Termination for a Storageworks 110/220?

[Javier Miinero]

Hi everyone,

Just got a Storageworks SDLT 110 tape drive. It seems the provider sold me a brown box model and just chipped in a cheap SCSI cable with no termination. The drive will be connected through a AHA2940UW card. No other device will be connected to this adapter.

I talked to him regarding this issue, and he said that the termination would be provided by the tape drive. I have identified the termination power dip switch in the drive, but there seems to be no further documentation on any other switch which might provide the termination.

Shall I demand the provider to come up with a termiantion for the cable, or can anybody point me out where might I find more documentation regarding which dip switch can provide the termiantion required by the cable?

[David Ruska]

> I talked to him regarding this issue, and he said that the termination would be provided by the tape drive.

They are confusing terms. The drive can provide "term power", which is a diode protected voltage source used by the terminator. This is separate - and completely independent - from providing termination. In SCSI both bus ends need termination. Only one device (usually the HBA since it doesn't get turned off) needs to provide term power, however if you have long cables it is good to provide term power on the last device on the bus (to minimize voltage drop across the cable).

> I have identified the termination power dip switch in the drive, but there seems to be no further documentation on any other switch which might provide the termination.

You are correct - there are no swtiches for embedded termination.

> Shall I demand the provider to come up with a termiantion for the cable, or can anybody point me out where might I find more documentation regarding which dip switch can provide the termiantion required by the cable?

There are a few devices with 'auto' termination. The HP Ultrium standalone drives, for example, can do this. The SDLT drives do not.

I wasn't sure if you have an internal or external drive. The external drive has two connectors. The second connector is used either for the terminator, or to daisy-chain to another device.

If you have an internal drive, then it has only one connector. You either need to use an inline terminator, have the drive in the middle of the bus (i.e. on a middle connector on a ribbon cable), or have a cable with a built-in terminator at the end (pretty common for internal SCSI drives).

Hope this info helps. BTW, you should be using an active LVDS/SE terminator.

[Javier Miinero]

Thanks David.

I connected it as you suggested, to the middle of the bus (single device connected in it). It did work, but now the backups seem to be slower than in my old DAT unit (HP Surestore 20).

Can this be attributed to the lack of termination or shall I be looking at other issues here?

BTW my tape is internal.

[A. Clay Stephenson]

SCSI buses with poor termination can behave exactly this way and termination in the middle of the bus is, by definition, poor termination. The terminators MUST be on the physical ends of the bus. There MUST BE EXACTLY 2 TERMINATORS per bus. (One is typically located on the controller but must be enabled either by software commands or jumper/DIP switch settings). Assuming your tape drive or enclosure has two SCSI connections for each bus, the cable is from the host is connected to one SCSI connection and a terinator is plugged onto the other connector. I don't view it as the vendor's responsibility to supply you with a terminator; it was your responsibility to order it in the first place.

By the way, it is possible for a DAT to outperform a DLT - even one properly terminated. If the DLT is not supplied with data at a fast enough rate, the drive must stop, rewind, and restart; it never is able to actually stream. In those cases, A DAT that is able to stream (because its demands are less) will actually outperform a DLT that is not able to stream because the underlying software and hardware is unable to feed its voracious apetite fast enough.

[David Ruska]

The internal drive will have only one connector, which is appropriate when using an internal SCSI ribbon cable that has intermediate connectors attached to the cable. The last device on the bus should have a terminator, or the cable should be self terminating. With the self-terminating cables, you want to make sure that the last connector on the cable is used with the last device on the bus, so you don't have and "stub" length on the bus.

If you are getting poor performance, it could be a number of factors. Key to good performance with DLT/SDLT/LTO is a large block size. It should be 64K or greater. If your block size is OK, then I'd suggest running the performance tools in HP Library & Tape Tools (www.hp.com/support/tapetools).