The S-100 ATX Motherboard uses a 550W or greater ATX PC power supply (PSU) to generate common S-100 Bus voltages for S-100 computers and floppy disk drives. It can be mounted in ATX Mid Tower and larger cases that provide at least 7-1/4" (or >185mm) of space from motherboard mounting plate to the top of the case (as indicated in the Maximum CPU Cooler Height specification for such cases). More height is almost always better to accommodate top-of-card connectors or that rare, extra-tall, S-100 card.

Table of Contents:

• Quick Start Guide
  • Component Pieces
  • S-100 Card Guide Assembly
  • ATX Case and Power Connections
  • Optional LCD Panel Install
    • Prepping the LCD
    • Downloading STM32 Programmer and Current LCD Firmware
    • Programming the STM32
• Project Overview
  • STM32 Microcontroller Features
  • S-100 ATX Motherboard Features
  • S-100 Voltage Outputs and Currents
  • ATX PSU Requirements
  • S-100 ATX Motherboard Jumper and Connector Assignments
• S-100 ATX Motherboard Thermal and Current Limits
  • S-100 Bus Current and Thermal Limits
  • 8-Inch FDD Current and Thermal Limits
  • Fan Requirements under High Loads
• Installation and Wiring Requirements
  • S-100 ATX Motherboard Connector Locations
  • Connector Pinout Tables
    • J1: ATX Main Power Connector
    • J3: ATX CPU Power Connector
    • J5: ATX PCIE Power Connector
    • J13-14: 8in. Disk DC Power Connectors
    • J15: S-100 Bus Power Pins
    • J16: Voltage/Current Monitor Connector
    • J23-26: PC FAN Connectors
• S-100 ATX Motherboard V1 Schematics
• S-100 ATX Motherboard V4 3D Printer CAD Files

1. Examine the S-100 ATX Motherboard kit you received and make sure you have all of the component pieces.
2. Locate your Side Rails and mount them to the PCB using 16 #4-20 x 3/8" tapping screws (screwing in from the bottom).
3. If you are not mounting the Optional LCD, use the two Rear Cross Braces and mount one in front and the other in the rear using 8 #4-20 x 3/8" tapping screws.
4. If you are mounting the Optional LCD, mount the Front Cross Brace (badged "S-100Projects") in the front, and the Rear Cross Brace in the rear using 8 #4-20 x 3/8" tapping screws. Next, mount the Lower LCD Mount using two #4-20 x 3/8" tapping screws and then proceed to the Optional LCD Panel Install section below before mounting the motherboard in your PC case.

Connect the 24-Pin ATX Power Connector, the 4-Pin ATX CPU Power Connector, and the 6-Pin PCI-E Power Connector from the ATX Power Supply to the S-100 ATX Motherboard Connectors.

Connect up the labeled PC Connectors to the following headers on your S-100 ATX Motherboard:

The LCD Panel is not supplied with the S-100 ATX Motherboard, but it is readily available from Digikey and Adafruit. You will also need a flat flex connector (listed below).

If you would like a longer Flex Cable (say to move the LCD Display to another part of the ATX enclosure), you can search Digikey or Adafruit for a cable with the following specifications:

• Pitch: 0.5mm (0.020in)
• Positions: 18
• Termination: Top on One Side, Bottom on Other, Backers on Both Sides

Here is a 12in (304.8mm) example: Molex 0151660204

You will need to install some "Solder Jumpers" on the 2.8in LCD to enable SPI mode. You can find the guide for soldering the correct jumpers here: Adafruit SPI Mode Jumpers

You may need to create an account on st.com to download the programmer. Follow this link to download and then install the STM32 Cube Programmer: STM32CubeProgrammer.

You will also need an ST-Link V2 Programmer. which will need a modification for Rev 1.0 motherboards to support a hardware reset from the ST-Link V2.
I am supplying the modified ST-Link V2 adapters with new orders. For current owners of the S-100 ATX Motherboard, I will send out the modified ST-Link V2 Programmers upon request.

For the current S-100 ATX Motheboard Firmware with LCD support, download it here: S100_ATX_MB_CONT_V4_R-07.zip.

If you would like to revert back to the non-LCD enabled firmware, you can download it here: S100_ATX_MB_CONT_V1_R0-2.elf.

