External SCSI enclosure grounding.

[Demon-Xanth]

I'm looking into building an external SCSI enclosure (68 pin). The main question I have is in the power supply grounding:

Should it be grounded to the case?
Should the shield of the SCSI cable be grounded to the case, power supply, neither, or (if the above answer is yes) neither?
I'm assuming the case should be connected to earth ground, is this a correct assumption?

Also, is there a power on sequence that must be met, or is the only concern that the power is on before the SCSI card's BIOS? And what is the potential for hot swapping it in and out with an Adaptec AHA-2940W SCSI card, and an AAA-131U2 SCSI card?

 

[dkozloski]

If you are going to go to the trouble of building a hot swap enclosure be sure that it is 80pin SCA and not 68pin. The pins in the 80pin SCA connector are of varying lengths so as the connection is made, power and signals are sequenced so as to prevent zapping the equipment.

 

[Demon-Xanth]

Although the plan has been scrapped because the PS I was using needs to be about 3x larger on the 12V line to handle spinup, it wasn't going to be hot swap. It was basically going to act like a pair of external HDs. But the grounding still has me curious.

 

[dkozloski]

In general terms, to prevent ground loops and circulating currents in electronic equipment, powerline neutral, safety ground, and signal ground should only interconnect in one place.

 

[Demon-Xanth]

The line in isn't even an issue. Remember this is (was) going to be an external device with it's own power supply. The power supply was essentially a very small AT power supply. The question was based on the device side grounds. I'll try and put a picture up:

".."=isolated, "--"=connected
One ground, all connected
line--PS-->PC-->SCSI cable<--drive<--PS--line

Components isolated, but on the same plane
line--PS..>PC-->SCSI cable<--drive<..PS--line

Components isolated on different planes
line--PS..>PC..>SCSI cable<..drive<..PS--line

neither component isolated, however grounds not attached via the SCSI cable
line--PS-->PC..>SCSI cable<..drive<--PS--line

 

[dkozloski]

The basic rules still apply. In the AT supply the power line neutral and DC ground are isolated but the DC and safety grounds should be connected at one point only. In the enclosure the DC and signal and safety grounds should be connected at one point. If all currents are low the problems are minimized beacuse what you are trying to avoid is noise signals impressed on the data. With digital equipment as long as the data signals are above background by a substantial amount the whole exercise is inconsequential. If you were dealing with low level audio for example you could spend a lot of time trying to minimize hum but here practically anything goes.