View Full Version : Help! I'm running out of slots!

December 6th, 2009, 10:00 PM
My NorthStar Horizon has 12 slots. A complete 256 color Microangelo system takes 9, 10 if the "genlock" (what is a "genlock"?) also takes a slot. I think it supports up to 8 CPU boards. Then there's the FPU, floppy controller, hard drive controller, tape backup controller, 6 or 8 RAM boards, a Cromemco 8PIO, a clock, Screensplitter video board, A/D board, sound board(s), cassette, video digitizer...

Maybe I should quit while I'm only up to 38 slots.

I don't want to replace the motherboard. I want to keep it original. Has anyone here ever connected a second motherboard, using heavily shielded and buffered cables, to an S-100 system with limited slots?

Oh yeah, the interface board in each box would take another slot.:wow:

December 7th, 2009, 12:24 AM
I doubt you could get past 2-3 MHZ with that arrangement. May want to consider boards with multiple functions.

December 7th, 2009, 08:01 AM
Ever heard of a "slot box" for a pee sea? If they can do it, so can we.

First, I will probably have to build the board myself. I would put buffers at each end of every signal. I would use the kind of ribbon cable with the shield on one side, like an Apple disk drive cable. Every other wire would be a ground. I've seen ribbon cables with shielding around them and insulation over that.

It still might not work. But if I can add slots to my pee sea, Why can't I add them to my S-100 system?

December 7th, 2009, 09:38 AM
Hi! Bus length, termination, cross talk, reflections, signal propagation delays, voltage drop, floating grounds, etc. There are many reasons but John is correct. Large S-100 bus motherboards/chassis generally have problems with signal quality.

The PC expansion slot took a <5 MHz bus with 5 or 8 slots and expanded it with 5 or 8 more. Total slot count is between 10 to 16. Typically slow IO boards went in the expansion chassis like serial or parallel ports not memory or video boards. That is a much more forgiving situation than trying to add another general purpose 12-24 S-100 slots to a NS Horizon S-100 bus.

S-100 bus termination and careful design can limit the problems but not make them go away altogether. Many of the large format S-100 chassis had a lot of slots but often many slots were left empty for cooling, etc. Completely full large S-100 bus system (>12 slots) are pretty unusual and tricky to get working reliably. Adding an expansion chassis to that would be extremely difficult.

I'm not saying it is impossible but nearly so -- I would love to see it happen. IMO, the practical limit is about a dozen slots (what's available in a NS Horizon). There are exceptions but not very common.

Thanks and have a nice day!

Andrew Lynch

December 7th, 2009, 10:05 AM
I'm not saying it is impossible but nearly so -- I would love to see it happen. IMO, the practical limit is about a dozen slots (what's available in a NS Horizon). There are exceptions but not very common.

Thanks and have a nice day!

Andrew Lynch
Most Cromemco systems had 22 slots; a multi-user configuration could indeed fill most of those, especially if you used small memory cards and I/O cards with few ports.

But I'd agree that an external bus extender would be a bit of a challenge to keep clean, considering that even some ordinary single small-count backplanes had problems with noise once they were heavily loaded (not to mention that you're going to need some serious power and cooling).

But hey, more power to ya; Andrew, don't you have some backplanes to sell?

December 7th, 2009, 10:28 AM
22 S-100 slots, given the uneven quality of third-party boards would be impossible in a mixed-board system. It's conceivable that a single vendor's boards (i.e. Cromemco) might be made to work in such a system. But most vendors didn't pay too much attention to bus loading and capacitance that tends to many-card backplanes.

You could bridge two backplanes using some transcievers and latches, synchronized to the bus, but it would cost you at least one wait state.

December 7th, 2009, 12:44 PM
Hi Mike! I designed a small actively terminated S-100 backplane PCB for benchtop repair/debugging of S-100 boards. Its not really intended for building a full blown S-100 system although you could use it if you wanted.

The N8VEM S-100 backplane PCB supports 6 slots. I kept it small to fit on the benchtop and also to minimize noise issues. That being said, S-100 systems are notorious for noisy backplanes.

Thanks and have a nice day!

Andrew Lynch

December 7th, 2009, 01:08 PM
Never had any bus problems with the Cromemco systems, but the quality (and price) was a little higher than most of the other S-100 stuff, and yes, they were all exclusively populated with Cromemco cards; for some of the high-speed stuff they used a secondary bus across the opposite card edge using ribbon cable and headers.

Just thought the OP might be interested in one of your boards (if there are any left); might be just the thing for this kind of experiment.

December 7th, 2009, 04:47 PM
Ok, maybe I should limit it to one or two CPU's, and limit the Microangelo system to 16 colors. The Cromemco 8PIO is a permanent part of the system - it controls a "switch register" (wannabe front panel) installed in the front of the computer. I will probably remove the Screensplitter. So except for the 8PIO, all boards will be NorthStar or Microangelo. Did NS ever make a Z80 equivalent of their 8088 Single Slot Computer?

What about one of those backplanes like the Archive or SOL-20 used? It would fit in the back slot, but the cables would get a little crowded.