There is no communication protocol.
RS-232 is a serial data interface standard, initially all developed and published by the Electronic Industries Association (EIA), RS-232 was released in 1962, named EIA-232-E, as an industrial standard to ensure compatibility between products of different manufacturers.
RS-232 is one of the communication interfaces on personal computers, and is a standard interface for asynchronous transmission developed by the Electronic Industries Association (EIA).
Usually, the RS-232 interface comes in either a 9-pin (DB-9) or 25-pin (DB-25) form factor, and there are two sets of RS-232 interfaces on a typical PC, called COM1 and COM2.
Extended Information:
Disadvantages of the RS232 interface:
Because the RS-232-C interface standard appeared earlier, there are inevitably shortcomings, there are mainly the following four points:
(1), the interface signal level value is higher, easy to damage the interface circuit of the chip, but also because of the TTL level is not compatible with the level of the need to use the level conversion circuit can be connected to the TTL circuit.
(2), the transmission rate is low, in asynchronous transmission, the baud rate of 20Kbps.
(3), the interface uses a signal line and a signal return line to form a *** ground transmission form, this *** ground transmission is prone to produce *** mode interference, so the weak anti-noise interference.
(4), the transmission distance is limited, the maximum transmission distance standard value of 50 feet, in fact, can only be used in about 50 meters.
Source: Baidu Encyclopedia - RS-232 Protocol