Start the STM32CubeProgrammer: (you should see an initial display like below).
Make sure the "Reset Mode" in the "ST-LINK configuration" dialog box is set to "Hardware reset" and then click on "Connect":

The programmer should now connect to the STM32 and show you a quick dump at address 0x08000000 (the contents do not matter at this stage).

Next, select the '+' tab next to "Device memory" and select "Open file" and navigate to where you downloaded the S100_ATX_MB_V4_R0-7.elf file and select it and click "Open".

Next, click "Download" and once complete, select the "Disconnect" button.

At this point, you may need to power cycle the ATX PSU to get the display to come up.

To take advantage of the low-cost and availability of PC Cases and Power Supplies, the S-100 ATX Motherboard was created and implemented as a 12.0" x 9.6" ATX standard form factor to match commonly available PC Mid-Tower and larger cases. At the same time, additional features were added to lower the system costs for a first time S-100 system builder.

An STM32 Microcontroller was added to enable control of non-vintage PC Interfaces and Power Supply Sequencing. Additional features will be added as the maturity of the project increases and different STM32 Software Modules are added and updated.

CURRENT RELEASE STM32 SOFTWARE FEATURES:

• PC Power Switch Control (allows you to turn on/off the ATX PSU and S-100 ATX Motherboard Bus and 8" Drive voltages)

Q1 2025 RELEASE STM32 SOFTWARE FEATURES:

• Target Availability of March 31st, 2025
• PWM Fan Control via current usage feedback (uses current monitors to drive fan speed)
• ARGB Color LED Output to give a visual indication of system current load (i.e. heat)
• Front Panel Touchscreen LCD System Monitor (uses Adafruit TFT LCD Display 2090) for:
  • Power Sequencing System Setup
  • Live Display of Voltages, Current. Wattage and Safe Drain-Down Voltages

Q3 2025 RELEASE STM32 SOFTWARE FEATURES:

• Target Availability of September 31st, 2025
• Extended Front Panel Touchscreen LCD System Monitor Features:
  • Keyboard and Terminal Settings
  • Power History (low/over-voltage event counters)
• USB Keyboard to PS/2 or Serial Conversion (enables use of common PC Keyboards with vintage systems)
• VGA Terminal Emulation (VT100/ANSI to start, others to follow)

The S-100 ATX Motherboard replaces several discrete power supply setups using efficient switching regulator power sections (+7.5V or +8V, +/-16V, and +24V) and a readily available ATX PSU of 550 Watts or greater (See ATX PSU Requirements below).

Current Features:

• ATX Motherboard Form Factor (12.0" x 9.6")
• 12-Slot S-100 Bus with Series Terminated Bus Signals and an Active Termination Option
• STM32 Microcontroller (see STM32 Microcontroller Features above)
• ATX Power Input Connectors (24-pin, 8-pin and 6-pin)
• Supports common S-100 Bus Voltages of +7.5V/8V and +/- 16V
• Supports common 8" Disk Drive Voltages of (+24V, +5V, and -5V)
• Three 3/4-pin 12V PC Fan Connectors (with Tach and PWM controlled by the STM32)
• Microprocessor level (0 to 3.3V) voltage and current monitors for S-100 Bus and 8-in FDD voltages
• Over-current and over-temperature shutdown protection
• Input fuses for each power supply section

* = 0.3A when -5V Regulator is supplied by -16V Switcher.
* = 0.5A when -5V Regulator is supplied by -12V from ATX PSU.
** = Limited by ATX PSU.

Since not all ATX PSU's supply a -12V voltage (used by the on-board -5V regulator for the 8-in. Floppy Drive Power Connectors), a jumper is provided to optionally supply this -5V regulator from the -16V supply that the S-100 ATX Motherboard generates. Due to dissipation reasons, the current output of the -5V regulator is reduced to 0.3A when driven from the -16V supply.

The main determination of which ATX PSUs are acceptable are driven by the 12V load of the S-100 ATX Motherboard itself, the maximum load present on its outputs (S-100 Bus and 8-inch FDD), and any external +12V accessories. If you will be using the S-100 ATX Motherboard at its maximum load limits (as shown above) then you will need at least 32 amps at 12V. However, additional 12V loads such as Fans, Hard-Drives, LEDs, et cetera, will raise this requirement. As a general rule, it is advised to add 5% or 10% margin to the current requirements to account for unanticipated loads or losses.

As shown above, the S-100 ATX Motherboard S-100 Power Supply Board can provide >350W of S-100 Bus and 8-inch FDD voltages, typically at 95%+ efficiency.

