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View Full Version : What's the max capacity for a SCSI HDD for IBM PS/2 Model 70



Springbok
September 11th, 2013, 10:33 AM
Hey guys,

I posted this question in another thread I had going, but I fear it got drowned. I was wondering if there is a limit the the HDD size you can connect to a SCSI card installed in a PS/2 Model 70.

I have a 16bit Adaptec AHA-1640 SCSI installed.

Thanks.

amadama
September 11th, 2013, 10:54 AM
Specs are here:
http://www.adaptec.com/en-us/support/_eol/scsi_host_mc/aha-1640/

I just wondered, are there any regular power connections for internal Hard drives in the model 70?

Clint
September 11th, 2013, 11:19 AM
More to the point what OS are you using ? Later DOS allows a 8GB disk and 4 partitions and they can be 2gb in size.

Springbok
September 11th, 2013, 11:25 AM
Thanks for the reply. I have a couple of follow up questions then based one this

"Supports SCSI hard disk drives under DOS of up to:1 GByte with on board BIOS, and up to 8 GByte in capacity as secondary device using Adaptec EZ-SCSI"

"Supports up to 7 SCSI devices"

1) I will be running windows for work groups 3.11. So should I use the SCSI drive as the boot drive? I use it as a boot drive, do I only get a 1GB drive?
2) How is that possible? Is there a splitter of some sort that I need to get? Can I install 2 drives. A 1GB to boot from, and an 8GB as a secondary?
3) If I install a HDD, how on earth do I run power to it in the model 70?

Clint
September 11th, 2013, 11:49 AM
I am not sure, the problem is its obscure being microchannel back in the day these were very expensive so not common place, my only thought would be the documentation may have been written before MSDOS6 and therefore limited to 1GB - I would just get it and see :) - report back so we know.

Clint
September 11th, 2013, 11:51 AM
1) I will be running windows for work groups 3.11. So should I use the SCSI drive as the boot drive? I use it as a boot drive, do I only get a 1GB drive?
2) How is that possible? Is there a splitter of some sort that I need to get? Can I install 2 drives. A 1GB to boot from, and an 8GB as a secondary?
3) If I install a HDD, how on earth do I run power to it in the model 70?


2&3) A splitter (or power connectors cut off an old PSU) is one thing you need to check the power requirements are not exceeded, old SCSI drive did tend to be hungry for Watts !

SomeGuy
September 12th, 2013, 07:02 AM
Is there an existing non-SCSI drive in the system?

If you want to add an internal SCSI, you would probably be better off just removing any existing internal drive and exclusively using and booting from the SCSI drive. That would also reduce the risk of using too much power.

A 50-pin SCSI drive is just powered via a normal 4-pin molex connector.

That card is probably limited to 8GB max due to early CHS (Calendar/Head/Sector) BIOS limits.

Under DOS 4-6.22 and Win95 DOS 7.0 the maximum FAT partition size is 2GB. Not sure why the specs say 1GB. Of course you can have multiple FAT partitions to fill a larger drive. (Or being a 386 you could use Win95OSR2/Win98's DOS 7.1 and FAT32 for a single 8GB partition, but you would have to hack it a bit to make Windows 3.1x happy)

Krille
September 12th, 2013, 09:58 AM
Not sure why the specs say 1GB.

It's a limitation in the 6 byte Command Descriptor Block used by old SCSI devices. They use 21 bits for Logic Block Addressing: 2^21 sectors * 512 bytes/sector = 1 GB.

SomeGuy
September 12th, 2013, 10:44 AM
Ok, I see now, by default the maximum accessible drive size is 1GB, so the boot partition must fit in there. To address more than that you must first load a DOS SCSI driver, and apparently that can go up to 8GB. Any boot floppies will need the DOS driver, or they will only be able to see the first 1GB.