The TRS-80 Color Computer 1 still needed a good display solution after my earlier composite experiments fell short, so I went ahead and tried the analog add-on board for the RGBtoHDMI setup. This version used the same Raspberry Pi Zero-based adapter from the earlier builds, but with the RGBtoHDMI analog board sandwiched in between to handle analog video sources. The goal was to take the CoCo’s native video signals and get a clean HDMI output on a modern display.
I had already assembled the analog board, so I focused on what was actually on it and how it fit into the stack. The main parts were a MAX529 8-bit octal DAC, a pair of quad TTL comparators, a 74LVC4066 bilateral switch, and a 3.3V regulator, all packed onto a board small enough to sit between the Pi and the RGBtoHDMI board. I did have one assembly mistake when I installed the voltage regulator backwards, but I removed it with solder wick and corrected it without any other trouble.
Here is what I found when I traced through the design. The analog signals came into the RGBtoHDMI analog board, the comparators evaluated those levels against programmable reference voltages, and the resulting digital-style signals fed the RGBtoHDMI board above it. Those reference levels mattered because different machines output different video levels, so the DAC let the system adjust for the source instead of hard-wiring the board for only one computer.
Loading the software was straightforward. I downloaded the current release files from the RGBtoHDMI GitHub page, extracted them to a FAT32 microSD card, inserted that into the Pi Zero, and powered the stack up. On first boot the CPLD was blank, so I used the on-screen menu to program it with the 6-bit firmware, and after the reboot the board came up normally and was ready for configuration.
To connect it to the CoCo, I opened the machine back up and used the pinout from the project page to wire directly to the Motorola 6847 video chip. I had already added a keyed connector, so I knew pin 1 was ground, and I soldered the wires carefully to the tops of the chip pins so I could still remove the chip later if needed. I bypassed the RF modulator entirely and powered the Pi from the CoCo’s own 5V supply.
The first power-up gave me a signal, but the colors were wrong and there was no usable profile because I had loaded the RGB firmware instead of the YUV one. I went back into the update menu, programmed the CPLD with the YUV firmware, rebooted, and then the proper Color Computer 1/2 profile showed up. After selecting that, the machine came right up over HDMI and the keyboard response felt excellent.
I also added separate audio since the adapter didn’t pass sound through HDMI at that point, but sound passthrough is now available. I found the audio pin by clipping headphones onto the board and running a simple BASIC sound loop, then wired that signal out to an RCA jack for connection to a TV or receiver. On the big screen, the difference between the original RF output and the HDMI output was obvious: the RF picture was fuzzy and dim, while the HDMI image was bright, sharp, and much easier to read. I loaded Dungeons of Daggorath and Starblaze, adjusted the sampling phase to clean up some artifacts, and both games looked great on the flat panel.
The CoCo 1 ended up running cleanly over HDMI with separate audio, and this was the best display result I’d gotten from the machine.
— Aaron
Links
Timestamps
00:00 – Intro
01:54 – Overview
10:24 – How It Works
21:20 – Loading The Software
27:42 – Connecting To The Coco
40:09 – The Big Screen
48:32 – Wrap Up