Even at those efficiency levels though, you will still have close to 18-watts of heat dissipation at full load. The board can dissipate, with natural convection (under 25C ambient conditions), up to approximately 12-watts of heat safely without the need of a fan in a typical 25C environment. (Note - most of this heat is generated by the 7.5V/8.0V and +24V regulator sections of the board). Above that limit, a 90mm or larger fan, blowing across the power supply section, is required to avoid thermal shutdowns and possible damage to the S-100 ATX Motherboard. Fans mounted on the 3D printer S-100 Card Cage Guides can usually meet this requirement, but the final installation should be checked for air flow across the power supply section using a piece of tissue to gauge the air movement.

Also, depending on the direction and flow rate of the PC Case fans, the additional fan mounts on the S-100 Card Cage Guides may not be needed, however the 90mm fans on the Card Cage act as insurance again localized heat buidup from the S-100 Cards' on-board regulators.

Due to the thermal constraints shown above, the current limits for the +7.5V and +8V versions of the S-100 ATX Motherboard are catagorized into four catagories according to these operational limits. The categories are +7.5V vs +8V and Fanless vs Fan cooling.

These are characterized limits, but convection currents, airflow patterns, and venting efficiencies within different enclosures can vary wildly, so please error on the side of caution. More airflow is generally better.

The +16V section takes advantage of a lower voltage and more efficient MOSFET than can be used in the 24V section (primarily because the higher switching voltages in the 24V section did not allow for using the lower voltage MOSTFET).

However, since the +16V section does use the more efficient MOSFET, it does not require a fan to achieve its full loading of 4-amps. A fan may still be required depending on the loading for the other output voltages.

The -16V section (at 0.5A maximum) is more limited in its output current, since this supply rail is generally just used for RS-232 I/O output and has limited current draw.

The +24V 8-Inch floppy disk drive power supply section behaves similarly to that of the +7.5v/+8V section. Above 2.7-amps of continuous loading, a fan is required. Floppy disk driver startup currents are generally not a factor in the thermal limits described here, but continuously driven head stepper motor current can be excessive. If in doubt, check the current indication on the current monitor port with one drive connected before adding additional drives to the secondary 8-Inch FDD power port.

Regardless of the individual current loadings, whenever the +7.5V/+8V or +24V output currents are 70% or higher of the total rated current limits, a fan should be used.

For the lowest voltage drop (cable loss) of the power cables to the 8in. Drives, please follow the following chart for wire sizes according to load current:

NOTE: On PVC vs THHN - THHN (Teflon Coated Wire) does not increase the current carrying capability of a wire. It does increase the temperature rating before failure. Simply put, it can dissipate more heat so a smaller wire can be used, but it comes at the cost of a larger voltage drop and more losses in the cable. However, THHN can be used at the above wire guage limits as it offers better abrasion resistance, heat tolerance, and is easier to pull through tight spaces due to its Teflon coating.

The following diagram shows the various connector locations:

For a zoomable image, please download the PDF file below and zoom into the smaller details as required.

S-100 ATX Motherboard Connector Locations PDF File

It is recommended that the S-100 ATX Motherboard be mounted in a case with standoffs of at least 0.25" to allow for airflow underneath the PCB for cooling at high loads, and to reduce the possibility of a connector short to the main chassis.

* = Supplied by ATX -12V (@ 0.5A) or -16V S-100 Supply (@ 0.3A)

• = These pins are additional ground pins in the IEEE-696 Specification to provide a lower impedance ground path for IEEE-696 compliant S-100 Cards. The ground connections for these pins are through 0-ohm 0603 surface mount resistors. If required for front panel S-100 systems, these resistors can be removed to enable the UNPROTECT (pin 20) and RUN (pin 70) signals.

The Voltage/Current Monitor signals allow for the use of a micro-controller or programmable Panel Meter to monitor and display the S-100 and 8-inch Drive power supply voltages and curents. These signals will also be used to calculate FAN RPM according to system current load.

n.b. - Pin 1 is at the bottom of the connector in the above diagram.

The following schematic is provided for reference.

S-100 ATX Motherboard V1 Schematics PDF File

The following ZIP file (S100_ATX_MB_CAD_Files_v4.zip) contains the current 3D Printer STL files for the following S-100 Card Cage Components:

S-100 ATX Motherboard V4 3D Printer CAD Files