Yeo Kheng Meng's Spin on the Graphics Gremlin Brings High-Quality HDMI Graphics to Vintage ISA PCs

Maker and vintage computing enthusiast Yeo Kheng Meng is aiming to make it easier to get vintage PCs connected to modern displays — by taking an open source ISA graphics card and respinning the design for HDMI compatibility.

"HDMI is a relatively modern video connector we take for granted on modern PCs and monitors," Meng writes, referring to the video and audio connectivity standard first introduced in 2002 by a partnership between Hitachi, Panasonic, Philips, Silicon Image, Sony, Thomson, and Toshiba. "Now vintage PCs can join in the fun too with a native connection to modern HDMI monitors without any additional adapter."

Creating a brand-new graphics adapter for modern computers is one thing, but Meng was aiming for broad compatibility with vintage systems — which meant going back to the Industry Standard Architecture (ISA) bus, introduced in 1981 and leading to the birth of the "IBM compatible" and the modern PC era as we know it today.

"Two years ago, I learned of an open source project called Graphics Gremlin (GG) by Eric Schlaepfer," Meng explains. "It is an eight-bit ISA graphics card that supports display standards like Color Graphics Adapter (CGA) and Monochrome Display Adapter (MDA). The frequencies and connectors used by CGA and MDA are no longer supported by modern monitors hence it is difficult for older PCs of the 1980s era to have modern displays connected to them without external adapters. GG addresses this problem by using techniques like scan doubling (for CGA) and increasing the vertical refresh rate (for MDA) then outputting to a relatively newer but still old VGA port."

With many modern displays having already ditched the VGA port, as they did connectivity and support for EGA, CGA, and MDA before them, Meng was after something which would make his IBM 5155 and other ISA-compatible devices more future-proof — so set about adding HDMI connectivity to the Graphics Gremlin design, including a revised PCB and modified Verilog source code for the graphics card's on-board FPGA.

"HDMI (and DVI) use Transition-Minimized Differential Signaling (TMDS) lines. The ICE40 FPGA however does not have that. Online searches garnered that some people have used workarounds to generate the differential signals however I’m not sure how compatible or spec-compliant those are," Meng writes of his redesign. "I decided to go for more reliable option by using a dedicated DVI transmitter, [the Texas Instruments] TFP410."

In addition to gaining an HDMI-compatible digital output supporting modern displays, Meng's modified Graphics Gremlin also supports an interesting twist: selectable colors for the monochrome output. "In the original GG, the MDA output is a constant amber. I didn’t particularly like amber," Meng explains. "I wanted to have a customizable color option depending on my needs. Since two of the physical switches are unused in MDA mode, I programmed the ability to change the colors depending on what switch is selected."

Meng's full write-up is available on his website, while the hardware design files and source code for the modified Graphics Gremlin With HDMI are available on GitHub under the reciprocal Creative Commons Atrribution-ShareAlike 4.0 International license.

Meng has also suggested other improvements which could be made in the future, including support for other display modes like EGA, VGA, and Hercules, RAM expansion mode, and even adding sound card capabilities.