[v1.2] MECB Backplane

[Editor]

This follows on from my Introduction of the Minimalist Europe Card Bus (MECB), where I outlined my preferred design, for recreating some of my 8-bit CPU experiences from the late 1970’s and early 1980’s.

For this modular Bus based system, the first thing we’ll need is a Backplane for connecting our modular boards together.

Join me, as I first walk-through the design of my MECB Backplane, and I then assemble my first Backplane.

You'll find all this here: Minimalist Europe Card Bus (MECB) – Backplane Design

Subsequent Version Updates:

• v1.2 - Implements a LED Power indicator, updates to the silkscreen to further clarify Capacitor orientation, and also added a new Tindie Store option for a PD (Power Delivery) compliant smart USB-C module.

[bugeyedcreepy]

I have a question that would hopefully allay my anxiety over this, but what's the likelihood two backplanes could be connected by a 20cm ribbon cable? I've designed a "MECB Backplane Extender", two of which could be used to connect two (or more) backplanes via a 64 way ribbon cable - is this doable, or is a ribbon cable not up to spec? Also, would I need to beef up the tracks if I wanted to connect another whole bus and set of cards?

[Editor]

I have a question that would hopefully allay my anxiety over this, but what's the likelihood two backplanes could be connected by a 20cm ribbon cable? I've designed a "MECB Backplane Extender", two of which could be used to connect two (or more) backplanes via a 64 way ribbon cable - is this doable, or is a ribbon cable not up to spec? Also, would I need to beef up the tracks if I wanted to connect another whole bus and set of cards?

Firstly, great to see you've got 3D models assigned in KiCAD. Great to see the in-line images are working as well! The 3D renders look awesome!

Unfortunatly there isn't a straightforward answer to your question. Whether it all works depends on the transmission line impedance, also affected by the capacitance added to the signal routes by the additional connectors and ribbon cables.

Essentially each connector signal path transition will be adding transmission line reflections to the bus. Whether this leads to any signal integrity loss would also depend on the operating frequency and added load of connected Cards.

In short, you'd only know for sure through testing and in-use trial.

Another complication is that as part of my "Minimalist" design intention, each MECB Card (designed to date), does not include bus drivers, which would have added another four IC's to each card. Effectively, a couple of '244 for the Address bus, a '245 for the Data bus, and drivers for the control signals.

Along with using a single PLD for the glue logic (also ensuring minimal signal load), the direct non-driven bus signals should be fine for all of my Minimalist intentions with the existing Backplane. Possibly also okay for an extension backplane, directly connected to the R/A connector. But, even this would be dependent on some in-use testing and also determining the "extended bus" usable frequency and usable load introduced by the added Cards on the extended bus.

Apologies that I couldn't provide a simpler answer to your question. Unfortunatly, the simpliest answer is just: Try it and see.

[bugeyedcreepy]

Firstly, great to see you've got 3D models assigned in KiCAD. Great to see the in-line images are working as well! The 3D renders look awesome!

lol! Thank you! though sadly, it doesn't translate in any way into an understanding of the electronics I'm rendering therein... -_-

Unfortunatly there isn't a straightforward answer to your question. Whether it all works depends on the transmission line impedance, also affected by the capacitance added to the signal routes by the additional connectors and ribbon cables.

Essentially each connector signal path transition will be adding transmission line reflections to the bus. Whether this leads to any signal integrity loss would also depend on the operating frequency and added load of connected Cards.

In short, you'd only know for sure through testing and in-use trial.

Lucky it's relatively cheap to get a prototype done up then, I guess I'll grab some boards and plug them all up to see how it goes - I'm no legend with electronics, but I have a 40Mhz oscilloscope that I have no idea how to use, so perhaps I should get serious about learning how that works too, I guess....

Another complication is that as part of my "Minimalist" design intention, each MECB Card (designed to date), does not include bus drivers, which would have added another four IC's to each card. Effectively, a couple of '244 for the Address bus, a '245 for the Data bus, and drivers for the control signals.

Along with using a single PLD for the glue logic (also ensuring minimal signal load), the direct non-driven bus signals should be fine for all of my Minimalist intentions with the existing Backplane. Possibly also okay for an extension backplane, directly connected to the R/A connector. But, even this would be dependent on some in-use testing and also determining the "extended bus" usable frequency and usable load introduced by the added Cards on the extended bus.

Hmmm, perhaps I should look at incorporating something like this into an extended bus design in a way that would allow existing cards and designs to largely work unmolested? That would of course require me to know what I'm doing though - not something that is viable in the short term...

Apologies that I couldn't provide a simpler answer to your question. Unfortunatly, the simpliest answer is just: Try it and see.

Done! I'll just "try it and see" - what could possibly go wrong??

All these question & answer posts are a big help, I'm certainly learning, and that's the goal, right?

[Editor]

Hmmm, perhaps I should look at incorporating something like this into an extended bus design in a way that would allow existing cards and designs to largely work unmolested? That would of course require me to know what I'm doing though - not something that is viable in the short term...

In theory, you could perhaps make a Backplane extender with bus driver ICs included, for driving the "extended" bus.

This would be rather unusual, but perhaps might make some sense in our Minimalist design?
i.e. Retaining the "no buffers needed" for the original 4 otr 5 card Backplane, but introducing buffers for driving an extended system.

But... One catch I can immediately think of is in controlling the enabling of the Data Bus buffers...

Assuming any CPU Card was restricted to only residing on the primary Backplane, then all the Address and Control bus buffers would be outward driving. But the Data bus is bidirectional!
The WR signal is normally used to control the data bus driver direction, which is fine when all peripheral Cards are on the other side of the buffer (as would be the case if the driver ICs were on the CPU Card).
But, if a peripheral Card was also located on the primary Backplane (i.e. inside the buffers), then a read from these Cards would create a bus conflict, with both the peripheral Card and the Backplane extension located bus drivers, both trying to drive the primary Backplane's Data bus at the same time!
Then, if we restricted the primary Backplane to only containing the CPU Card, we would be negating at least 3 card slots from our overall extended Backplane system. Doh!

Hmm.. Sounds like a bad idea. Probably better to design a range of CPU Cards with bus drivers on-board, specifically for supporting an extended Backplane system.

Sorry.. just thinking out loud. LOL

[bugeyedcreepy]

Sorry.. just thinking out loud. LOL

lol! That's the entire point! I wouldn't know if I didn't ask...!!