I’m calling a SCELBI 8H that includes a 4K memory card from a SCELBI 8B a SCELBI 8H-B. See the original 8H-B here.

Attaching a single 4K memory card to an 8H is easily done because of the way that SCELBI memory cards are addressed. While reading the following description, keep in mind that on a SCELBI, a bank of memory is considered 256 bytes.

On a SCELBI 8h, there are 4 bank select lines connected to each slot for a total of 16 bank select lines. This provides addressing for 4 banks or 1K per slot and 16 banks or 4K for the entire system. The are 4 bank select lines connected to each of the 4 slots at different points as can be seen in this image of part of the 8H backplane.

8H backplane bank selects

On a SCELBI 8H 1K memory card, each of the four banks available on the 1k card, is connected to the backplane at four different points. In the picture below, the line for the first memory bank on that card is highlighted in green.

1KSRAM 1st bank select

Depending upon which slot the card is plugged into, it will pick up the 4 bank selects for that slot, thus automatically providing a different address range for each slot, without requiring extra decoding logic on a memory card.

The SCELBI 8B memory design is a little different in that the memory expansion card decodes 16 banks for each slot (4k) and a total of 64 banks (16K) for the entire system. This image shows the 64 bank select lines going to the 4 memory slots on a SCELBI 8B backplane.

8B memory selection

The 4K memory card is also different, with the 16 bank lines all feeding into the on board circuitry separately.

4K memory selects

This design also requires extra complexity on the memory card. The 16 bank selects on the back plane must be demultiplexed into 4 chips selects, each of which selects one of the four rows of memory chips.

So how do you connect a 4K SCELBI memory card to the 8H? A few of you may have already deduced how to do this. You simply jumper all 16 banks selects available on a 8H backplane to a single slot, making sure you leave the other slots empty. This still leaves you a 4K system memory capacity, but will significantly reduce the cost of creating a 4K SCELBI 8H.

The SCELBI 8B memory addressing architecture is unique in my experience. I think it was deliberately designed to allow use of a single 4K memory modules in a SCELBI-8H. Perhaps Nat and Bob were considering a cost reduced 5 slot SCELBI 8H backplane that would take advantage of this architecture.