[dicriseg]

> Unsure if our go-external plan made sense or not (does an internal hardware modem run its own UART or communicate over ISA to the board's serial port?).

Internal hardware modems had their own UART. A lot of them had DIP switches or jumpers where you'd set the IRQ and COM port. You needed to set them to a free IRQ/COM pair.

This will take you back in time: https://support.usr.com/support/5685/5685-files/spvc336.pdf

[Scoundreller]

It was probably unusual for this ISP to deal with a bargain basement computer, but a premium external modem, and the incompatibilities that can result.