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Old 08 October 2019, 15:20   #1
GadgetUK
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SCSI Termination Question

Hi, sorry if this has been answered elsewhere - I see conflicting information etc.


I've got an A2091 SCSI interface and plan to connect up a CD-ROM drive (no HDD or other internal or external devices). Do I need to terminate the 50 way IDC connector or not? It has been 25 years since I last looked at SCSI and I cannot remember.
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Old 08 October 2019, 15:23   #2
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If RN1 RN2 and RN3 are in place, you should not do anything.

http://amiga.resource.cx/photos/phot...res=hi&lang=en

Please remember to terminate your external SCSI chain.

This is actually an interesting dilemma.. Most Amiga SCSI controllers are set up to only have an internal SCSI chain or an external one. but not both at the same time.
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Old 08 October 2019, 15:25   #3
Glen M
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I've been looking an answer to this question myself lately.

From what I understand yes you need to terminate the scsi bus regardless of what devices are connected.

Now, that said... I have a squirrel scsi with cd drive in a drawer that I've not used in years but I was looking at it the other night and it doesn't have a terminator fitted. I'm almost sure it had one before and it does (or did) work but I've no idea what I've done with the terminator so maybe it didn't have one. Who knows haha. Would be interested to hear what others say.
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Old 08 October 2019, 15:31   #4
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Quote:
Originally Posted by Glen M View Post
From what I understand yes you need to terminate the scsi bus regardless of what devices are connected.
Yes, both ends (and only the ends) must have terminators. The terminators for the controller end are next to the internal SCSI header on the A2091.

However in the A2091 example, if you have a multi device internal chain as well as an external chain, you will have terminators in the middle of the chain as well.

Now continuing to think: IIRC the SCSI spec allows for a 10cm pigtail cable between the target and the bus, so technically if you have only one internal drive on the bus with a max 10cm cable, and leave your internal drive unterminated, it would still be set up according to the spec as long as your external chain is terminated properly.


Quote:
Now, that said... I have a squirrel scsi with cd drive in a drawer that I've not used in years but I was looking at it the other night and it doesn't have a terminator fitted. I'm almost sure it had one before and it does (or did) work but I've no idea what I've done with the terminator so maybe it didn't have one. Who knows haha. Would be interested to hear what others say.
If the CD drive's enclosure has two SCSI connectors and no obvious terminator switch, you should have a terminator on one SCSI connector and the bus connects to the other.
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Old 08 October 2019, 15:34   #5
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Thanks! The next question is - what terminator do I need to fit an 50 pin IDC connector? The cable I have is quite long =/
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Old 08 October 2019, 15:58   #6
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Originally Posted by GadgetUK View Post
Thanks! The next question is - what terminator do I need to fit an 50 pin IDC connector? The cable I have is quite long =/
Do you have internal devices connected to the A2091?
Do you have RP1 RP2 and RP3 populated on the A2091?
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Old 08 October 2019, 16:00   #7
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Quote:
Originally Posted by Glen M View Post
I've been looking an answer to this question myself lately.

From what I understand yes you need to terminate the scsi bus regardless of what devices are connected.

Now, that said... I have a squirrel scsi with cd drive in a drawer that I've not used in years but I was looking at it the other night and it doesn't have a terminator fitted. I'm almost sure it had one before and it does (or did) work but I've no idea what I've done with the terminator so maybe it didn't have one. Who knows haha. Would be interested to hear what others say.
My Squirrel SCSI CD-ROM (Toshiba 2x) came with a Terminator from new.
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Old 08 October 2019, 16:00   #8
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there should be a termination jumper on the CD-ROM enable that.
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Old 08 October 2019, 16:01   #9
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Quote:
Originally Posted by Jope View Post
Do you have internal devices connected to the A2091?
Do you have RP1 RP2 and RP3 populated on the A2091?

I've not connected up the drive yet, but RP1, RP2 and RP3 are populated. The cable I have has 4 SCSI connectors on it (IDC) - the plan was to use the nearest ones to connect the interface to the drive, and leave the other connections just floating, but I am not sure if those will need terminating.


Maybe I should wait to try and see if it works without any additional termination!
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Old 08 October 2019, 16:39   #10
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Ah, so the CD-ROM you are installing is internal? For some reason I thought you had an external drive.

Put it on the end of the SCSI cable, last connector, and set the jumper for termination enable behind the CD-ROM. No need to do any changes on the A2091 end.

