A brief discussion of RS232 voltage levels and signals
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A brief discussion of RS232 voltage levels and signals

Posted Date: 2024-01-15

The RS232 standard includes defined line levels and operating modes for handshaking.

In this way, correct operation of any RS232 system can be ensured. If the voltage falls within a defined level range, the receiver is able to correctly detect the data being transmitted or the status of other lines.

If the line exceeds the required limits, there may be uncertainty and data errors.

RS 232 voltage level

In order to enable RS 232 transmitters and receivers to be designed according to common standards, it is necessary to define the voltage levels that constitute the two logical states required for data transmission. The definitions of these two states are shown in the table below.

It is also necessary to define the voltage states of the control signals, since these states are widely used in RS 232.

RS232 serial data transmission

Data is sent serially on RS232, each bit being sent one after the other as there is only one data line in each direction. This data transfer mode also requires the receiver to know when the actual data bits arrive so that it can synchronize with the incoming data. To achieve this, a logic 0 is sent as the synchronized start bit. Next is the data itself, usually seven or eight bits. The receiver obviously has to know how many data bits to expect, and there's usually a small dip switch on the back or inside the device that sets this information.

Data on RS232 is usually sent using ASCII (American Standard Code for Information Interchange). However, other codes, including Murray codes or EBCDIC (Extended Binary Coded Decimal Interchange Code) can be used equally well.

After the data itself, a parity bit is sent. Again, this needs to be set, and since it's optional, it can be even or odd parity. This is used to check the correctness of the received data, it can indicate whether the data has an odd or even number of logics. Unlike many systems today, it has no error correction capabilities.

Finally the stop bit is sent. This is usually one bit long and is used to indicate the end of a specific byte. Sometimes two stop bits are required, and this is usually an option that can be set on the device.

RS232 data transmission is usually asynchronous. However, the transmission and reception speeds obviously have to be the same. Allow a certain amount of tolerance. After the start bit is sent, the receiver samples the center of each bit to see the level. Within each data word, synchronization must not differ by more than half a bit length, otherwise you will see incorrect data. Fortunately, this is easy to achieve with today's accurate bit or baud rate generators.

RS232 handshake

In order to exchange data on an RS232 link, control signals must indicate that the devices on both ends of the link are ready to send data and ready to receive data. This can be accomplished in a number of ways, but one of the more common is using RTS, CTS, and DTR lines.

These lines are located in data terminal equipment, DTE, and data communications equipment DCE, as follows:

The handshake exchange that initiates the data flow is very simple and can be thought of as a number of different stages:

RTS is turned ON by DTE

The DCE then turns the CTS line ON

The DTE then responds by turning the DTR line ON.

The DTR line remains open while data is being transmitted.

RS232 handshake signal

At the end of the transfer, DTR and RTS are pulled to the OFF state, and then DCE pulls the CTS line to the OFF state. This series of handshake controls is designed to allow the DTE to request control of the communications link from the associated modem and then have the modem notify the end device that control has been obtained. This way, communication will only occur when both ends of the link are ready.

RS-232 voltage levels are important because they enable system design to ensure data errors are minimized. Ensure data errors are minimized by assigning levels to mark and space conditions, and setting a band between the two. In addition to this, using lines including RST, CTS, and DTR for handshaking operations, the system can operate reliably and only send data when all devices are ready.

Review Editor: Huang Fei


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