[Coco] 50 pin SCSI adapters?

[gene heskett]

On Thursday, January 19, 2012 12:37:39 AM Tony Cappellini did opine:

> Date: Thu, 19 Jan 2012 03:55:21 +0000
> From: Mike Pepe <lamune at doki-doki.net>
> Subject: Re: [Coco] 50 pin SCSI adapters?
> To: CoCoList for Color Computer Enthusiasts <coco at maltedmedia.com>
> Message-ID:
>        <9A1A4185BE833D4AB31ADF4066AE4B2DE5641E3D at CONCORD.lamunet.local>
> Content-Type: text/plain; charset="us-ascii"
> 
> >>Keep an eye out for ACARD AEC-7720UW IDE / SCSI adapters or bridges.
> >>They
> 
> sometimes appear on ebay >>for reasonable prices. Never used on a CoCo
> but they worked fine on the SCSI RAID cards I was using years >>ago.
> 
> Just looking at the part number you posted:  AEC-7720UW gives me the
> feeling this is not compatible with 50-pin SCSI devices. The UW to me
> implies Ultra Wide, which means 80MB/s, 16 bit data transfers.
> 50 Pin SCSI is only capable of 8-bit data transfers, and much less than
> 80 MB/s. It's been too many years since I've worked with the SCSI
> protocol, I can't remember the max transfer rate for 8-bit SCSI, but
> I'd guess around 30 MB/s.
> 
> An Adaptec 1540 is definitely an 8/Bit SCSI card, but finding one may be
> touch.
> Try Ebay or Weird stuff.

I don't have a 1540, which IIRC is not a scsi-2 IIRC, but I do have an 
Advansys ABP-970UA laying in front of me, used with a scsi tape drive I did 
use for backups about a decade back.  Its linux drivers are a bit iffy, but 
the windows drivers I am told were better. Std pci, 50 pin scsi-2.  
Internal 50 pin IDC, or external 50 pin mini db, or both if the cables are 
properly terminated only on the ends.  I don't recommend it as getting that 
truly right can cost one some hair.  So use one or the other.

This card, like most single ended scsi cards, absolutely needs a psu 5 volt 
line that is if anything a bit hot, up to 5.10 volts, and will not function 
at all with a psu that has sagged below 4.90 volts.  This is generally a 
characteristic of all scsi cards that aren't using LVDS signaling like the 
wider 80 pin drives used.  Some of this psu voltage sensitivity can be 
alleviated by locating the bus isolation diode that the design engineers 
specified as a schotkey type, which has a forward voltage drop in the 
0.15-0.20 volt range but was replaced by the bean counters with a much 
higher forward drop of 0.65 - 0.75 volts of a cheaper silicon diode on the 
way to the production line.  This drops the scsi bus logic 1 voltage from 
the specified 3.0 volts, to as little as 2.6 volts because of this diodes 
additional voltage drop.  Since the scsi-2 bus is still a TTL level bus, 
with open collector style drivers, and the minimum voltage to be legally 
called a logic 1 is 2.4 volts, the formerly .6 volt logic 1 noise margin is 
reduced to only .2 volts.  Any ringing on the bus is guaranteed to raise 
hell with the data integrity.  And with 10 nanosecond rise & fall times, 
there is bound to be a bit of ringing on the signal edges.

This isolation diode, is intended to keep external equipment such as the 
drive itself from powering the whole system should the pc be turned off 
while the attached drive, disk or tape, is still powered up and supplying 
terminator power from its own possibly separate power supply.  This diode 
can be replaced with a schotkey type, and really should be but I never did 
it to this card as my psu's were apparently pretty good.

This is the single biggest reason that scsi gained a reputation as a 
cantankerous bus.  The idea was designed very well, but too many 
compromises were made between the designers and the production lines by 
people who really should not have had the authority to change what they 
didn't understand.

If it is to be used in a situation where the power supply always powers 
everything, this diode can safely be jumpered, and the bus will bow and 
give thanks in your direction.

It was working fine the last time I had it in a system.  Since nearly 
everything I have running here now has sata drives, I don't really have a 
need to maintain the ability to run scsi drives other than the pair of 1Gb 
drives on my coco3.  They run on a TC^3 from cloud 9.

Make me an offer starting at $20.  No cables, no driver floppy, just the 
card.  The driver should be on any windows driver disk.  Or it should 
autoload at boot time on any linux system.

Cheers, Gene
-- 
"There are four boxes to be used in defense of liberty:
 soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
My web page: <http://coyoteden.dyndns-free.com:85/gene>
Old Mother Hubbard went to the cupboard
To fetch her poor daughter a dress.
When she got there, the cupboard was bare
And so was her daughter, I guess...