Connecting U320 LVD/SE HBA with narrow SCSI-1 device?

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James Youngman

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May 24, 2021, 5:56:54 AM5/24/21
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I (will soon) have a LSI LSI20320IE SCSI PCI-E HBA. That has, I think, a single VHDCI ("Alt 4, P-cable connector") connector. According to the manufacturer's manual, it features LVD/SE termination.

I would like to use it to access a tape drive capable of reading DDS-1 and DDS-2 tapes.

Because backward compatibility for DDS seems like a bit of a lottery, it seems reasonable to get a DDS2 tape drive (so that I have the best shot of being able to also read the DDS1 tapes). IOW, I'm worried that if I get, say, a DDS-3 drive it may no longer read DDS-1. The DDS-2 tape drives that I can spot on eBay mostly seem to have the Centronics / CN50 / SCSI-1 / "Alternative 2, A-cable connector" connectors. I suppose I'm going to end up buying one of these, though I haven't chosen one in particular. The HBA and the tape drive will be the only devices on the SCSI bus.

What cabling and termination am I going to need in order to make this work? Any tips on where to buy those?

Can I, for example, just use a cable like this one <https://www.ebay.ie/itm/361858629574> and use a normal CN50 active terminator on the tape unit? Or might that not work? If it won't work, why won't it work? What should I do instead? (Can I assume that such a cable would terminate the upper byte of the SCSI channel?)

(I'm not concerned about the performance aspects of this, just whether it works at all).

Thanks,
James.

Michael Bäuerle

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May 24, 2021, 6:34:55 AM5/24/21
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James Youngman wrote:
>
> I (will soon) have a LSI LSI20320IE SCSI PCI-E HBA. That has, I
> think, a single VHDCI ("Alt 4, P-cable connector") connector.
> According to the manufacturer's manual, it features LVD/SE
> termination.
>
> I would like to use it to access a tape drive capable of reading
> DDS-1 and DDS-2 tapes.
>
> Because backward compatibility for DDS seems like a bit of a lottery,
> it seems reasonable to get a DDS2 tape drive (so that I have the best
> shot of being able to also read the DDS1 tapes). IOW, I'm worried
> that if I get, say, a DDS-3 drive it may no longer read DDS-1.
> The DDS-2 tape drives that I can spot on eBay mostly seem to have
> the Centronics / CN50 / SCSI-1 / "Alternative 2, A-cable connector"
> connectors. I suppose I'm going to end up buying one of these, though
> I haven't chosen one in particular.
> The HBA and the tape drive will be the only devices on the SCSI bus.
>
> What cabling and termination am I going to need in order to make this
> work? Any tips on where to buy those?
>
> Can I, for example, just use a cable like this one
> <https://www.ebay.ie/itm/361858629574> and use a normal CN50 active
> terminator on the tape unit?

Yes, this should work.

The HBA has a LVD/SE (multimode) interface. Old DDS drives normally use
SE (only) interfaces. If such a device is connected to a SCSI bus, all
multimode interfaces will be switched to SE mode automatically.

The maximum bus (segment) length for SE is 3 m for the fastest SE
transfer mode (Fast-20), as long as there are not more than four
devices on this segment. The cable above has a length of 2 m and
should therefore not be too long.

> Or might that not work?
> If it won't work, why won't it work?
> What should I do instead?
> (Can I assume that such a cable would terminate the upper byte of the
> SCSI channel?)

No, there are cables with and without terminators for the additional
signal lines of a wide bus. Both types should be usable for your
configuration.

As long as there is one terminator present (you can configure the HBA to
terminate the whole wide bus at one end), the signal lines are not
floating and this should be sufficient when they are unused (no wide
data transfers are possible with your configuration).

The signal lines for the narrow bus subset must be correctly terminated
at both ends. For external drive enclosures plug a terminator into the
second bus connector (and disable potential internal termination of
the DDS drive).
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