Tang Nano 20K FPGA board upgrades TRS-80 CoCo 1/2 to CoCo 3 specs with HDMI

Felipe Antoniosi has developed a modern upgrade path for vintage Radio Shack TRS-80 Color Computer (CoCo) 1 and 2 systems using a Tang Nano 20K FPGA board. The design emulates the CoCo 3's paged memory controller and enhanced graphics chip, which were originally Motorola custom silicon, through a set of daughter boards that physically replace the original motherboard chips. The upgrade expands addressable memory to 512 KB and adds HDMI output alongside analog video, plus enhanced audio capabilities. The FPGA implementation also includes a secondary mode that can emulate an Apple II, giving the modified hardware additional versatility beyond the Tandy ecosystem.

The modification requires direct chip replacement on the original motherboard, making it one of the more invasive approaches to vintage computer restoration. The Tang Nano 20K, built around a Gowin GW2AR-18 FPGA with 20K LUT4 logic elements and embedded SDRAM, provides sufficient resources to replicate the Motorola MC6847 video display generator and the CoCo 3's advanced memory management unit. The daughter board architecture suggests modularity in addressing different original chip footprints across CoCo 1 and 2 variants.

FPGA architecture and hardware implementation

• FPGA platform: Uses the Tang Nano 20K with Gowin GW2AR-18 FPGA, providing 20K LUT4 elements and embedded SDRAM for chip emulation without external memory chips.
• Emulated subsystems: Replicates the CoCo 3's paged memory management unit supporting up to 512 KB of addressable RAM and the enhanced graphics capabilities beyond the original Motorola MC6847 video generator.
• Physical interface: Daughter boards connect to the exact socket locations of removed original chips on the CoCo 1/2 motherboard, requiring desoldering or socket extraction of factory components.
• Video output: Generates both digital HDMI and analog video signals simultaneously from the FPGA, replacing the original RF or composite-only output of early CoCo models.
• Secondary emulation: FPGA bitstream can be reconfigured to emulate Apple II hardware, indicating the design uses programmable logic rather than fixed-purpose replication.

Why it matters

This project contributes to the growing corpus of open-hardware retrocomputing preservation tools that use low-cost Chinese FPGAs rather than scarce original semiconductors. For the TRS-80 community specifically, it addresses a historical gap where no factory upgrade path existed between CoCo generations, potentially reducing reliance on donor machines for replacement chips. The choice of the Tang Nano 20K, a sub-$20 development board with active community support, lowers the barrier for builders compared to prior FPGA retrocomputing projects that required more expensive Xilinx or Intel platforms.