S-100 Bus Systems
I've been thinking about S-100 bus systems and their pros and cons. Certainly the SOL-20 is one way to go. But you should be aware of its shortcomings and its limitations. First, it is slow. It runs at something less than two megahertz clock speed for reasons that are part of the overall design. Lee was working against space constraints and a horrible deadline and so he made some compromises. I think I remember that the clock rate was dictated by the video display sub-system, not vice versa. Second, there are only four slots, and in a maximum 16K ram world, and the need for a floppy disk controller and maybe some special purpose board you might want to use, you are pretty tightly constrained.
As for floppy disk systems, the only two that make any sense at all are the North Star and maybe a Morrow Designs Thinker Toys system. But by far the best choice is the North Star. The PTC disk subsystem never got off the ground, and while I've seen them on Ebay once or twice I know that they are as rare as hen's teeth, and expensive as the dickens. The problem with the Thinker Toy disk with the SOL-20 is that there aren't many around and never were. And the CP/M must be configured before it will run. For me, it would be a dual North Star disk system or nothing. Also remember that the display is only 64 characters per line so you need a compatible word processor. The one basic fact that is the most positive about the SOL-20 is that it is so desirable to the collecting public. Oh yes, if you buy one you must get the ALS-8 and other PTC software somewhere. Again, not impossible to get, just hard to find.
Other S-100 bus systems to consider are the IMSAI 8080, the Morrow designs system or the Northstar system itself. The most valuable is the IMSAI 8080 and the best is the North Star (either the cabinet version or the desk-top unit). Here again for ease of use and every other aspect of ownership one can think of, the North Star is the choice for me. The desk top unit with display built in is neat, well designed and has excellent documentation. It isn't as limited in available expansion space as the SOL-20 is, and not as clumsy as the IMSAI.
Of course the IMSAI is always great to have. If you want to speed it up to Z80 speeds all you need is a good mother board like the Cromemco or the Morrow Designs or a half dozen other well designed and/or terminated boards. With 22 slots to fool with, there is nothing you can't install in the darn things. Here again, the thing to watch out for is the boards which comprise the basic computer, which are the cpu card, memory boards, I/O interface, display board and disk controller board. The trick is to try to avoid IMSAI boards except for the MPU-A 8080 board and the front panel board if you aren't using the Cromemco one. IMSAI I/O boards, memory boards and display boards were mostly pure crap. We used PTC 3P+S boards or Solid State Music's version of the same specifications board.
For memory we used anything but IMSAI; the best documented were the PTC 8KRA8 and 16KRA16 boards. Other memory boards that worked fine were Vector Graphics and Morrow Designs. Cromemco boards worked fine but were a little more complex to set up. As for video display boards, there were a number of them on the market, but my favorites were the PTC VDM-1, the Vector Graphics display boards (80 characters per line as opposed the VDM 64) and either the SSM board or the Polymorphics module, another 64 character per line board. The trick was to get a display board and an I/O board that felt at home with each other. The Vector Graphics board was a real winner, and was especially effective with the Vector Graphics Serial/Parallel I/O board. (I assume that if one got an IMSAI system that included dual floppies and a full complement of their cards, all factory installed and tested with their software, it would work as well as any of the other stuff I've mentioned. But if one is going to buy an antique, one wants the earliest antique available.)