohm resistor already at R130. Do I leave that alone or change it for the 1.2K?

 

[dankcomputing]

Pulling R137 just makes it think it's a Quadra 610 (Gestalt 53), changing R130 to1.2K doesn't make any kind of difference. Ideally I'd want to change this machine's Gestalt ID to 63 in order to get the serial clock correct, but I don't know what resistors to pull/add for that.

 

[dankcomputing]

R129 and R130 are swapped on the diagram. Pulling R129 got me a Machine ID of 63 - the gestalt ID of the “unreleased 33 mhz WLCD derivative”. Here’s hoping it’ll be stable.