A forty-year-old TRS-80 Color Computer gets the upgrade path the official line never offered, courtesy of a Tang Nano 20K development board. Felipe Antoniosi's approach takes over the original chip locations on the CoCo 1/2 motherboard, emulating the CoCo 3's paged memory and enhanced graphics, and adds HDMI output.
The TRS-80 Color Computer came in three versions, and the CoCo 3 was clearly stronger than the first two in memory and graphics, but CoCo 1/2 users never had an official upgrade path. Felipe Antoniosi's approach turns the Tang Nano 20K into a set of daughterboards that connect to the locations where the CoCo motherboard's chips originally sat, with the FPGA emulating the CoCo 3's paged memory system and graphics chip. The official claim is that the graphics chip is compatible with the original Motorola part while also providing enhanced graphics capabilities. That means the upgrade is not a chip swap but an FPGA standing in for multiple original components on the board.
The design provides HDMI output while retaining analog video and enhanced audio capabilities. For a 1980s machine, HDMI is the practical way to connect it to a modern display, while analog output preserves the use case with the original monitor. The material gives no specific figures beyond resolution, audio specifications, or memory ceiling; the only capacity information stated is that the CoCo 3's pageable memory can expand to 512k. The real issue is the invasiveness of the modification: the daughterboards must sit at the original chip locations, meaning the motherboard has to be substantially modified. This is not a plug-in upgrade.
The material notes that if you tire of the CoCo, the board can also be used to emulate an Apple ][. This is another soft-core configuration on the same Tang Nano 20K, showing that the design's reconfigurable space is not fully consumed by CoCo 3 emulation. For people working on FPGA retro computing, this is more instructive than the CoCo itself: a single development board hosting multiple machine emulations relies on spare logic resources and the generality of the daughterboard interface. The material does not disclose the compatibility level of that emulation or the peripherals required.
For engineers in China working on high-speed interfaces and boards, the direct value here is not the CoCo itself but the daughterboard form factor: using an FPGA to stand in for multiple original components on a motherboard and tapping signals at the original chip locations is a common route to extending the life of legacy equipment, involving level matching, clock domains, and video output. The Tang Nano 20K is a development board that is readily available in China, meaning the barrier to this kind of modification is lowering. However, the design has not published schematics or pin constraint details, so whether it can be reused directly depends on what the original author releases; on the supply chain side there is no domestic substitution issue, and the toolchain remains the vendor's own.