S-100 backplane board spacing

There are two considerations, airflow and bus length. Wider gap helps airflow to a point then wider doesnt help as much. Wider spacing makes the bus longer for a give number of connectors and longer has more problems with reflections(ringing) and other transmission line behavours. Longer may also impact power distribution as well. Last items is the height of boards limits how close or forces skipping slots. Some fo the taller boards in my collection suggest greater than 0.75" center to center would barely be adaquate because of tall heatsinks contacking the next board. Look at the better backplanes, Compupro would be one. Also the later boards (more modern) were dense enough that only a few slots are required. I have at least two machines that are: Zpu-B z80 cpu 64K ram (ram 16, 17 or 22) Interfacer (multi serial plus parallel) DISK1A FDC DISK3 HDC Systemm Support Thats only 6 boards and the smallest backplane I have is 8 slots. It's as loaded a system as most or more so. The limiting factor for the box is fans, power supply and mountings for the disks. Older antiques like an altair filled with then current board may have: CPU 8080or later Z80 Ram boards anhywhere from 4k per to 16K per (as many as 8 for 64K) Some form of rom boot card possible a dual board FDC (altair disk was a two card set) multiple IO baord for serial possible multiple boards for one or more printers or other parallel devices. And cooling issues. Hope that helps with some history. I'd say 6-8 slots spaced 1" and also on the board terminator. Allison