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, the Data Terminal Equipment (DTE) and data communication equipment (DCE) as assigned by the standard.

E.g., the computer (DTE) and the modem (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 communication: method to data 1 bit at a time over a communication channel (like wire), thus being "serial"
modem (modulator-demodulator): hardware bridging digital devices (computers, routers) and analog infrastructure (phone lines, cable systems, fiber optics)
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. It's also referred to as RS232C, [6] where 'C' refers to one of the three RS232 revisions, alongside RS232A, RS232B. [7] Since this adoption was so long ago, most manufacturers, like National Instruments have dropped the 'C.'

[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.

RS-323

Structure

Data terminal equipment

Data communication equipment

DB9

Operation

DB9

Voltage level sequences

Properties and specifications

Electrical range and binary states

Cables and wires

Data and slew rates

Current

Baud rate, bit rate, and bit period

RS232C pinout

RS232 control signals

Applications

References

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, like a computer. Its name refers to its role as the terminal device (endpoint) at the end of a communication link that generates/processes/receives data.

E.g., others are:

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

Definition of 'terminal' The term 'terminal' originates from the fact that back, then computers were often accessed through terminals.

In a general context, a terminal is an endpoint/boundary.

In telecommunication contexts, a terminal is a device with a keyboard and screen used to send/receive data from a computer. It was literally the terminal, or endpoint, of the communication system.

A terminal device is a device at a communication system's end where data is created, entered, displayed, or used.

A non-terminal device only forwards/converts/manages communication IO being the endpoint where users/applications interacts with the data. E.g., (often DCEs)

Modem (connects the terminal to a communication network)
Serial-to-Ethernet converter
Network switch
Router
Signal repeater

Data communication equipment

A data communication equipment (DCE) or data circuit-terminating equipment moves/carries data to another location/network. Its name refers to its role to communicate data over a transmission medium, like a modem.

E.g.,

Telephone modem
Some network adapters
Some industrial communication devices

DB9

The DB9/D9 connector is a type of 9-pin electronic D-sub connector, the most common of its 9-pin connector, which has 9 pins.

It's essential to know the role of pins of a DB cable.

[4.3]

[4.3] Pin 1 - (DCD) Data Carrier Detect indicates a carrier signal's presence.
Pin 2 - (RXD) Receive Data carries data from the receiver to the transmitter.
Pin 3 - (TXD) Transmit Data carries data from the transmitter to the receiver.
Pin 4 - (DTR) Data Terminal Ready indicates the readiness of the transmitter to communicate.
Pin 5 - (GND) Signal Ground acts as the ground reference for all signals.
Pin 6 - (DSR) Data Set Ready indicates the receiver's readiness to communicate.
Pin 7 - (RTS) Request to Send is a signal from the transmitter indicating an intent to send data.
Pin 8 - (CTS) Clear to Send is a signal from the receiver indicating readiness to receive data.
Pin 9 - (RI) Ring Indicator indicates an incoming call signal.

Operation

DB9

[4.3] For 2 devices to communicate to each other, one of them transmits data (from TXD) while the other one one receives (RXD), which in this case is only a simplification of the process. In real life, both devices are both actively sending and receiving data simultaneously, both being able of full-duplex communication..

[4.3]

DTR and DSR's roles [4.3] For some applications, the standard emphasizes to verify receiver presence by connecting of the transmitter device's DTR to the receiver DSR via a control wire to indicate the receiver's presence then a voltage is supplied across the wire, before data can flow from the receiver's DTR to the transmitter's DSR.

[4.3]

Hardware flow control [4.3] At times, the receiver can fail to match the transferring pace, causing possible data loss. This is fixed by connecting a control wire to from the transmitter's RTS to the receiver's CTS to enable coordination during this. This allows the receiver to indicate if data transfer is should pause or resume via a voltage. This method is known as hardware flow control.

[4.3]

Software flow control [4.3] Software flow control involves the sending a packet, Xon or Xoff, to regulate data flow. In this case the transmitter's RXD terminal connects to the receiver's TXD terminal to receive the packets, thus connecting both TXD and RXD to one another.

If the receiver is ready to receive additional datas, it sends an Xon packet to the transmitter. To pause data flow, it sends an Xoff packet.

[4.3]

[4.3] This configuration to connect the transmitter's TXD and RXD to the receiver's RXD and TXD respectively is the basic RS232 data transfer setup, with the both GND terminals connected to accuracy and functionality.

[6] Fig. 1

Fig. 1 illustrates the handshaking process between a computer (DTE) and a modem (DCE) prior data transfer starts.

Step 1: RTS (Request to Send): The computer initiates the process by activating the RTS signal, letting the modem know it has data ready to transmit.
Step 2: DCD & CTS (Data Carrier Detect & Clear to Send): In response, the modem activates the DCD signal to show it is connected, followed by the CTS signal to tell the computer it is officially ready to receive the data.
Step 3: TXD (Transmit Data): Once the computer sees the CTS signal is active, it begins sending the actual data over the TXD line.
Step 4: Deactivation: After all data is transferred, the computer deactivates RTS, prompting the modem to deactivate CTS, closing the transmission loop.

[4.3]

Voltage level sequences

[4.3] RS232 communication datas are transmitted through a wire as signals, which includes a sequence of voltage levels encoding the data.

The voltage levels are positive/negative relative to a 0 V reference.

