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RS-232 specifies two connectors, a 25-pin (DB-25) and an alternate 26-pin connector (pin 26 is "No Connection"). RS-232 provides a list of circuits and says if you use a circuit; this is what it must do and it must be on this pin of the connector. However, when you take into account full list of features and application benefits it can provide, there is no competition than can rival its flexibility and power in this cost bracket. If the outside serial device is RS485 standard, you can attach this unit onto M5stack, therefore, to implement the communication with RS485 device by TTL protocol. Again many thought that the UART protocol used in the serial port were part of the RS-485 standard, but the RS-485 standard contains even less of the OSI model layer one than RS-232 does. However this contradicts the facts that an idle mark state is a logical one and the termination polarization puts B at a higher voltage in Profibus guidelines. The diagram below shows potentials of the A (blue) and B (red) pins of an RS-485 line during transmission of one byte (0xD3, least significant bit first) of data using an asynchronous start-stop method.

Bit and framing protocols are in layer two of the OSI model. Layer two of the OSI model includes the bit and framing protocol, which is outside of these EIA/TIA standards. This means that it is necessary to wait until the signal has stabilized before the bit can be sampled. The physical layer may include a connector, wires, and electrical levels, but does not include the bit and framing protocol of the UART. With Modbus, BACnet and Profibus, A/B labeling refers A as the negative green wire and B as the positive red wire, in the definition of the D-sub connector and M12 circular connector, as can be seen in Profibus guides. The RS-232 standard defines a DB-25 and a 26-pin alternate connector, the function of each of the pins in the connector (called circuits), and the electrical characteristics of the signals that are on the "circuits". It defines a means of exchanging control signals and binary serialized data signals between data terminal equipment and a data communication channel in cases where interchange point. This defines all 25 pins of the connector. This standard is for up to 21 circuits on a 25 pin DB-25 connector.

Laser PWM (K) - This is a three pin Molex Tripeg connector with ground, 1 input and 1 output. Circuit AB is on pin 7 and is described as "Signal Common". Another figure in the annex (again, the annexes are not considered to be part of the standards) discusses connecting the "Green Wire Ground of Power System" or "Protective Ground or Frame Ground" to "Circuit Common or Circuit Ground" and "SC Signal Common". These annexes are discussed in this document but it should be clear that they are not a part of the standard even though they are included in the standard. The RS-485 standard is obsolete and has been superseded by TIA-485, but many if not most engineers and applications guides continue to use the RS designation even though it has officially changed. And, even if the diff-mode impedance is given, the common-mode impedance is never mentioned (on high-performance STP cables, you can hazard a guess - see below). So I measured the cable's impedace figures: 70 Ω diff-mode and about 17 Ω common-mode. This connection may be used to limit the common-mode signal that can be impressed on the receiver inputs. The receiver is designed to operate with voltages between 3 and 15V in magnitude (i.e. both positive and negative voltages) but must be able to handle an input of 25V without damage.

The balanced receiver sensitivity treshold is high enough that in many scenarios, ingress EMI is not an issue (some point out that RS485 is more immune than CAN). Grounds between buildings may vary by a small voltage, but with very low impedance and hence the possibility of catastrophic currents - enough to melt signal cables, PCB traces, and transceiver devices. It does not mean that this protocol is part of the standards or that only this protocol may be used with the 232/422/485 standards. Just because a protocol is used with a particular implementation of RS-232 or RS-485, does not mean it is part of the standard. When configuring an IBM PC’s serial port, the parity, number of data bits, and number of stop bits must be set; therefore, many think this must be part of the RS-232 standard. When the driver is transmitting a 1 (from the UART for example), then the voltage on the wire must be less than -3V. The ratio of resistance will match the ratio of voltage. Again the amount of loss depends of the values of the series and parallel resistance and the loss will increase with frequency.