The Mil8 64 is a custom memory board designed for the Mil Mod 8/80 computer system. It serves as the sole memory card, combining two 2716 EPROMs for firmware storage with a 62128 SRAM chip for read/write operations. Although the SRAM supports up to 128K addressing, only 16K or 64K is actively used. This board enables flexible booting between different monitors and interpreters, such as MOD8 MONITOR-8 and SCELBAL, preserving the spirit of early 1970s modular microcomputer experimentation. In the MicroBasement, the Mil8 64 demonstrates how modern recreations can keep these rare systems operational and accessible for hands-on exploration.
The Mil8 64 uses two 2716 EPROMs and a 62128 SRAM chip. EPROM selection is handled by two comparators. A 74LS85 combined with a 74LS02 forms a “less than or equal” comparator for the lower address range. Switches SW2 and SW3 set the number of blocks to select. When address lines A8, A9, and A10 are equal to or less than the setting on SW1, the OR’d outputs drive the enable line of the 74LS688 low. The 74LS688 then compares A11, A12, A13, A14, and A15 against the block selected on SW2 and SW3, outputting LOW on a match. In 8008 mode, A14 and A15 are pulled high to ensure they always match. The EPROMs are directly selected from their respective comparator outputs. When neither EPROM is selected, 74LS00 NAND gates enable the SRAM chip.
The Mil8 64 is populated with two 2716 EPROMs containing MOD8 MONITOR-8 and the SCELBAL boot loader. These can be independently selected using Switch 1. In 8008 mode, set position 3 on and position 7 off to boot MOD8 MONITOR-8 on reset. Set position 3 off and position 7 on to enable the SCELBAL boot loader.
Connect the Raspberry Pi Mil Loader using two ribbon cables: Mil Loader INPUT to Mil Mod8 O0 (Output Zero), and Mil Loader OUTPUT to Mil Mod8 I0 (Input Zero). Attach a terminal via an RS232 to current loop converter. A Prolific USB-to-serial adapter is recommended, as FTDI types often do not support the required 110 baud rate.
Configure the terminal (Tera Term recommended) for 110 baud, 7 data bits, no parity, 1 stop bit.
Follow these steps:
MilLoader scelbal.pkg and press Enter. The loader initializes the output port to the starting state.SCR to initialize SCELBAL’s variable space.Enter a simple test program:
100 PRINT “Hello WORLD!” 110 GOTO 100
Use LIST to verify the program, then type RUN. A continuous display of “Hello WORLD” confirms successful operation.
The Mil8 64 memory board extends the life of the Mil Mod 8/80 system, allowing it to run classic software environments like SCELBAL in an era when original hardware is increasingly scarce. Preserving and demonstrating these early machines is essential because they represent the foundational efforts of hobbyists and engineers who created the pathways for modern computing. The modular designs, simple TTL-based logic, and hands-on programming experiences of the 1970s laid critical groundwork for personal computers, embedded systems, and the open-source hardware movement that followed. By keeping these systems running and documenting their operation, the MicroBasement helps ensure that the ingenuity of those first steps remains visible and inspiring for future generations.