Why choose RS232?
We have come a long way in communications from simple analog communications on telephone lines to typical USB cables for data exchange. RS232 is the first milestone reached in this journey. It is a standard introduced by the EIA Radio Division in 1962 for electromechanical typewriters and modems for digital data exchange. It makes data exchange on analog channels more reliable. Standard-defined voltage levels make them immune to noise and reduce errors in data exchange.
With the development of technology, many electronic devices were developed during this period, such as computers, printers, test instruments, etc. For a while, manufacturers felt the need to exchange information between these electronic devices. For example, data exchange between a computer and a printer or between two computers. But there is no standard or method for accomplishing this task. RS232 was the only available standard for data exchange at that time. Therefore, they considered adopting this standard for digital data exchange in electronic devices. But the standard fell short because it was developed specifically for modems and teletypes. t
o To overcome this problem, designers started implementing the RS232 interface to that was compatible with their devices. Computers like , HP, and can only use HP peripherals. Because of this, the market is flooded with different manufacturers who have their own standards for equipment. This leads to common problems, such as non-standard pin assignments for circuits on connectors, and incorrect or missing control signals. The lack of compliance with standards has spawned a booming industry of breakout boxes, breakout boxes, test equipment, books and other aids for connecting disparate devices. So, to put an end to all these differences in devices, the manufacturers combined to make a transmitter offering both +5V and -5V and marked them as "RS-232" compatible, and they are all identical to date. The standard has been revised several times since the original standard and has been updated by the Electronic Industries Association . The standard name also changed from RS232 to EIA232. The Electronic Industries Association has issued three modifications, the most recent being EIA232F in 1997.
What is RS232 – “Recommended Standard 232”
RS-232 is a standard communications protocol used to connect computers and their peripheral devices to allow the exchange of serial data. Simply put, RS232 defines the voltage of the path used for data exchange between devices. It specifies common voltage and signal levels, common pin line configurations, and the minimum number of control signals. As stated above, the standard was designed based on specifications for electromechanical teletype machines and modem systems, and does not define elements that are essential for data transfer between computers and printers, such as character encoding , character frames, errors Testing protocols, etc. Without it, data cannot be transferred between the computer and the printer. To overcome this problem, an integrated circuit called UART called Universal Asynchronous Receiver/Transmitter is used in conjunction with RS232.
This is how the whole arrangement works.
Figure 1: Schematic diagram of serial data exchange between PC and device using RS232 protocol
From this figure it is clear that UART, line driver and RS232 are three independent parts in the system and each part has own characteristics. UART and line drivers are the parts of RS232 used to improve system quality during serial data exchange.
EIA gives a standard definition, defining RS232 as "the interface between data terminal equipment and data communication equipment." A typical RS232 system looks like this.
Figure 2: Image explaining a typical RS232 system
DTE - DTE stands for data terminal equipment, which is a terminal instrument that converts user information into signals or re-converts received signals.It is the functional unit of the station, serves as a data source or data sink, and provides communication control functions according to the link protocol. The male connector is for DTE and has pin configuration.
DCE -A DCE stands for Data Communications Equipment. It is located between the DTE and the data transmission circuit (such as a modem). DCE devices use a female connector with holes in the surface to hold the male connector in place. Communication between
DTE and a DCE device requires at least three signals. These signals are transmission lines, receiving lines, and ground. The two devices communicate with each other via a handshake. It allows DTE and DCE equipment systems to acknowledge each other before sending data.
handshake is the process by which a DTE device sends a signal to a DCE device to establish a connection between devices before actually transmitting data. It sets the parameters of the communication channel established between two devices before normal communication over the channel begins. It follows the physical establishment of the channel and precedes normal information transmission. Handshaking makes it possible to connect relatively heterogeneous systems or devices through communication channels without requiring human intervention to set parameters. The same concept is used in RS232 to allow two devices to communicate with each other before actually exchanging information.
All these terms put together give a complete picture of the RS232 system from DTE to DCE with UART, line driver and RS232 as the connection between them.