RS232 transmits logic 0 via positive voltages, known as "spacing".

It sends logic 1s via negative voltages, known as marking.

[4.3] A standard RS232 frame consists of 11 bits, starting at a 0-valued start bit followed by 8 data bits. Then a parity bit is included for error detection, then ends with a one-valued stop bit to lower the voltage.

Some RS232 frames can be 9-12 bits, such as the widely used 10-bit frames.

Properties and specifications

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 5 V, RS-232 uses negative and positive voltage levels to encode bits in messages.

It represents logic 1 via negative voltages of -15 to -3 V and logic 0s via positive voltage of +3 V to +15 V. So, RS232 traditionally employs negative voltages and uses an inverted representation of the transmitted bits. The range between -3 V and +3 V is undefined — but, many modern controllers interpret this range as logic 1s.

[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 frame 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).

[TBA]

[4.2] Low Idle voltage: -3 to -12 V or logic 1
Is active low:
- low voltage = 1
- high voltage = 0
Always start bit of 0.
Always stop bit of 1.
Often 8-9 data bits
Bit order undefined

D0, D1, D2, etc refers to the binary position values. E.g., D3 bit has binary value of 3 and place value of 23=8.

Bit "a)" (bit -) is the idle bit and is at logic HIGH, without anything transmitted, and has no bit binary value. An idle bit is always at a bit of 1 or in MARK state (-3 to -15 V).
Bit "b)" (NOT bit D0) is the start bit, the first transmitted bit, one of the 2 control bits. It lets the receiver synchronize its timing to start, with a value of 0 (NOT D0) and no binary place value. A start bit is always at a bit value of 0 or in SPACE state (+3 to +15 V).
Bit "c)" (bit D0 or b0) is the LSB with a binary place value of 20=b.
Bit "d)" (bit D1) is the bit with a binary place value of 21=2, thus binary position value of 1.
Bit "e)" (bit D2) is the bit with a binary place value of 22=4.
Bit "f)" (bit D3) is the bit with a binary place value of 23=8.
Bit "g)" (bit D4) is the bit with a binary place value of 24=16.
Bit "h)" (bit D5) is the bit with a binary place value of 25=32.
Bit "j)" (bit D6) is the bit with a binary place value of 26=64.
Bit "k)" (bit D7) is the bit with a binary place value of 27=128.
Bit "l)" (bit D8) is the bit with a binary place value of 28=255.

[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.

Cables and wires

[6] The RS232C max cable length is 15.24 m (50 ft) with a 2500 pF capacitance. Limits for the impedance of wires ranges from 3 to 7 ohms.

[4.3]

RS232 cables can come in:

A straight-through cable connects one device's TXD to the other's TXD. The same for both's RXD.
- Connection logic: Uncrossed
A null-modem cable connects one device's TXD to another one's RXD and vice versa.
- Connection logic: crossed
A crossover cable connects
- Connection logic: Crossed

Data and slew rates

Data transmission rate through RS232C is up to 20 Kbps. The change rate in signal levels ie. slew rate is up to 30 V/μs.

Current

Max current rating is 3 A at the max operating voltage of 250V AC.

Baud rate, bit rate, and bit period

The baud rate, measured in symbols per second, is the speed at which data communication devices transmit signals, i.e., the speed signal changes states/symbols per second over a communication channel.

Not to be confused with the bit rate, the actual number of total raw bit transmitted by per second, measured in bits per second, calcultated as:

"bit rate" = "baud rate" x "bits per symbol"

"Bits per symbol" refers to how many bits (1s or 0s) are packed per transmitted physical signal.

A symbol represents an individual state/change in a signal.

[4.2] The bit period is how much time both processor must wait between bits, calculated as the inverse of the baud rate.

E.g., If the baud rate is 19.2 kbps, the bit period is 1/19.2k = 52.08e-6 s.

Symbol and bit difference A bit is the smallest digital data unit, while a symbol is the physical signal waveform carriing 1+ bits through a communication channel.

In standard electronics (like Arduino or modem communication), 9600 baud rate is often used, meaning the signals change 9600 times per second.

Misalignment fails If the sender transmits at 9600 baud but the receiver listens at 115200 baud, it breaks the rhythm. The receiver will sample the wire too quickly, misread the voltage states, and output unreadable, scrambled text ("garbage data")

RS232C pinout

Historically, the original RS-232 standard explicitly specified as a 25-pin connector (the DB-25).

[6] It needs 25 pins connector to connect DTE and DCE.

Breaking this down:

[6] TXD and RXD on the DTE are the serial data lines, which have opposite functions on a DCE. TXT sends outgoing data to DCE. RXD receives incoming data from DTE.
RTS & CTS: Transmitter activates the Request to Send to transmit data over the line. The line itself gets deactivated if communication stops. Receiver activates the Clear To Send to tell the transmitter whether it is ready or not to receive the data. It remains active during the transmission.

RS232 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

Page updated

Google Sites

Report abuse

RS-323

Structure

Data terminal equipment

Data communication equipment

DB9

Operation

DB9

Voltage level sequences

Properties and specifications

Electrical range and binary states

Cables and wires

Data and slew rates

Current

Baud rate, bit rate, and bit period

RS232C pinout

RS232 control signals

Applications

References

Site navigation

Main Site

Science Site

Math Site

Computer Science Site

Usual headlines

Definition structures

Headlines for references

Webcull - to store all links

Legend: