Suffering with Backplane

Excuse the awful pun but I wanted to post about one of my recent projects. Back when I used to buy the great Spectrum magazines like Your Spectrum and Crash, I read an article about adding peripherals to the rear edge connector with a device called a Back Plane. I managed to find the article in a collection of YS magazines I bought on CD.

Your Spectrum Issue 18 (Pages 20/21)

Your Spectrum Issue 18 (Pages 20/21)

I had always remembered this image and recently, wondered if I could build a modern variation.

I determined that the best way to go, for the first iteration anyway, was to design a PCB with straight through connections. This would mean that it would be compatible with all Spectrum models, including the new¬†Spectrum Next. I was aware that the internal 7805 regulator used for powering the Spectrum and it’s associated¬†peripherals might be a little under-powered to cope with much more than a few added devices so I made provision for an external power source too. One final touch was to add a reset switch. Mostly because it’s an easy thing to add.

I had used KiCad library parts to make previous interfaces so I went ahead and used them again. I didn’t want to have a board as long as the one illustrated in the YS article so I shortened it a little. I use a ZX-HD display interface at the moment, in lieu of having a suitable Composite Monitor so I knew I’d need at least one pass-through connector. In the end, I decided on two.

I plan to make several other interfaces to fit on the backplane so a way of connecting them easily was vital. On the YS illustrated backplane, the board has sockets which add-on boards plug into. I thought this might be a little limiting, especially for interfaces that already exist. What I have used is rows of header pins that can be used directly with ribbon cable or fitted with an extra board to emulate the Spectrum’s rear edge connector. These extra boards will be soldered on to the header pins to make a permanent extra ‘Edges’ to mount horizontal interfaces on.

Spectrum Backplane v2.00

Spectrum Backplane v2.00

Edge Connector Extender Board

Edge Connector Extender Board

The Spectrum is connected to the left hand side of the Backplane board. I added a jumper in the top left to select internal or external power. The mini psu is a TRACO unit, the same as is commonly used in Spectrum refurbishments to replace the 7805. The power socket is a standard barrel with center as positive (reversed to the original Spectrum PSU!).

Why version 2.00? Well, in my haste to get the boards fabricated the first time around, I made a critical mistake. I had got the Spectrum edge connector inverted. This meant that the pass through connections were also inverted! I managed to fry my ZX-HD interface. Hence my rather bad pun as the title. But, a very valuable lesson was learned. Check, check and check again!

The new version is in production and, hopefully, with the right connections! I will double and triple check before connection this time. Once I have proved the design and tested it thoroughly, I will publish the design files on the project page.

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