RS-323

[2] RS-232/RS232 (Recommended Standard-232) is a standard in serial communication introduced in 1960 by the Electronic Industries Alliance (EIA), one of the oldest and most influential serial data communication standards between devices. E.g., the computer (called the Data Terminal Equipment (DTE)) and the modem (Data Communication Equipment (DCE)). It defines the protocol and physical interface for data exchange between devices like computers and peripherals.

[4] It's the standard to connect AV devices to control system. The connection in audio visual equipment is often a 9 pin D subconnector.

serial commnication: method to data 1 bit at a time over a communication channel (like wire), thus being "serial"
audiovisual (AV) equipment/system: integral gear enhancing sound and visuals. The term encompasses devices designed to integrate and deliver high-quality audio and visual content seamlessly.
9-pin D-sub connector/DB9: electrical connector type with 9 metal pins arranged in two rows in a D-shaped metal shell, a shape ensuring it's plugged in 1 way.

RS232 standard defines things like:

Electrical voltage levels used to represent 0s and 1s.
Connector pins' meanings (TX, RX, GND, RTS, CTS, etc.).
Signaling used in serial communication.

Though some call it RS232, the standard's newer revisions are called EIA-232 or TIA/EIA-232 after updates by industry organizations.

[3 In the AV (audiovisual) industry, RS-232 doesn't transmit audio and video signals, but specifically controls devices in AV systems; so source devices like BluRay players and digital media players, display devices like televisions and projectors and signal control products like signal splitters and matrices.

Structure

As the RS-232 standard is written, it assumes/assign 2 different roles:

Data terminal equipment (DTE): The device creating/using data.
Data communication equipment (DCE): The device helping to transmit data between DTEs

[4.1] It assigns the 2 roles as 2 DTEs/DCEs devices can't contact each other independetly, so both a DCE and DTE rely on each other, via a reverse (null modem) cable to contact the devices.

E.g.,

Data terminal equipment

A data terminal equipment (DTE) is often the device one uses, e.g., a computer.

E.g.:

Computers
Terminals (a keyboard and screen connected to a larger computer)
Microcontrollers
PLCs

Data communication equipment

A data communication equipment (DCE) or data circuit-terminating equipment moves/carries data to another location/network.

E.g.,:

Telephone modem
Some network adapters
Some industrial communication devices

Properties

Electrical range and binary states [5] Unlike other protocols, like I2VC or SPI, which encode digital zero via 0 V and represent the logic high state via a positive voltage level, often 3 or 5V. But RS-232 uses negative and positive voltage levels to encode bits in messages. It represents logic 1 via negative voltages between -15 V and -3 V and encodes 0s via positive voltage between +3 V and +15 V. Thus, RS-232 traditionally employs negative voltages and uses an inverted representation of the transmitted bits. Traditionally, the range between -3 V and +3 V isn't defined — but, many modern controllers interpret this range as a logic 1.

[5] While such voltage choices presents challenges, especially in modern microcontroller-based projects, the differing voltages representing the 2 binary states provide great noise immunity even if transmitting signals in big distances. But they also imply projects must somehow generate negative voltages to adhere to the RS232 standard, which may increase a build’s complexity.

[5] Message frames RS232 defines that asynchronous serial communication is used to exchange data between devices, meaning info is transmitted a bit at a time without a dedicated clock signal synchronizing exchange between communication partners. Instead, a message has start and stop bits separating individual frames (indivisible packets of bits).

[5] Other than that, RS-232 doesn’t precisely define message frames' composition. This is often done via a start bit, followed by the data bits, an optional parity bit for error checking, and 1+ stop bits.

The sending device shifts bits over the TX line into the receiver’s RX input per transmission. Data can only travel from the sender's TX output to the recipient's RX input, meaning the protocol needs 2 wires if bi-directional communication is needed.

RS-232 control signals

[5] We often only use the TX, RX, and GND signals in a DB-9 connector, a standard connector often present on devices using RS-232 communication. Using such 3 signals suffices often. But, the standard depicts more control lines once widely used in computer-to-modem communication.

Data Terminal Ready (DTR), Data Set Ready (DSR), Request to Send (RTS), and Clear to Send (CTS) are 4 control signals still relevant in more modern applications. Via the DTR signal, the DCE (e.g., a computer) can indicate to the DCE or receiver, (the modem) e.g., that the recipient must be ready to receive data. Conversely, the DSR informs the computer that the modem is ready. The RTS and CTS lines indicate to the computer and modem, respectively, that they should prepare to receive data frames.

Applications

[4.1] PLCs used RS232 to contact other PLCs or modules also using RS232 (e.g., HMIs, motor controller, robots).

Between a computer and older printers, RS232 is still used via a cable to allow the computer to send instructions to the printer via voltage signals then the printer decipher the commands to print, thought USB became the standard in modern versions.

[4.1] Downsides

Slow data transmission: RS232 is slow at transfering data, at 20 kps, unlike modern data transmission.
Limited length: The usual cable length is 50 ft, as wire resistance and voltage drop are an issue in cables any longer, so it isn't used as much in modern techs.

Thought adapters can be used to assist in device communications via new and old standards.

Upsides TBA

References

[1] Wikipedia
[2] Lenovo
[3] CIE Design Supply Support
[4] What is RS-232? - TheIntegrationExchange (YouTube)
- [4.1] What is RS232 and What is it Used for? - RealPars (YouTube)
[5] Digikey

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RS-323

Structure

Data terminal equipment

Data communication equipment

Properties

RS-232 control signals

Applications

References

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