James asks:

> Then the system hangs when booting.  I can get the system to boot 
>  up from this point by turning off both external tape drives.  
>  However, if I connect the devices individually then the host boots 
>  and the system can access which ever drive is attached to the host. 
>   
>  terminator --> DLT --> scsi_hostadapter (boots, device accessible)
>  XOR
>  terminator --> DAT --> scsi_hostadapter (boots, device accessible)
>  
>  My question is: Is it even possible to daisy-chain these two 
>  devices to the host in manner that permits the system to boot and 
>  access both; and if so, how does one accomplish this?  

If you can get either device to work by itself then both devices should be 
readily daisy-chainable. Nor should the reason why it's not working be 
complicated.

There are really only a few possible reasons:

     o you have a SCSI address conflict, either with your two devices or with 
one of the other internal devices that are on the same SCSI bus.

     o one or both of the devices have an internal terminator (active or 
passive). Your adding a second terminator on the single device configuration may 
not make too much difference, although it does cut the effective terminating 
impedence in half. Adding a third terminator is the straw that breaks the 
camel's back. If this is the case, you need to disable the internal terminators and 
go with only the single external terminators.

     o you have a bad SCSI cable connecting the the two devices, leaving the 
most distal device not only unable to communicate but also at least one wire 
in the entire chain unterminated.

Proper termination is critically necessary for a proper impedance match. It 
serves the same purpose as a shock absorber on a diving board. Without the 
shock absorber, when the diving board is struck with a one or a zero transition, 
it rings for a substantial period of time, possibly looking like an errant one 
or zero to whoever's reading the position of the diving board. But with the 
shock absorber/terminator, the diving board moves to its desired position and 
stays there, with only a little wobble.

But the size of the terminator/shock absorber is also critical. Too little 
and the diving board wobbles wildly, ringing. Too much and the diving board 
hardly moves. Either condition will cause data transmission errors.

It's important to remember that your two devices are not the only devices on 
the SCSI channel. If you screw up the transmissability of information on the 
SCSI bus by causing an impedance mismatch, you're affecting every device on the 
chain.

Wirt Atmar

 

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