external scsi, terminating

[orange]

I would like to connect SCSI HDD in external case to Amiga. How should termination be done with those external scsi devices? The scsi controllers used are Oktagon, a2091, fastlane and MK1. Do they all work if nothing is attached (not even terminator) at external connector?
Which (3.5") case would you reccomend me to buy?

[Brian]

You need terminasion in both ends of a SCSI chain no matter what controller you use. However some controllers have builtin terminasion or space for terminasion resistors where others don't. If the card have an external port but no external terminator and still work that meens the SCSI chain is already terminated most likely on the controller. If that's the case remove visible resistors (usually comes in 3 rows of resistors) or jumper the board to not be terminated... and then terminate the external harddriver or it's passthourghport.

[Castellen]

Most (all?) of those boards have on board termination which needs to be disabled if you're using external SCSI devices.
For more info on individual boards see http://www.amiga-hardware.com

External SCSI boxes tend to have either a 25 way D-range, or the special high density 50 way connector.

So you'll need a cable which plugs into the Centronics or 25 way D-range connector of your SCSI board with the appropriate connector for your external SCSI box.

The cables are cheap and easy enough to get from computer cable suppliers.  Be sure to buy good quality, well shielded cables.

Most external SCSI boxes have two ports on the back.  The computer plugs into either one.  You'll need to connect an active terminator into the other unused port.

Lots more SCSI info here:
http://amiga.serveftp.net/A4000_HardwareGuide/SCSIExamples.html

The place who sells the cable should have the terminators as well.

I use these guys most of the time:
http://www.cablesdirect.co.nz/

Not sure if they deal outside NZ or not.  You should have something similar local to you anyway.

[orange]

I see, so that means that *every* time I want to connect/disconnect external HDD, I need to open Amiga and disable/enable termination. agh

edit: or maybe I could use external terminator?

[fillosauru]

Well, I have used external&internal SCSI devices, and the rules that apply are: If you only use internal or external, the terminators have to be activated both on the controller and the last device on the cable; If both internal and external, the last devices (both in or ex) got to have the terminators activated (you have to deactivate the terminators on the controller).
 :-)

[Brian]

@orange

Correct... use an external terminator that goes right on the port if no external device is connected.

[orange]

OK, thanks all. I would like to know just one more thing about this scsi: will ACTIVE terminator work with Amiga scsi cards?

(supposedly its much better than passive)

[Castellen]

Yes, providing the external port on the SCSI card follows standard SCSI specifications.

Some active terminators have a LED built into them, which is good as it lights up to show the power is correctly applied to the termination power line by the controller.

[Chain]

and remember, many drives have own termination  8-)

[melott]

I've had an external SCSI box on my A3000
for years using the C= SCSI.Device and have
never had any problems. I've tried it with a
terminator on the pass-thru port and without.
Works fine either way. I just terminate the
last device on the chain. My external box has
a 9 gig drive and a CD Rom.

Also an old AT PC tower case makes a nice external
SCSI box (with very little hacking).
 

[Chain]

ho hacking, HAMMERing




and it has own 90W PSU + SCSI IN and OUT, i got it for $4 on our auction site  8-)

perfect stand for monitor, better than very high cases

[melgross]

I don't know what kind of boards require the removal of termination from them under ANY circumstances. I've installed scsi systems since the middle '80's and never seen that kind of instruction. I can't even imagine why they would suggest that.

Do the board manufactures here reccomend that?

Internal drives have to be terminated, external drives have to be terminated - as long as they are the last one on the chain (there are times when this doesn't work). The board terminates the first drive. It is not removable.

Drives may be self-terminatable, or not. An internal drive cable must be terminated at the LAST connector, whether or not a drive leaves one or more connectors between itself and the last one.

Scanners may or may not pass scsi power to later devices in the chain. Be careful with that one! Put a scanner last, just in case. External Zip drives come with a special cable, you may have trouble with it if you don't use that cable. Zip's should be the FIRST device in an external chain, if possible.

Scsi I drives get no benefit from active termination. Scsi II and III drives can benefit. Drives with scsi 160 or higher speeds need special cables and MUST have active termination, preferably digital termination.

[orange]

oh {bleep}, I've just spend 20e on terminators I don't need. duh

[Chain]

20€ ? oh {bleep}

[Hyperspeed]

SCSI can be a major pain. I have a Blizzard SCSI-IV and it has given
me quite a bit of grief.

I suspect most of the trouble I've had is due to it being manufactured
by DCE and there being a bit of tarnish on the contacts - however I'm
sure my cabling is just as much a problem.

The problem is that SCSI-2 is so fast the data becomes a radio wave
and noise can bounce down the cable, reflecting back off all adaptors
in the chain. The terminators sort of filter or regulate things but
TermPower is another spanner in the works.

The big trouble is going from a 50-pin ribbon to a scanner and back
(since a scanner will either require a terminator block or a pass-thru
cable for more devices.

I've managed to get hold of a Honda 50-pin internal SCSI ribbon with
an active terminator on the end, which is handy, as I have no idea if
my CD-ROM drive is supplying active or passive termination and
TermPower (and the SCSI-IV must be using active as it's
SCSI-2).

Also, whilst I have kept my chain relatively short (under 2m) I get
horrible data unreliability with Synchronous mode (7MB/s).

AmigaOS 4 has a special HDToolBox section dedicated to SCSI bus tests,
the rest of us just have to get pot lucky!

:-)