Ted Fried's Teensy-Powered XTMax "Software-Defined ISA Card" Is a Must-Have Add-On for Vintage PCs

Vintage computing enthusiast Ted Fried has designed an add-in board for ISA bus computers, which can replace up to three original pieces of hardware, and potentially more: the XTMax 8-bit Software-Defined ISA Card.

"XTMax is a software-defined 8-bit ISA card which uses a Teensy 4.1 microcontroller board that provides the functionality of THREE vintage ISA cards," Fried explains of his creation. "It can expand 'conventional' motherboard RAM up to 640kB, adds 4MB of Expanded RAM, and provides hard-drive access using a microSD card. A small PCB is used to allow nearly all of the ISA bus signals to attach to the Teensy 4.1."

The XTMax is designed to be compatible with as a broad a number of vintage machines as possible, requiring only a single eight-bit ISA slot. To prove its capabilities, Fried has showcased the board commented to an early revision-A IBM Personal Computer 5150 — increasing the conventional memory from 64kB on board to the maximum 640kB, placing the rest of the 4MB as Expanded Memory, and turning a microSD card into a solid-state storage device.

"It is worth noting that the first BIOS version of the IBM PC did not support extension ROMs and therefore do not support hard disks," Fried adds, "so XTMax is currently the only way to have a hard disk equivalent on these machines!"

While the above are the features of the XTMax as it exists today, there's nothing to stop anyone expanding its capabilities — with the Teensy 4.1 microcontroller at its heart offering plenty of expansion potential, hence the "software-defined" part of Fried's project. "The Teensy 4.1 has two memory footprints," Fried notes of one possible future expansion, "so 8MB or more can be added if drivers are written to support it."

The board is documented on Fried's website, with PCB design files and source code available on GitHub under an open source license.