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Common gotchas (Today I learned...)


Sep 28, 2019
Recently I acquired a H9276-A backplane, and decided that I wanted to give it a try. I looked through the LSI Systems Service Manual Volume I (https://manx-docs.org/collections/antonio/dec/ek-lsifs-sv-005-vol1.pdf) to understand what all of the jumpers and things are for since they're not labelled as they are on my H9270 backplane.

On pages 364 and 365, it's easy to see that the screw terminals on the D side of the backplane are where the normal power connections go. And, handily enough, they're labelled 1-8, with the functions of each listed. Easy! I connected the screw terminals to the proper wires, following the guide on page 365, inserted my CPU board, and turned on the power...and immediately blew fuses on my power adapter!

When I started beeping out the pins, it seems that the H9276 that I have is actually pinned BACKWARDS from the way the service manual listed! Terminals 7 and 8 are +5, terminal 1 is +12, and terminals 2-5 are GND. I guess I learned again to always, ALWAYS, trust but verify. Hopefully I haven't broken my CPU board...

This wasn't intended to be a rant, but maybe what could be a sticky topic here that lists gotchas like these so that others don't run into the same problems. Unless there's something like this that exists already?
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Always assume that the documentation is suspect.

I have come across schematics for equipment that we use at work where the pinout is documented from the sense of the connectors on the board not the connector you plug into it. The numbering is all wrong as a result.

There are also connectors that are 'upside down' when compared to the pinouts on the schematics. I tell people about this, bt everyone still gets it wrong!

Check the wiring with a multimeter for continuity.

Never, ever power anything up with boards in it to start with (unless you are forced to).

But you know this now - so lesson learnt.

What CPU board have you got? You might be lucky...

Thankfully I just got a good memory test from the boot ROM...it's a 11/23+, and it still works thankfully. Still quite a shock...and very thankful for the fuses on the power board!