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.

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.

My first computing experience was with an IMSAI 8080 that class assembled the year before.

It had a keyboard and video board, rather than a terminal. The monitor was open chassis to boot (ah the old days when we didn’t protect children from lethal electricity).

It had a ROM monitor and cassette tape. You had to type in (in hex) a short machine language program into the monitor to load BASIC from a cassette. We simply never turned it off.

I tried ti enter the bootstrap through the front panel once, but I made some mistake, and it didn’t work. It was an awful enough experience I never tried again.

One of the interesting aspects of the Altair was that it was based on a bus called the S-100 bus. You would have a CPU card and a memory card at least, but everything else was optional. The serial board was separate, and strictly not absolutely necessary to play with the computer, since you could enter simple programs directly from the front panel.