Where did this serial protocol originate, and what is it called?
What is described in the second paragraph is not a "serial protocol", but a description of the abilities to form a transmission word of most integrated asynchrones serial units, not any specific protocol.
A protocol would include at lowest level the definition of a specific word format and work up from there.
On what transmission systems was it originally used,
Most prominent on teleprinters (tty).
While serial signals were already in use since the late 1800s, all transmission formats had to keep sender and receiver synchronized for a longer period, which became the main problem (*1). There were several proposals for automated resynchronisation using start/stop codes, but it wasn't until the early 1920s that the Morkrum-Kleinschmidt Company brought the first teleprinter operating on a word based synchronisation, that is using a fixed word size encapsulated by a start state and a stop state or in modern technical terms:
- 1 start bit
- 5 data bits
- No parity
- 1 stop bit
In 1928 Morkrum-Kleinschmidt changed their name to "Teletype Corporation" .. and the rest is well known history.
and when did it start being used on RS-232?
Right from the beginning. The whole purpose of RS-232 is to standardize serial communication connections past a simple 4 wire TTY. It defines the basic electric and signal standards for such. So it's rater natural to connect those devices which already used such formats based on this standard.
RS 232 defines a
- connector with
- signal lines and their
- signal level and
- relation between signal lines as well as
- subsets (configurations) used for certain application cases.
Due the lack of dedicated clock signals it's only for self clocking (including asynchronous) specified. It does not define any specific asynchronous word format, families of word formats or protocols using them.
There's a standard protocol we use over these connections that's asynchronous (thus not needing any of the lines that RS-232 provides for synchronous protocols),
Nonetheless it does not supersede any of the RS-232 lines. CA/CB/CC/CD are required by RS-232 to be operated, otherwise it's not an RS-232, but a non standard connection (*2).
sends marks at idle, starts a data word ("byte") with a space, sends the data bits of the word (5-8, usually) from least significant to most significant bit, optionally adds a parity bit, and ends the word with 1, 1.5 or 2 spaces.
This describes the ability of most integrated asynchronous serial units, not any specific protocol. If at all it can be called a basic asynchronous word structure.
Additionally it defines a "break" signal as a relatively long (longer than a "byte") sequences of spaces.
Which is the basic definition of an inactive/broken line.
On serial lines are not bytes but words. Also it's not a 'sequence' of spaces, but a continuous space, as any division would mean having a mark inbetween.
The whole 'continuous space' is a carry over from Teletype times where a space is equal to no current, which is what happens when the line is inoperable (for various reasons). While RS-232 woudl allow to detect this independently, applications continued to interpret this as a bad line for compatibility with teletype sections.
*1 - Early ticker systems had to stop operation after a certain time and literally send boys running around the city to reset all devices before restarting operation.
*2 - See section "Required Interchange Circuits for Standard Interface Types" of the linked document