Questions about SCSI

[melott]

I know this has been talked about from time to time
BUT can  someone explain about SCSI in terms
that us putzs can understand.
In my case the A3000 SCSI controller or WarpSCSI,
and SCSI controllers in general.
Like rules for 'Termination'
 (yes I know it needs to be terminated but I have several drives connected)
Do you set power to the bus and Terminate the
drive or just terminate the drive?
What about 'Parity' ?
What about 'ReSelection'???
And all those neat little jumpers on the drive  
It seems that most of us just do the
"Trial & Error" method. It would be nice if I knew
something like "If this is this, then that has to be
that"  rule of thumb.

Mel Ott

[Tigger]

Basic rules:

Each unit on the SCSI bus has its own address, with the controller on the A3000, there are 8 valid addresses (0-7).    The controller takes one of the addresses, the remaining 7 are available for drives, I have in the past ran 7 devices off of the A3000.   The A3000 actually lets you set the SCSI address of the controller to any of the valid addresses, usually however, the controller is set to 0 or 7.  

Termination:
Terminate the last internal unit on the SCSI cable, terminate the last external unit on the SCSI chain, do not terminate any of the other units.  

Parity:
Parity off is how I usually run everything, usually you put reselection on for a removable as I remember, though its been awhile since I've hooked SCSI removables up to my system.
     -Tig

[Lo]

Terminate the last internal unit on the SCSI cable, terminate the last external unit on the SCSI chain, do not terminate any of the other units.

Yes, all good general advice, there is some Amiga hardware that strays from this as in my GVP '030@50 in my A2000.  This unit has no termination.  The answer I got at the time was that the scsi cable is so short, it worked better without termination? :-?

[melott]

Yes, we are fumbling in the dark most of the time.
Answers like above " the cable is short enough " ect.
don't help much.
And what about termination power to the bus, do I set it
on a device or all devices, or none of the devices??
There are lots of little things that add up and you
need  some kind of guide line.
We need some indepth answers here.

Mel Ott

[Piru]

The answer I got at the time was that the scsi cable is so short, it worked better without termination?

Always terminate. Short cable is no excuse.

And what about termination power to the bus, do I set it on a device or all devices, or none of the devices??

Term power to single device only.

See www.faqs.org scsi-faq and www.scsifaq.org.

[Lo]

Always terminate. Short cable is no excuse.

 Apparently if the HD is mounted directly on top of the controller card, it is a good excuse. :-P

[Wain]

Apparently if the HD is mounted directly on top of the controller card, it is a good excuse.

Then you should enable jumpered termination.

[melott]

OK ..... now whats the difference between the
internal port and the external port on my A3k
besides the internal port is 50 pins and the
external port is a 25 pin and SCSI-1 ( I think ).
And why, on the origonal commodore installed
harddrive did the drive have a label on it saying
the tremination was removed when infact it
was the last drive on the chain and according
all popular opinion it should have been terminated??

Mel Ott

[Lo]

And why, on the origonal commodore installed

the tremination was removed when infact it

-edit- I must be having a bad cut 'n' paste day, sorry.  I'm not popular opinion..*sniff* :cry:   Gee, your so demanding! :-P

[JimS]

The 25-pin external scsi port uses the macintosh unofficial scsi pinout. They used the DB25 connector because it was smaller than the official connector. Commodore went along, I suppose because it was a defacto standard by then.  Anyway, the internal and external scsi ports are directly connected  - they are on the same scsi bus. (half of the pins on that internal connector are grounds..)
There is a set of terminator resistors on the motherboard.  Normally, the scsi bus is supposed to extend from one terminated end to the other, with no branches. However, it is permitted to have a short stub off the bus.  Like the distance between the SCSI chip and the terminators themselves. ... or the cable between the motherboard and the internal drive.  Those motherboard terminators can be considered as being on the internal drive... (assuming you don't have more than a few inches of cable to the drive)
The problem comes in if you have external scsi devices. The last one of those will have termination.  If you had the internal drive terminated, that would put two terminatons in the computer... a no-no.  Ideally, in that case, you should remove the terminators on the motherboard and put terminators on the internal drive.  But the motherboard terminators are not removable.
I seem to recall that the 3000 came with an external terminator.  That would put the termination on the other end of the bus.  
Clear as mud, eh? :-)

[Lo]

Clear as mud, eh?

Exactomo, Senor! :-)

[shaf]

Just to add more to this discussion.
I used to repair Amigas and also worked for a Canadian Amiga Hardware Manufacturer and wrote SCSI device drivers for their line of Comspec SCSI Controllers.

I have probably configured at least 10 Dirrerent Manufacturers Amiga SCSI Controllers and have found that unlike the PC World no standards were conformed to when it came to SCSI on the Amiga.

Controller Termination is handled differently on almost every SCSI controller I have seen.

The CBM A2091 had the Terminators Soldered onto the controller card, these Terminators should be removed if using both internal and external devices.

The Comspec Controllers used removable SIPP Resistor packs as termintors. Installed if using internal drive only.

Its been years since I have seen the Supra Controllers so don't remember their configuration.

Cheers

Shaf

[JimS]

Lo wrote:

Clear as mud, eh?

Exactomo, Senor! :-)

Hey, scsi is a mystic art. You have to know the secret handshake to learn all the rules.   :-D  :-D  :-D  :-D  :-D

[melott]

Hmmm..... :-D

So it sounds like I made a mistake..
I added a 'HD50' pin adaptor on a bracket using
ribbon cable and the internal C= SCSI port, thinking
the 50 pin internal port would be better, but it
sounds like unless I removed the MoBo resistor
pack its not going to work,  this is for external
drives as I am using the WarpSCSI for the internal
drive. And if I understand correctly the C= 25 pin
external port  does not restrict me from using
a 68 or 80 pin drive with the right adaptor ??

The MUD is thinning
Feel free to impart any bits of wisdom you have

Mel Ott
 :-D  :-D  :-D

[vortexau]

Warren Block goes into it better than what I've seen many places!

SCSI Examples


Then, there's Gary Field's SCSI FAQs

and this SCSI Termination Tutorial


The RAM Electronics SCSI Cables page shows external HD68 TO HD50 and HD68 TO DB25
cables!