Clarification on the Quadra 950's SCSI bus

Hi all,

There is a lot of confusing info floating around this and Apple never really documented it. 

Most webpages I find online say that the early Quadras (700, 900 and then 950) got an updated SCSI bus compared to the II line (which came with Narrow SCSI 1 as standard). 

The internal bus of the 950 is (according to those sources) rated up to 10mbps. The external one up to 5mbps.

But SCSI I is a 8 bit bus offering a maximum of 5mbps (3,5mbps average). So what kind on controller is on the 950? Could it be an early Apple implementation of SCSI2 in its Narrow form?

Thanks in advance for your thoughts on this...

Replying to myself here.

Apparently the chip for the internal bus is an NCR 609-3400508. Can't seem to find a data sheet but @joethezombie listed it in this thread as an Ultra SCSI controller, which seems period correct but that would mean the transfer speeds would be of 20mbps or more. Doesn't really stick with what I've found online.

And apparently, the second SCSI chip for the external bus is the exact same one. So how would this 10mbps internal / 5mbps external scheme work?

SCSI-2 comes in 2 forms Fast, and Wide. Fast being 8 bit 10Mbs and Wide being 16 bit 20Mbs. A Jackhammer Nubus card with the 68 pin connector would be Wide 16 bit 20Mbs SCSI-2.

I think the external speed was dropped in half because of the SCSI spec for cable length and the fact that users would daisy external chain drives and scanners exceeding the spec.

There is only one SCSI chip on the 950.

Well apparently the Q950 has two. 



Original picture posted by beachycove in the thread linked in the second post. 

Found this Wikipedia page for the NCR 53C9x series of ASICs. Apparently the 53C96 (the one in the Q700, Q900 and Q950) is SCSI2 and HVD compatible. 

The bus is Narrow (0-7 IDs selectable) for both the internal and the external busses, so 8 bit only. SCSI2 + 8 bit = 'Fast' SCSI.

According to this chart found on https://computer.howstuffworks.com/scsi2.htm, that would allow for transfer speeds of up to 10MBps, which seems coherent with what I found online.



Which brings me to the SCSI2SD... The v6 hardware supports SCSI-2 Narrow (8-bit) synchronous transfers, which allows speeds of up to 10MBps (Up to 20MB/s for the SD card?). That means a v6 actually hits the limit of the Quadra's hardware. Best storage upgrade for the unaccelerated bus? I think so! The best part is that the 53C96 is found on most of the Quadras, including the Centris 650 and the LC475...

The only thing is that the 'external' bus is rated at 5MBps... This could be due to the fact that the external DB25 connectors Apple used only work at SCSI 1 speeds. But then, does that mean a SCSI 2 Narrow device connected to that second internal port on the 'external' bus would work at the full speed? No idea. 

Unknown_K said:

SCSI-2 comes in 2 forms Fast, and Wide. Fast being 8 bit 10Mbs and Wide being 16 bit 20Mbs. A Jackhammer Nubus card with the 68 pin connector would be Wide 16 bit 20Mbs SCSI-2.

I think the external speed was dropped in half because of the SCSI spec for cable length and the fact that users would daisy external chain drives and scanners exceeding the spec.

There is only one SCSI chip on the 950.

Click to expand...

Actually, SCSI-2 comes in 3 flavors.  Unenhanced, Fast, and Wide, and combinations of the aforementioned.

SCSI-2 does not mean Fast.   Plain, unenhanced SCSI-2 is just 5 MB/s, it just adheres to the more explicit SCSI-2 standards.

There are two SCSI controllers on the 950, but they form one logical SCSI bus, until/unless SCSI Manager 4.3 is loaded.

trag said:

they form one logical SCSI bus, until/unless SCSI Manager 4.3 is loaded.

Click to expand...

I didn't know this was possible without the AWS95 upgrade card. Thanks for the tip. I very much doubt I'll need more than 4 IDs to be honest but I'll definitely give that SCSI Manager a try!

trag said:

SCSI-2 does not mean Fast.   Plain, unenhanced SCSI-2 is just 5 MB/s, it just adheres to the more explicit SCSI-2 standards.

Click to expand...

So we still don't know how SCSI is implemented. Bummer.

BadGoldEagle said:

Found this Wikipedia page for the NCR 53C9x series of ASICs. Apparently the 53C96 (the one in the Q700, Q900 and Q950) is SCSI2 and HVD compatible. 

According to this chart found on https://computer.howstuffworks.com/scsi2.htm, that would allow for transfer speeds of up to 10MBps, which seems coherent with what I found online.



The only thing is that the 'external' bus is rated at 5MBps... This could be due to the fact that the external DB25 connectors Apple used only work at SCSI 1 speeds. But then, does that mean a SCSI 2 Narrow device connected to that second internal port on the 'external' bus would work at the full speed? No idea. 

Click to expand...

Yeah, that chart is wrong.   SCSI-2 can be unenhanced without Fast.  I.e. only 5MB/s.

The Q950 only has 5MB/s SCSI support.  The 53C96 does not provide support for Fast SCSI-2.   You need the 53CF96 for that.

The 53C94 is normal voltage signalling only.  The 53C95 is high voltage differential only.   The 53C96 supports both regular signaling and HVD.

The 53C96 was licensed to a number of manufacturers.  Also, NCR became Symbios, which became LSI Logic (IIRC).  I strongly suspect that Apple's MESH chip is just a licensed 53CF96.  I've never done the transplant experiment to test it though.

https://www.prismnet.com/~trag/DataSheet/AM53C94_AM53C96.pdf

https://www.prismnet.com/~trag/DataSheet/AM53CF94_AM53CF96.pdf

View attachment NCR_53C94_53C95_53C96_Data_Sheet_Feb90.pdf

View attachment 53CF94_96-2_Fast_SCSI_Controller_Data_Manual_Apr1993.pdf

So the Wikipedia page is wrong then... You're right. 5MBps max. for the Quadras. I wonder if the the CF96 chip could be installed on the motherboard...