Glenn - You are making a COMMON Mistake / Mixup, and New developments don’t always fit older equipment. Digital Modes (like FT8) Require AUDIO, from the Radio (MIC/INPUT; SPEAKER/OUTPUT), to be Routed to a Digital Signal Processor (DSP). The DSP can be within your Computer, an Add-On Card, OR an External DSP (Timewave Navigator, etc.). https://timewave.com/product/navigator-sound-card-modem/ The mid-1990s was filled with various “sound card to radio” interfaces for PSK31 usage. == The mini-DIN 6-pin connector (Rear Panel) on TM255 was originally provisioned for Packet Radio (AX.25 usage) of mid-1980s to 1990s ... that connected to a Packet Modem/TNC. Suitability for your usage, unknown. K1JT — FT8 https://physics.princeton.edu/pulsar/K1JT/wsjtx.html == The USB to Serial Bridge Cables ONLY Convert from one Serial Communication Standard (TTL/UART, 1973), to Another (USB, 1995). USAGE: Remote Computer Control of a modern Radio (that has a microprocessor inside).