NEEDED: Back plane schematic.

[Jay Cotton]

Hi Guys:

I am working on my XXU unix box and I am chasing a bus problem that is causing

core bombs and system lockups.

I can't find any documentation for the back plane.  

I am trying to verify bus termination values and which lines are terminated.

tnx

jc

 

[Peter Higgins]

What make/model of backplane are you using?

[Jay Cotton]

I have a z2 system, This has termination resistors.  I think this one is the best choice.

and a backplane from a much newer machine, looks like the IMSAI 22 slot.  It does not have termination resistors on it.

tnx

jc

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[Peter Higgins]

Based on all the Z2 manuals I've read, I thought all those hugh Cromemco Z2 enclosures had Cromemco's 21 slot "Blitz Bus" backplane, which was unterminated. Cromemco also made a 12 slot version of the "Blitz Bus" backplane, and another still smaller version for their later compact systems, all without termination. I am not aware of any Cromemco backplane with built in termination, which is why I was wondering whether yours might be a third-party product.

[Mike Stein]

That's my recollection, at least about the systems I knew, that the backplanes were not terminated but pretty trouble free nevertheless.

IIRC the 21 slot boards were in the Z2/200, CS3/300 and 4xx, the 12 slot for the Z2 with internal 7710 hard disk, and the 8 slot for the CS-1/100.

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[Jay Cotton]

I think the backplane has pull ups on it.  Can't say for certain the board in 8 hrs drive from here.

There probably are not any termination resistors.   

The cromemco boards do not respond well to bus termination, probably because they are designed 

for a non terminated backplane. 

jc

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[Roger Hanscom]

I think I remember discovering that early revisions of the ZPU only work in non-terminated busses?  I believe that the "turning point" was rev. E ???  This could be all wrong.  I experimented with this a long time ago. *grin*

Roger

[Richard Muse]

FWIW, I can add this. I have a very early System 2 with 21 slot MB. It will not work with a DPU, only a ZPU. I have a second one several years newer and it works fine with a ZPU,  DPU and XPU. I have not tried an XXU in it. The early one does not even have the front panel hinge. I assume there to be some difference in the MB's. Whatever it is though, it's not obvious to my eye.

Richard

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[Peter Higgins]

One thing  that might contribute to this issue... both the XPU and XXU boards have built-in on board bus termination with data lines having active termination and address lines having passive termination. For this reason, I don't think either of these boards should be used with an S100 backplane that has its own built-in termination.

[Jay Cotton]

wow.  That is interesting.

I will have to go over the schematics and look at the interrupt lines specifically.   

tnx

jc

[Mike Stein]

Is there anything Cromemco that you don't know, Peter? You're amazing!

Sounds like that might be the issue; doesn't explain the DPU though. Weird.

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[Mike Stein]

Maybe the speed? The 680x0 boards are all faster (except the DPU in Z80 mode)

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[David Roberts]

I was having problems with my Z2, but it turned out to be 'dicky' slot connectors in the end. I found a set of 'good' backplane slots and my ZPUs, DPU etc. work fine.

Dave

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[Peter Higgins]

On Thursday, April 10, 2025 at 11:54:02 AM UTC-7 Mike Stein wrote:

Maybe the speed? The 680x0 boards are all faster (except the DPU in Z80 mode)

Totally agree. Getting S100 CPU boards faster than a basic 4MHz Z80 running reliably is sometimes a dark art. This issue, more than any other, has caused me countless hours of troubleshooting time over the years.

[curt mayer]

The blitz bus has unusually good inter-trace shielding, so crosstalk was much better than the more naive designs.   Termination of course is also sensitive to board placement, and cromemco favored putting bus masters in the middle of the bus, and slaves on the ends.  This goes a long way to mitigating the reflection and skew problems of an unterminated bus

one of the crossbeams has gone out of skew on the treadle

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