Interrupt daisy chain

[Jon Langseth]

I' forwarding a bit of correspondence I've had with Steve Cousins
about a quirk of the pinout on the System Expansion connector of the
LiNC80 SBC1, as it brings up a point that's important to have
documented ... (I hope the group formats this in a sensible, readable
way)

---------- Forwarded message ---------
Date: Fri, Jun 7, 2019 at 9:19 PM
Subject: Re: Interrupt daisy chain

On Wed, Jun 5, 2019 at 11:51 PM Steve Cousins
<steve.co...@gmail.com> wrote:
>
> Hi Jon
>
> I'm doing some work on Z50Bus backplanes and I've noticed an anomaly. That or I'm just getting confused.
>
> Your SBC1 and 5 slot backplane are consistent and the daisy chain is complete, but I think there is a slight problem.
>
> If you plug a card into the horizontal bus expansion socket it should work. For the daisy chain signal to arrive at the CARD's daisy chain input (IEI) it needs to be on the bottom row of the bus pins.
> Your SBC1 and 5 slot backplane have this signal on the top.
>
> My RC2014/Z50Bus backplane bridge follows your layout, so it is compatible. I think I just copied what you had done.
>
> If I'm right, then a card plugged in a horizontal backplane extension will suffer a conflict on the card's IEO line.
>
> Perhaps I'm confused!
>
> Either way, I guess future products need to be consistent with existing ones.
>
> Steve


You're right: the horizontal connectors on the ends of my 5-slot
backplane only function correctly if plugged into either another
backplane, or the SBC1.
And of the same reason/origin, the bus connector on the SBC1 is also
.... an anomaly..
On the backplane, the five slots (vertical connectors) pass the IEO of
one card to the IEI of the next, as expected. But for some reason, I
followed the pin-naming literally on the SBC1 and on the end
connectors, meaning the IEO pin on the SBC1 the same pin as the IEO on
the card... And I've kept this on the backplane ...

So, an expansion using the interrupt chain works correctly when in the
"slots of the backplane", but get their IEO connected to the
"previous" IEO on the "System/Expansion" connectors, and the IEI is
left unconnected.

Semantically, what I've done is not wrong; the IEO signal is on the
IEO pin of the bus, and the backplane connects that signal to the IEI
of the first slot.
In practice, it may be confusing, and it removes the possibility to
connect a card using IM2 directly to the system bus connector without
a backplane.

Hmm.

--
regards, Jon Langseth

[Jon Langseth]

If one wants to use an interrupt-enabled expansion card (using IM2 w/IEI+IEO) connected directly to the Z50Bus system expansion connector of the SBC1, a workaround or "hack" can be used:

On the expansion card, using either a bit of wire or a blob of solder, short the IEI and IEO pins. This will mean that the card will not work correctly on a backplane, it will only work correctly when used as either the only attached card (direct attach), or as the last/final card of a chain.

[Steve Cousins]

I don't see how that will allow the mode 2 daisy chain to work.

The output (IEO) of the modified card will still conflict with the output (IEO) from SBC1 (or backplane). When the SBC1 (or backplane) tries to block interrupts from the lower priority modified card, the card's output will be driving high and conflict with the SBC1 (or backplane) IEO signal driving low.

I think that we should say that only cards not supporting the mode 2 daisy chain should be used in the Z50Bus expansion connector. In practice that is not much of a limitation.

Steve

[Jon Langseth]

Yes, you are correct. When I thought about a workaround, I had implicitly "cut" the trace of the expansion card going to it's IEO pin on it's bus connector. So my "workaround" is even more of a hack, requiring both disconnecting the IEO signal on the expansion, and connecting it's IEO signal to the IEO pin on the connector.

The best solution is yours: being aware that IM2 enabled cards using the IEO/IEO signals do not work as direct attach and need a backplane or adapter to work.

The proper supported configuration of the Z50Bus system expansion connector, especially on the LiNC80 SBC1 and the LiNC 5-slot Z50Bus backplane, is with Z50Bus backplanes and expansion cards that do not use the IEO/IEO chain connectors.

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[Steve Cousins]

Attached is my design, complete with a solution to the IEI/IEO problem.

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