Implementation and Specification
Implementation of the RS232 standard
Figure 3: Block diagram explaining the implementation of RS232 in a device
The RS-232 interface is used in conjunction with a UART universal asynchronous receiver/transmitter. It is an integrated circuit integrated inside the processor or controller. It takes bytes and transmits individual bits in a sequential manner within a frame. A frame is a defined structure that carries a meaningful sequence of bits or bytes of data. It has a start bit, followed by 8 data bits, a parity bit, and a stop bit. Once the data is converted into bits, a separate line driver is used to convert the UART's logic levels to RS-232 logic. Finally, the signal is transmitted along the interface cable at the specified RS-232 voltage level. Data is sent serially via RS232. Each bit is sent one after the other. t
his transmission mode requires the receiver to know when the actual data bits arrive to synchronize with the incoming data. Therefore, a logic 0 is sent as the start bit. The start bit in the frame signals to the receiver that a new character is coming. Once the receiver acknowledges the next five to eight bits are sent, representing the character. This is followed by a parity bit used for error detection. The parity bit is used to specify an even or odd number in a set of bits. For error detection, we add an extra bit to the data word. The transmitter calculates the value of the bit based on the information sent, and the receiver performs the same calculation. It checks the parity value of the calculated value. The stop bit helps the receiver identify the end of the message. The start bit always has a space value, and the stop bit always has a mark value. Now, if the receiver detects a value other than the flag when the stop bit should be present, it knows there is a synchronization error. This can cause a framing error condition in the receiving UART. The device then attempts to resynchronize on the new incoming bits. At the other end, the line driver interface again converts this to UART-compatible logic levels. At the destination, the second UART reassembles the bits into bytes. This is how RS232 makes data exchange compatible and reliable.
standard RS232 specification
RS 232 is called a complete standard . It defines not only the electrical properties, but also the functional and mechanical properties, ensuring perfect compatibility. Such as voltage levels, slew rates, signal rates, pluggable connectors, pin identification, etc. All specifications, their values and examples are summarized in this block diagram.
Figure 4: Block diagram summarizing the standard RS232 specification
This section deals with a detailed understanding of each feature in the standard.
Electrical Characteristics
It includes voltage level specifications; slew rate and withstand voltage levels. The RS232 pin output signal is represented by the voltage level relative to common. It specifies a maximum circuit voltage of 25v. On the transmitter side, the driver output specifies the voltage +3v to +15v as high level, and -3v to -15v as low level . Similarly, the receiver outputs high-level voltage from +3v to +15v and low-level voltage from -3v to -15v. It should be understood that the receiver logic provides +2v noise margin. The dead band between +3v and -3v is designed to absorb line noise. In the RS232 specification, low levels -3v to -15v are defined as logic '1' as the ON state and are called "flags", while high levels +3v to +15v are defined as logic '0' as the off state and are called "flags". Called "interval". The RS232 standard also limits the maximum slew rate, thereby reducing crosstalk between the two signals. Slew rate is defined as the rate of change of output voltage with respect to time. The maximum allowed slew rate for RS232 is 30v/microsecond, which slows down rise and fall times and reduces crosstalk. The circuitry driving an RS-232 compatible interface must be able to withstand an indefinite short to ground or any voltage level up to 25 volts. Some computer equipment ignore negative levels and accept a zero voltage level as the off state. The output signal level typically swings between +12V and -12V.
RS232 Logic Specifications Diagram
Figure 5: Diagram showing RS232 electrical characteristics
Mechanical Characteristics
This area is related to the mechanical interface.
originally used a standard 25-pin connector. It specifies the minimum connector size that can accommodate all signals. Each pin is predefined to allow compatibility between host and peripheral systems. Data terminal equipment uses male connectors and data communications equipment uses female connector pins. Another important concept related to connectors is gender type. In the electrical and mechanical industries, every connector comes in a pair. One is a male connector and the other is a female connector. The male connector has pins that protrude from the surface, while the female connector has holes to secure the male connector. In RS232, DTE has a male D-25 connector, while DCE has a female D-25 connector. The combination of D-SUB 25 MALE and D-SUB 25 FEMALE connectors is used to connect DTE and DCE by specifying common voltage and signal levels, common pin configuration, and minimum control signal volume.
Figure 6: Image of D-SUB 25 MALE connector in RS232 based system
25 bit connector was widely used but now it is 9 pin connector used in many applications. In most cases, this is sufficient because many of the lines available on the RS232 25-pin connector are rarely used. The 9-way connector is able to provide all the required connections for most applications and allows the application to transmit and receive the necessary signals as required.
A 25-pin and a 9-pin connector are shown below.
