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/  Forum Index
   /  Classic Amiga Hardware
      /  SCSI Hard Drives - The Basics
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BigD 
SCSI Hard Drives - The Basics
Posted on 20-Feb-2017 15:13:44
#1 ]
Elite Member
Joined: 11-Aug-2005
Posts: 7307
From: UK

Setting up SCSI drives seems like a dark art and having never set one up from scratch it's a bit confusing when swapping out old drives with new.

Q1) Does there need to be an ID = 0
Q2) What is 'termination'.
Q3) What is 'parity'
Q4) Anyone care to outline the basic important facts?

I've got my new drive to work but after putting a jumper on ID bit 0 the address is now 1 rather than 0 as it was with the previous drive. It is the first drive in the chain but the only other thing selected is 'auto-spin up'.

The old drive had jumpers selected with regards to accepting molex power and powering both the drive and the the SCSI Bus from that source! I saw no such info on the new drive. After partitioning using WB 3.1, OS3.9 started behaving itself again after initially crashing repeatedly on installation of the second drive.

Last edited by BigD on 20-Feb-2017 at 03:15 PM.

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evilFrog 
Re: SCSI Hard Drives - The Basics
Posted on 20-Feb-2017 16:06:52
#2 ]
Regular Member
Joined: 20-Jan-2004
Posts: 397
From: UK

@BigD

1. Nope, just need all IDs on the chain to be unique (including the controller)
2. Terminators sit on either end of the physical chain of devices. They stop the signals echoing when they hit the end of the wire.
3. A way of checking if the data you're sending is in good nick. Can be slower that way, but I'd consider reliability the more important thing to have.
4. Active terminators are better than passive ones. Use good quality cables, as short as you can get away with. If you have an internal device and an external one, they're likely on the same chain with the controller in the middle, but not always. Power on external devices before your computer.

There's a million and one things to SCSI, I've forgotten most of it but if you ask the question, I bet I'll know the answer.

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Daedalus 
Re: SCSI Hard Drives - The Basics
Posted on 20-Feb-2017 16:20:55
#3 ]
Super Member
Joined: 14-Jul-2003
Posts: 1680
From: Glasgow - UK, Irish born

@BigD

It's not really black magic, but there are certain considerations that need to be followed, which many people seem to neglect. To answer your questions:

1) No. There does not need to be any ID 0. Any device can have any unique ID.

2) Termination is used at each end of the chain to keep the signal "clean", i.e., to aid proper rise/fall times and stop reflections on the cable.

3) Parity is the use of extra check bits to ensure the integrity of any transferred data.

4) Sure

For termination, the bus should be terminated at each *physical* end, so you should have exactly two terminations on a bus. No more, no less, no ifs, no buts. The logical unit IDs make no difference to the termination, it *must* be at the physical ends of the cable and *nowhere* else. Normally drives have termination activated with a jumper, but separate terminators are available when you don't have a drive on the physical end of the cable. The controller is typically the other end of the cable and normally has termination active.

For unit IDs, some devices use binary representation for the unit jumpers, so bit 0=1, bit 1=2, bit 2=4, so any combination can be made up for IDs from 0 to 7. On these drives (such as your new drive), ID 0 is selected by removing all ID jumpers. Other devices might have a jumper for activating the selected ID, in which case your choices are more limited (e.g. ZIP drives). Some drives have a rotary switch that you set the ID number on directly.

Bear in mind though that most systems scan the bus starting at ID 0 so if you set up your drives as 0, 1, 2 etc., it will be faster than if the computer scans each ID with no device attached. Also, the controller itself is a bus device, and usually has the ID 7.

Cable quality is very important. SCSI is far more sensitive to cabling issues than IDE, so make sure your cables are good quality.

Finally, once the setup is up and running, use some SCSI software to set the last unit flag on your highest unit, which will speed up the startup. Also, if your hard drives support synchronous mode, enable that for faster transfers, but only do it when you're sure they support it.

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Hypex 
Re: SCSI Hard Drives - The Basics
Posted on 21-Feb-2017 15:23:16
#4 ]
Elite Member
Joined: 6-May-2007
Posts: 11180
From: Greensborough, Australia

@BigD

What's your controller? One big issue you can get is with termination. Which has been explained by the answers here. So check the guide for your controller and make sure that termination is activated for the controller also. This may involve an external card screwed into the back inside of the Amiga that must be wired correctly.

Some drives have term and term power so be careful there. You need it active.

Sometimes it can be easier just to stick an active terminator on the end of your chain.

SCSI also allows you to place optional external devices on the chain. So you can hook an external SCSI socket to one of the links/plugs on the chain. This allows you to for example hook a SCSI CDRW into your setup and only turn it on when needed.

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BigD 
Re: SCSI Hard Drives - The Basics
Posted on 24-Feb-2017 0:14:59
#5 ]
Elite Member
Joined: 11-Aug-2005
Posts: 7307
From: UK

@evilFrog

Quote:
1. Nope, just need all IDs on the chain to be unique (including the controller)


That bothers me because in HD Toolbox there are now two entries for Unit 1. One is the new Hard Drive the other "is not accessible". Is this the controller? Would the system boot if the controller and one of the hard drives had the same unit number?

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thomas 
Re: SCSI Hard Drives - The Basics
Posted on 24-Feb-2017 8:25:41
#6 ]
Super Member
Joined: 28-May-2003
Posts: 1141
From: Germany

@BigD

Quote:

after putting a jumper on ID bit 0 the address is now 1 rather than 0


The ID usually is made of three jumpers. Each jumper represents a power of two, the ID is the sum of these values.

Jumper  0   1   2 
Value 1 2 4
ID 0 off off off
ID 1 on off off
ID 2 off on off
ID 3 on on off
ID 4 off off on
ID 5 on off on
ID 6 off on on
ID 7 on on on

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evilFrog 
Re: SCSI Hard Drives - The Basics
Posted on 24-Feb-2017 11:33:51
#7 ]
Regular Member
Joined: 20-Jan-2004
Posts: 397
From: UK

@BigD

It's possible that the HD is doing something wacky and presenting itself to the system twice. The quick way to tell is to move the drive to ID 2 or 4 (assuming there's nothing there already) and see what happens.

I used to have a HD that presented the first partition on every single LUN for the ID I'd set.
(LUNs - logical unit number - are sub-IDs that devices assign by themselves, mainly for CD changers to identify each disk individually).

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