This motherboard is an update to the publically available motherboard designs at http://www.s100computers.com/My%20System%20Pages/S100%20Motherboard/S100%20Motherboard.htm.
I really enjoy working on vintage S-100 systems, however they are very heavy/bulky systems and do not lend themselves well to testing small systems or performing bench diagnostics. I would really like to have a small motherboard that has all of the power requirements already taken care of. The previous motherboard design relies on an external power supply to provide the "odd" voltages (by today's standards): +8V, +16V and -16V. Generating these voltages usually means cobbling together a few Meanwell (or similar) power supplies. This version uses 3 different switched mode power regulator modules to get "close enough" to the S-100 voltages. I've gone with a 7.5V and 15V module from Pololu, and a negative module providing -15V made by TI. Each S-100 card has their own voltage regulator(s) which convert the motherboard voltages to +5V, often times ±12V. Due to this, being off 0.5-1V in our motherboard shouldn't cause any issues. In fact the slightly lower voltages should help the boards run slightly cooler. The motherboard receives base power from an external 24V laptop power supply, through a barrel jack connector.
One limitation of using these switching regulators is that current will be limited. It should be ok to power 3 or 4 standard S-100 cards. Any more than that and you'll start to push the limits. This motherboard is not intended to run large/power-hungry systems. Please be considerate of the amperage requirements of the cards you are using. I'd suggest using SIP socket strips for mounting the power supplies. That way if you need to provide external power for one of the voltages, say the +8V line, you could easily remove the 7.5V module and provide power from a bench supply.
If you do choose to populate the active termination components (which is optional, BTW) then I'd also recommend using SIP socket strips to populate the resistor packs. This way if you have a board set you want to use without termination, you can easily disable this feature by removing the resistors and the JP4 jumper.
I have a project set up through Mouser which has most of the parts required to build this motherboard: https://www.mouser.com/ProjectManager/ProjectDetail.aspx?AccessID=847fe5a657
There are a few parts not included in the Mouser project above:
- LM4250CN - this Op Amp IC is discontinued but can still be found on eBay
- 24V power supply (5.5mm x 2.1mm barrel connector). While I targeted a 24V input, any voltage between 15V-29V should work. I ordered the 7A version of this one and it's working great: https://www.ebay.com/itm/393979601717?var=662535600213
- 7.5V, 5A Step-Down Voltage Regulator D36V50F7 https://www.pololu.com/product/4093
- 15V, 2.7A Step-Down Voltage Regulator D30V30F15 https://www.pololu.com/product/4897
- S-100 slot connectors - sorry you're on your own for these - they can be expensive ($12-17 ea), so populate as few or as many as you'll actually need.
- 3 standard LEDs (I had plenty in my parts bin)
- Jumpers (will need up to 8)
- 2.54mm pitch pin headers for jumpers
- 3 heatsinks for TO-220 parts (7805 and TIP29/30)
This revision has kept all the great features of the V3 and V4 motherboards including:
- Active Termination to accomodate fasters speeds on the bus
- 9 S-100 slots
- Expansion/Logic Analyzer hookups
- Fused external power supply connector
- Mounting holes for Altair/IMSAI cases
- Reset switch
And I've done the following:
- Updated schematic/pcb to Kicad 7.0 and included all necessary libraries
- Synced up all parts between schematic & PCB to allow execution of DRC
- Increased edge connector pin hole sizes to 0.042"
- Changed fuses to resettable fuses
- Added 3 On-board power supplies
- Added Power On/Off toggle switch
- Added 3-way jumpers for 3 legacy pins to choose if you want to ground those pins, leave them unterminated, or active terminated
- Added jumper to enable/disable active termination power and indicator LED
