8-bit acorn hardware • Re: New SCSI Host Adaptor

I was having a look at this earlier. Standalone terminators are still available, but most seem to use either 50 or 68 pin connectors that are rather expensive.

You used to be able to get them built into a 50-way IDC connector. But maybe hard to find now and/or as expensive as getting JLC to make you one. Such a standalone terminator doesn't need to be switchable of course - you only plug it in if you need it.

As currently configured, my host adaptor is set up to accept TERMPWR from the ribbon. The host adaptor currently doesn't put TERMPWR onto the ribbon. Should I change that?

Ideally yes, though it's a hassle and/or expensive to provide proper short-circuit protection (there was a thread on people setting their Filestore ribbon cables on fire due to its non-standard TERMPWR). The protection needs to allow about half an amp to power two terminators; maybe one of those USB power switches would do and not be too expensive; some of them come in SOT23-5 so not too much space either.

I think this termination issue is only an issue with the 80 pin drives, as the 50 pin drives all seem to have termination built in.

50pin drives can be expected to have termination _capability_ built in, but if the resnets were unplugged and lost 40 years ago that doesn't necessarily help you now.

Therefore, what I was thinking was that I would make a special 'end of line' backplate with the termination in it, and that termination would be powered either directly from the power supply that powers the drive, or pick up TERMPWR from that specific drive; so no need to pick it up from the ribbon. Any intermediate 80 pin drives would have a simpler backplate (like this) that doesn't have the termination. I also don't think bus isolation would be needed for this special 'end of line' backplate. You either need to use it because it's end of line, or just use the simpler intermediate backplate. Also, I don't really see a case where I would want to plug one of these plates into a cable to act solely as a terminator, so I don't really see the need to power it from the ribbon.

Agreed that you don't need it switchable ('isolation') if it's a special version just for the end device, but drawing power from TERMPWR would allow the same board to be used as a standalone terminator for a string of 50-pin drives.

Statistics: Posted by arg — Tue Feb 25, 2025 9:12 am

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