On Thursday 06 August 2009, rory_f wrote:
>Currently we are using this:
>
>03:0c.0 SCSI storage controller: Adaptec AIC-7892A U160/m (rev 02)
>
>And it is throwing up hardware errors every now and again so we want to grab
> another one to put in there. I understand it could also be cabling which is
> causing this and we'll address that too
>
>We're using a fc7 box, 32 bit.
>
>Can anyone recommend a card they have had good use out of ?
>
>rory

Rory, I get the impression that your problems are hardware setup issues.

1. A sagging psu that isn't supplying a good solid 5 to 5.1 volts to the scsi 
bus will impact the logic one noise margin, already only half the logic zero 
noise margin in a typical resistive terminated setup.  If yours has sagged to 
4.8 volts, there is no hope for a solid link.  Replace the psu with a good 
one.  They sag with time, I think to enhance future sales. :)  And one of the 
reasons I won't buy Antec's these days.

2.  scsi bus terminations need to be on the last device in the chain only, and 
_must_ be on the end of the cable, you cannot have surplus cable wound up in 
the corner while using the next to last connector.

3. "Active" terminations on both ends will alleviate much of #1, nothing but 
reconfiguring the cabling will fix #2.

4. In extreme cases one may have to sacrifice a virgin or 3, but once I've 
addressed the first 2, I never found I needed the virgin.  In case you haven't 
noticed, they seem to be in quite limited supply, even if not to be used in 
this manner.

What few folks understand is that the scsi/ide/atapi/sata buses are in fact 
transmission lines and must be properly terminated in order to control the 
standing wave ratios.  All the buses that use the same ribbon cable wire 
spacing as the original scsi/floppy/40 pin ide, have an impedance near 120 
ohms, and the std resistor pack terminator made from std jedec resistor 
values, is about 132 ohms..  It could be closer, but that is what the resistor 
pack makers can give us at a reasonable price, and which present a reasonable 
power dissipation.  Because the 'active' terms only use power for logic 0, 
this can have a significant effect on a laptops battery life, but its been 
ages since I last saw a lappy with a scsi connector on it.  However, the power 
considerations are what drove the active development years ago.


-- 
Cheers, Gene
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