Figure 7: Pin numbers in 25-pin and 9-pin connectors
Specifications continued
Functional Features
This is the third area related to the RS232 specification. It defines the functionality of the different signals used in the interface. These signals are defined into four categories: data, general, control, and timing. Few terms are used in the table, such as loopback, off-hook or on-hook, and auxiliary channel.
Loop Back - is a method for line transmission testing in the switching center. Loopback allows users to test their own network to ensure it is functioning properly.
Pick Machine – A situation that occurs when a telephone or other user instrument is used during dialing or communication. It was originally used on phones that had a separate headset (receiver) that hung on a switch hook until the user wanted to use it.
Auxiliary channels - These are data channels with the same functionality as the first channel. Examples include Supplementary Transmit Data (STD), Supplementary Receive Data (SRD), Supplementary Request to Send (SRTS), Supplementary Clear to Send (SCTS) and Supplementary Carrier Detect (SDCD).
The following table shows the different signals and their functions
Figure 8: Table summarizing the RS232 pin descriptions
As we can see, the standard provides a rich set of control signals. Only a few applications require all defined signals, otherwise very few are used, for example a typical modem only uses eight signals, some may only require four, two for data and two for handshaking, while others may only Use data signals instead of handshakes.
handshake and conclusion
handshake
The handshake in RS232 is commonly known as the "RTS/CTS" handshake. The data terminal equipment asserts the RTS pin to indicate a desire to transmit to the DCE, which then responds via the CTS pin to grant permission. Thereafter modems disable their transmitters when not needed and must send a synchronization signal to the receiver when they are re-enabled again. In the latest version of the RS232 E standard, the handshake has been redefined in which CTS (clear to send) is no longer a pair RTS response, but it represents DCE's permission for the DTE device. In a similar manner, RTS instructs the DTE to allow the DCE to send data. RTS and CTS are controlled by DTE and DCE and are independent of each other. A detailed handshake system with 8 signal lines is explained.
Figure 9: Block diagram showing RS232 handshake system details
When Data Carrier Detect is turned off, it indicates to the local terminal that the remote DTE has not opened its RTS and the local terminal can control the line. When this circuit is turned on locally, it indicates to the local terminal that the remote modem has received an RTS ON condition from its terminal and that the remote DTE controls the carrier line. RXD stands for receiving data from modem to DTE. TXD transmits data from the DTE to the modem. DTR The data terminal ready pin is usually turned on when the terminal is ready to establish a communication channel through its modem. But when the DTR does not want to accept calls from the remote terminal, the circuit is closed. When a communication path is established between two sites, both modems open their data set ready circuits. Now, when the terminal is ready to send, it opens the request-to-send circuit, indicating to the local modem that it is ready to send data. This request is passed to the remote modem. RTS controls the direction of data transfer. Once the terminal is ready to transmit the local modem, it opens the CTS circuit to indicate that it is ready to receive data from the DTE. It can also control telephone lines. Next, when the modem receives a call, the ring indicator turns on/off, notifying the DTE that a call is coming and indicating that the remote modem is requesting dialing. This is a simple handshake system with 8 signal lines. Once the terminal is ready to transmit the local modem, it opens the CTS circuit to indicate that it is ready to receive data from the DTE. It can also control telephone lines. Next, when the modem receives a call, the ring indicator turns on/off, notifying the DTE that a call is coming and indicating that the remote modem is requesting dialing. This is a simple handshake system with 8 signal lines. Once the terminal is ready to transmit the local modem, it opens the CTS circuit to indicate that it is ready to receive data from the DTE. It can also control telephone lines. Next, when the modem receives a call, the ring indicator turns on/off, notifying DTE that a call is coming and indicating that the remote modem is requesting dialing. This is a simple handshake system with 8 signal lines.
Conclusion
RS232 became a standard feature of personal computers connected to modems, printers, mice, data storage and other peripherals. Despite many new developments, RS232 still finds its applications. The first and most important reason is the simplicity of the standard. It allows users to communicate directly with serial port . There is a continuing need for RS232 in laboratories, automation, measurement, etc. due to the use of very expensive but aging equipment. Using RS232 is much cheaper than replacing equipment.Not only old equipment, but also modern automation equipment such as servo drives , CNC equipment, etc. can be programmed via RS232. Toshiba also reintroduced the DE-9M connector on laptops . A serial port with RS-232 is used to communicate with headless systems, such as servers that do not have a keyboard installed during boot. Some embedded systems use RS232 serial port as an alternative network mode monitor to communicate.
