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Old 14 January 2016, 18:14   #1
Arnie
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SCSI Termination

I'm going to use the CDTV SCSI card with a 50 pin SCSI cable with 3 connectors.

I connector to the SCSI card, middle to a SCSI2SD and the last to a HPDB5 external port.

The SCSI card has passive termination. Do I need any more termination on the external port?
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Old 14 January 2016, 19:01   #2
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SCSI chain must be terminated in each end.
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Old 14 January 2016, 19:04   #3
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Quote:
Originally Posted by idrougge View Post
SCSI chain must be terminated in each end.
I'm not sure the external connector is terminated, so would I then be better off using 4 connectors and terminating the last connector?

EDIT: Thinking about it, I'm sure the SCSI2SD will have termination, so I could put the external connector in the middle and the SCS2SD at the end of the chain.

Last edited by Arnie; 14 January 2016 at 19:18.
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Old 14 January 2016, 19:22   #4
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No, you can't put the external connector in the middle because that will mean that when you connect external devices, your SCSI chain will be Y-shaped.

It's easy to see if the external connector is terminated. If there's a terminator plugged in, it's terminated.
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Old 14 January 2016, 19:27   #5
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So the external connector has to be at the end of the chain.

Would you use passive or active termination? The SCSI card is passive, the SCS2SD is active.
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Old 15 January 2016, 01:28   #6
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I believe you shouldn't mix.
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Old 19 January 2016, 19:26   #7
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O.K The external adapter arrived today. I also scored a cheap Zip 250 external SCSI drive off of e-beh.

Using a 50 pin IDC cable, I plugged one end into the CDTV SCSI card and the other into the external adapter. I Plugged the Zip drive into the external adapter using a 25pin to HPDB50 cable.

I used a 100mb zip disk pre formatted and installed with WB2.1 as a boot disk, switched on the CDTV and, OMG I can't believe it, it worked first time.
Yep WB2.1 booted with no problems.
The biggest shock was how fast WB booted, 15 secs, wow.

Just waiting for the SCSI2SD card from Amigakit now to test it further.

I need to find a way of attaching the external adapter to the CDTV back plate. I might just cut a hole in it and feed it from the front, use the external adapter screw holes to attach to the CDTV plate and then use the CDTV screw holes to secure it in place.
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Old 19 January 2016, 22:21   #8
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For whatever reason, many Amiga SCSI controllers had hard-wired passive termination, including the Commodore A2091 and various GVP boards. Even though that means the SCSI chain will be incorrectly terminated if you use both internal and internal connectors.

If you're not averse to a bit of soldering, I'd suggest you desolder the terminating resistor packs on the SCSI controller board, and instead use a terminator (preferably active) on the external SCSI connector if you don't have any external SCSI drives connected.
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Old 20 January 2016, 03:56   #9
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The only terminator I have, at the moment, is an active 50 pin IDC pass through.
I need the SCSI2SD card before I can test further.

Initial testing with/without the Zip connected seems O.K but I'll see what happens with another device connected internally.

Am I right in assuming that incorrect termination could lead to data corruption?
Is there any possibility that it could damage the SCSI drives?
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Old 20 January 2016, 15:22   #10
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Yes I think incorrect termination could lead to data corruption.

However with slow SCSI devices/bus (we're not talking U160 or U320 SCSI here), and short SCSI chain the quality of termination doesn't seem to be as critical. Meaning, things might appear to work fine even though your SCSI chain is incorrectly terminated. The chance of problems would increase with a longer chain and faster transfers (e.g. synchronous).

With the GVP SCSI controller in my A2000 I used to have a couple of external MO drives (terminator on the last one), plus an internal hard disk and CD-ROM (CD-ROM's built-in termination enabled). That seemed to work OK, even though the SCSI controller had hard-wired passive termination.
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Old 23 January 2016, 01:04   #11
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SCSI2SD card arrived, using a 2gb microsd.

Everything works O.K.

CDTV SCSI card > passive Termination
SCS2SD > no termination
External SCSI port > pass through active termination.
(optional) Zip 250 on external >active termination.
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Old 17 February 2019, 11:20   #12
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Quote:
Originally Posted by mark_k View Post
For whatever reason, many Amiga SCSI controllers had hard-wired passive termination, including the Commodore A2091 and various GVP boards. Even though that means the SCSI chain will be incorrectly terminated if you use both internal and internal connectors.

If you're not averse to a bit of soldering, I'd suggest you desolder the terminating resistor packs on the SCSI controller board, and instead use a terminator (preferably active) on the external SCSI connector if you don't have any external SCSI drives connected.
Are these two the terminating resistor packs?

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Old 17 February 2019, 22:15   #13
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Yes. But also... GVP didn't publish schematics as far as I know, so I'm just going from the board pics at amiga.resource.cx.

Another couple of SCSI bus lines may be being terminated by four separate resistors. Look at R1, R2, R6 and R7 to the left of the WD33C93A chip.

I can't trace what they are connected to from the pics. But if you have a multimeter you could check yourself. Set it to continuity check mode so it beeps when there's a connection between the probes. Hold one probe to the right side of R6 or R7, then touch the other probe to each SCSI connector pin. Is there a beep? And similarly, hold one probe to the left side of R1 or R2, touch the other to the SCSI connector pins.
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