We had an Altair at home, that my father assembled from a kit. That version did not have a serial port. It had only the switches on the front panel. I did succeed in inputting some programs and in having them run. But of course it was appallingly limited.
There were two very clever I/O ideas that emerged for the Altair: (1) A radio on top of the housing would pick up a signal, allowing audio output! (2) A cassette tape recorder could be used as an external storage device, though I forget how it interfaced... the serial board, I think.
The serial port was its own separate board. https://www.youtube.com/watch?v=RaYTd3dbXeM
There were two very clever I/O ideas that emerged for the Altair: (1) A radio on top of the housing would pick up a signal, allowing audio output! (2) A cassette tape recorder could be used as an external storage device, though I forget how it interfaced... the serial board, I think.