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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Serial asynchronous adapter for COM port. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Computers Basic concepts and terms Almost every computer is equipped with at least one asynchronous serial adapter. Usually it is a separate board or located directly on the computer motherboard. It is also called an asynchronous RS-232-C adapter, or an RS-232-C port. Each asynchronous adapter usually contains several RS-232-C ports through which external devices can be connected to the computer. Each such port has several registers through which the program accesses it, and a specific IRQ line to signal the computer about a change in the state of the port. During the BIOS boot procedure, each RS-232-C port is assigned the logical name COM1 - COM4 (COM port number 1 - 4). The RS-232-C interface was developed by the Electronic Industries Association (EIA) as a standard for connecting computers and various serial peripherals. The IBM PC does not fully support the RS-232-C interface; rather, the connector marked on the computer case as a serial data port contains some of the signals included in the RS-232-C interface and have voltage levels corresponding to this standard. At present, the serial communication port is widely used. Here is a far from complete list of applications:
Basic concepts and terms Serial data transmission means that data is transmitted over a single line. In this case, the bits of the data byte are transmitted in turn using one wire. For synchronization, a group of data bits is usually preceded by a special start bit, followed by a group of bits, followed by a parity bit and one or two stop bits. Sometimes a parity bit may be missing. This is illustrated by the following figure:
It can be seen from the figure that the initial state of the serial data line is logic level 1. This line state is called marked - MARK. When data transmission begins, the line level goes to 0. This line state is called empty - SPACE. If the line is in this state for more than a certain time, it is considered that the line has switched to the BREAK state. The start bit START signals the start of data transfer. Next, the data bits are transmitted, first the lower ones, then the higher ones. If the parity bit P is used, then it is also transmitted. The parity bit is set so that the total number of XNUMXs (or XNUMXs) in the packet of bits is either even or odd, depending on the setting of the port's registers. This bit is used to detect errors that may occur during data transmission due to interference on the line. The receiving device recalculates the parity of the data and compares the result with the received parity bit. If the parity does not match, then it is considered that the data was transmitted with an error. Of course, such an algorithm does not give a XNUMX% guarantee of error detection. So, if an even number of bits has changed during data transmission, then the parity is preserved and the error will not be detected. Therefore, more complex error detection methods are used in practice. At the very end, one or two stop bits STOP are transmitted, completing the transmission of the byte. Then, before the arrival of the next start bit, the line again switches to the MARK state. The use of the parity bit, start and stop bits determine the data transmission format. Obviously, the transmitter and receiver must use the same data format, otherwise the exchange will not be possible. Another important characteristic is the data transfer rate. It must also be the same for transmitter and receiver. The data transfer rate is usually measured in baud (by the name of the French inventor of the telegraph machine Emile Baudot - E. Baudot). Bauds determine the number of bits transmitted per second. The start/stop bits as well as the parity bit are also taken into account. Sometimes another term is used - bits per second (bps). Here we mean the effective data transfer rate, excluding overhead bits. Hardware implementation Your computer may have one or two serial ports. These ports are located either on the motherboard or on a separate card that plugs into the expansion slots on the motherboard. There are also boards containing four or eight serial ports. They are often used to connect multiple computers or terminals to a single, central computer. These boards are called "ultiport". The serial data port is based on the Intel 8250 chip or its modern counterparts - Intel 16450, 16550, 16550A. This chip is a universal asynchronous transceiver (UART - Universal Asynchronous Receiver Transmitter). The microcircuit contains several internal registers accessible via I/O commands. The 8250 chip contains the transmit and receive data registers. When a byte is transmitted, it is written to the transmitter's buffer register, from where it is then rewritten to the transmitter's shift register. A byte is moved out of the shift register bit by bit. Similarly, there are receiver shift and buffer registers. The program has access only to the buffer registers, copying information to the shift registers and the shift process is performed automatically by the UART chip. The registers that control the asynchronous serial port will be described in the next chapter. The asynchronous serial port is connected to external devices through a special connector. There are two standards for RS-232-C interface connectors, they are DB25 and DB9. The first connector has 25 pins and the second has 9 pins. Here is the DB25 serial connector pinout:
Along with the 25-pin connector, a 9-pin connector is often used:
Only two pins of these connectors are used for transmitting and receiving data. The rest transmit various auxiliary and control signals. In practice, a different number of signals may be needed to connect a particular device. The RS-232-C interface defines the exchange between two types of devices: DTE (Data Terminal Equipment - terminal device) and DCE (Data Communication Equipment - communication device). In most cases, but not always, the computer is a terminal device. Modems, printers, plotters are always communication devices. Let us now consider the signals of the RS-232-C interface in more detail. RS-232-C Interface Signals Here we will consider the interaction between a computer and a modem, as well as two computers directly connected to each other. First, let's see how the computer connects to the modem. The TD and RD inputs are used differently by DTE and DCE devices. The DTE device uses the TD input to transmit data and the RD input to receive data. Conversely, a DCE device uses the TD input for receiving and the RD input for transmitting data. Therefore, to connect the terminal device and the communication device, the pins of their connectors must be connected directly:
The remaining lines when connecting a computer and a modem must also be connected as follows:
Consider the process of handshaking between a computer and a modem. At the beginning of a communication session, the computer must make sure that the modem can make a call (is in working order). Then, after calling the subscriber, the modem must inform the computer that it has made a connection to the remote system. In more detail, this happens as follows. The computer signals on the DTR line to indicate to the modem that it is ready to conduct a communication session. In response, the modem sends a signal on the DSR line. When the modem has made a connection to another remote modem, it sends a signal on the DCD line to inform the computer. If the voltage on the DTR line drops, this tells the modem that the computer can no longer continue the session, for example, because the computer's power is turned off. In this case, the modem will terminate the connection. If the voltage on the DCD line drops, this tells the computer that the modem has lost connection and can no longer continue the connection. In both cases, these signals give a response to the presence of communication between the modem and the computer. We have now looked at the lowest level of communication control, the handshake. There is a higher level that is used to control the baud rate, but it is also implemented in hardware. In practice, data rate control (flow control) is necessary if large amounts of data are being transferred at high speed. When one system attempts to transmit data at a rate faster than it can be processed by the receiving system, the result can be a loss of some of the data being transmitted. To prevent the transmission of more data than can be processed, a communication control called flow control "(flow-controll handshake) is used. The RS-232-C standard defines the possibility of flow control only for a half-duplex connection. Half-duplex is a connection in which data can only be transmitted in one direction at a time, but in fact this mechanism is also used for duplex connections, when data is transmitted along the communication line simultaneously in two directions. Flow control On half-duplex connections, the DTE device sends an RTS signal when it wishes to send data. The DCE signals the CTS line when it is ready and the DTE begins the data transfer. Until both RTS and CTS are active, only the DCE can transmit data. For full-duplex connections, the RTS/CTS signals have the opposite meaning from those they had for half-duplex connections. When the DTE is able to receive data, it signals on the RTS line. If the DCE is also ready to receive data, it returns the CTS signal. If the voltage on the RTS or CTS lines drops, this tells the transmitting system that the receiving system is not ready to receive data. Below we give an excerpt of the dialogue between the computer and the modem that occurs during data exchange.
Of course, this all sounds good. In practice, everything is not so simple. It is not difficult to connect a computer and a modem, since the RS-232-C interface is designed just for this. But if you want to link two computers together using the same cable that you used to connect the modem and the computer, then you will have problems. To connect two terminal devices - two computers - at least a cross-connection of the TR and RD lines is required:
However, in most cases this is not enough, since for DTE and DCE devices, the functions performed by the DSR, DTR, DCD, CTS and RTS lines are asymmetric. The DTE device sends the DTR signal and waits to receive the DSR and DCD signals. In turn, the DCE device sends DSR, DCD and waits for DTR. Thus, if you connect two DTE devices together with the cable you used to connect the DTE and DCE devices, they will not be able to negotiate with each other. The handshaking process will not run. Now let's move on to the RTS and CTS signals, flow control. Sometimes, to connect two DTE devices, these lines are connected together at each end of the cable. As a result, we get that the other device is always ready to receive data. Therefore, if the receiving device does not have time to receive and process data at a high transmission rate, data loss is possible. To solve all these problems, a special cable, colloquially called a null modem, is used to connect two DTE devices. Having two connectors and a cable, you can easily solder it yourself, guided by the following diagrams.
To complete the picture, let's consider one more aspect related to the mechanical connection of the RS-232-C ports. Due to the presence of two types of connectors - DB25 and DB9 - adapters from one type of connector to another are often needed. For example, you can use this adapter to connect a computer's COM port to a null modem cable if the computer has a DB25 connector and the cable terminates in DB9 connectors. We show a diagram of such an adapter in the following figure:
Note that many devices (such as terminals and modems) allow you to control the state of individual RS-232-C lines through internal switches (DIP-switches). These switches can change their value on different models of modems. Therefore, to use them, you should study the modem documentation. For example, for hayes-compatible modems, if switch 1 is in the "off" (down) position, this means that the modem will not check for a DTR signal. As a result, the modem can answer incoming calls even if the computer does not prompt the modem to establish a connection . Technical parameters of RS-232-C interface When transmitting data over long distances without the use of special equipment, due to interference induced by electromagnetic fields, errors may occur. As a result, there are restrictions on the length of the connecting cable between the DTR-DTR and DTR-DCE. The official length limit for the RS-232-C patch cable is 15,24 meters. However, in practice this distance can be much larger. It directly depends on the data transfer rate. According to McNamara (Technical Aspects of Data Communications, Digital Press, 1982) the following values are defined:
The voltage levels on the connector lines are -15..-3 volts for a logical zero, +3..+15 volts for a logical one. The interval from -3 to +3 volts corresponds to an undefined value. If you connect external devices to the RS-232-C interface connector (as well as when connecting two computers with a null modem), first turn off it and the computer, and also remove static charge (by connecting ground). Otherwise, you can damage the asynchronous adapter. Computer ground and external device ground must be connected together. Publication: cxem.net
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