How not to build a computer, step 1: Planning

 

Building an S100 computer requires more decisions upfront compared to modern computers. While the classic S100 machine was a Z80 based machine, CP/M only used the i8080 subset of instructions. The S100 architecture allows for multiple CPUs to share system resources, you could choose any of the following as the system primary board (master) available from S100computers

• Zilog Z80
• Intel i8080
• Intel i8086
• Intel i80286
• Intel i80386
• Intel i80486
• Intel Edison
• Motorola 68000/68010
• Motorola 68030
• MOS Tech 6502
• Digital PDP-11

A Z80 is an excellent place to start, there are a number of board options available. Another excellent choice would be a replica MITS Intel i8080 board. One of the 16 bit CPUs would be an ok choice, I recommend waiting until you have a working system before moving on to a different CPU. The 32 bit CPUs (the '030 and the i486) require a second memory card; I highly recommend any other CPU first. I don't know enough about the PDP-11, 6502 and Edison CPUs to make a recommendation there. My choice is the Z80 FPGA SBC and an additional 4MB RAM card.

The next decision is a case for your system. S100 cards are 10" wide, 5.5" tall and are spaced every ¾ inch. A recycled 4u server case could work nicely. S100Computers has a very good article on choosing a good case. I am going with an aluminum and laser cut case, details to follow.

There are three main systems that take up space in your case:

• The mainboard, cards and card cage
• Power
• Storage

Power is also interesting in S100 computers. Prior to low cost kilowatt grade switching power supplies, state of the art was large transformers and linear regulators. To 'simplify' the power supply design, unregulated power is distributed and individual cards are required to regulate their own power. I am going with a 300W Flex ATX/Mini ITX power supply and two buck/boost converters to generate the +8V and ±16V supplies.

Finally there is storage. To begin with, I expect to use an IDE to CF converter card. I am going to create mounting points for two drive cages, a 3.5" and a 2.5" cage. I am not anticipating or planning on mounts for any 8" drives.

One last thing to keep in mind, there should be enough space for ventilation. Modern cards with modern regulators should run pretty cool. Legacy cards have a reputation of generating a lot of heat. I am going with oversized fans for cooling.

Disclaimers:

• I am not receiving any form of compensation from any of these sites. I am just enumerating the places I find useful or have discovered. 
• The opinions listed here are my own and do not necessarily reflect the official policy or position of my employer, Apple Computer.