The other end (again the very edgemost connector) of the cable is connected to the A2091 internal SCSI header. The connectors between the drive and the A2091 do not need any attention, just leave them empty.
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Old 08 October 2019, 16:41   #11
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You need termination at the (physical) ends of the chain, and *only* at the physical ends of the chain, so leaving the last connectors floating will not do you any favours. Connect a drive to the end connector and use its termination to terminate the chain at that end, and use either the resistor pack termination on the controller *or* an external terminator at the other end. Using both at the controller end will also cause you problems.
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Old 08 October 2019, 16:57   #12
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Quote:
Originally Posted by Daedalus View Post
You need termination at the (physical) ends of the chain, and *only* at the physical ends of the chain, so leaving the last connectors floating will not do you any favours. Connect a drive to the end connector and use its termination to terminate the chain at that end, and use either the resistor pack termination on the controller *or* an external terminator at the other end. Using both at the controller end will also cause you problems.
I feel this reply makes the matter more confusing to the recipient. In the A2091, the card based terminator is not optional for a regular user, as the terminator packs are soldered in. In my opinion in this case it is better to recommend that no additional termination is done on the A2091, instead an unterminated end of the internal scsi cable should be connected.
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Old 08 October 2019, 17:03   #13
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In this scenario, the middle drive (which is marked as configured with address 1 in the illustration) is the same thing as the empty connectors.



Note how the ends of the cable are connected, there are no loose bits on the other side of the SCSI card or on the other side of the last target (would be the CD-ROM).

Please note, the drive addresses have absolutely no relation to their physical location or order on the bus, neither are they in any way relevant to whether termination is on or off! Termination is to be turned on at the physical ends of the chain, where the cable stops. With a modern SCSI device, there are no terminator packs to install or remove, instead you use jumpers or dip switches to turn the internal terminator for that target on or off.

Last edited by Jope; 08 October 2019 at 17:09.
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Old 08 October 2019, 17:28   #14
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Apologies for the confusion, I thought they were socketed on the 2091... Yep, just make sure one physical end of the cable is connected to the controller.
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Old 08 October 2019, 21:27   #15
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Thanks for the great posts there Jope! Much appreciated! Will let you know how I get on! The next challenge might be to try and get the right drivers to support CD, and also maybe a BIOS update might be needed too.
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Old 08 October 2019, 22:52   #16
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Best way to deal with SCSI termination is remove terminators from devices (those funny resistors nets/groups in DIL or SIL package) and use Active Terminators on both ends of cable. Active Terminator improve overall signal integrity and SCSI is always properly terminated. IDC50 Active Internal 50-Pin Male SCSI Terminator
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Old 10 October 2019, 20:53   #17
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Thanks! Update: The A2091 seems to detect the SCSI CD-ROM OK (terminated at the 2091 end and at the drive. However, the system won't boot with the CD ROM drive connected - it's almost like the autoboot ROM is trying to boot from the CD =/ rather than the IDE CF interface. If I remove the CD from the cable it boots to IDE OK again.

I was going to try rev 7 ROMs but I cannot find them anywhere. They seem to be in the "Zone" however, I have no idea how to access the Zone.

Is it normal behaviour do you think for the A2019 to not allow the system to boot without a HDD connected to the chain?

EDIT: I found rev 7 here:- http://eab.abime.net/showthread.php?...88#post1350388

I will give that a try and see if it makes any difference!

EDIT2: Also ordered a version of the newer DIP controller IC too (ends -08).

Last edited by GadgetUK; 11 October 2019 at 08:18.
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Old 11 October 2019, 10:55   #18
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It certainly doesn't hurt to have the latest firmware and a more recent controller chip. Hope you get it going.
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Old 14 October 2019, 22:58   #19
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It certainly doesn't hurt to have the latest firmware and a more recent controller chip. Hope you get it going.

Thanks! Getting nowhere fast it seems =/ v7 ROM didn't make any difference. I find that the system does not boot when the CD ROM is connected, but disconnect it and it boots from IDE normally.


My final attempt is to try a new WDC chip (-08 rev). I will try that, if that doesn't work I might get a SCSI to SD adapter and try that on its own before I throw the towel in.
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Old 15 October 2019, 05:07   #20
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Are you trying to use the IDE interface on the A2091 with a CF card? If so it won't work:

Quote:
A little-known and mostly-unused function of the A2091 is a built-in XT-IDE controller. The IDE connector and LED are not installed, but visible on the board to the left and upper-right of the SCSI chip. Unfortunately, this port will only work with 8-bit (XT) IDE hard drives, not the common 16-bit AT-IDE hard drives. This option was apparently mostly used for the 20M drives supplied with the A590, the A500 hard disk option which shared the 2091 design.
(from here

Or if is the IDE interface on a separate card (in which case might be issues with scsi.device compatibility).
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