Troubleshooting port errors. COM port. Wiring, pinout and circuit

Serial port (serial port, serial port or COM port-, communications port) is a dual-directional serial interface.

Why is the port called serial? Because all information on this port is transmitted in steps equal to one bit. It transfers data bit by bit, unlike a parallel port.

Despite the fact that some other interfaces, such as Ethernet, FireWire and USB, use serial data exchange, the name “serial port” is assigned to a port that has a standard RS-232C.

This port, in comparison with other "serial" technologies, has a distinctive feature: it does not have any timing requirement between 2 bytes. Timing requirements exist only between the bits of a single byte. The reciprocal of the time pause between the bits of one byte is called the “baud rate” (baud rate). In addition, in this technology there is no such thing as a “package”. Other "serial" data transfer technologies (X.25, USB or Ethernet) use "packets" and also have strict timing requirements between bits of one packet.

Regarding communication protocols with industrial equipment, there are strict timing requirements between serial port bytes. Implementation of these protocols in multitasking operating systems with weak real-time support is very difficult. These systems include Windows. That is why MS-DOS or older software is often used to work with these protocols.

The most common standard for a serial port is RS-232C. Previously, the serial port was used to connect a terminal, later it was used to connect a modem or mouse. Currently, it is used as a means of connecting to uninterruptible power supplies, as well as as a means of communication with embedded computing system development hardware, satellite receivers, cash registers and security system devices.

Thanks to the COM port, it becomes possible to connect two PCs to each other using a so-called “null modem cable”. This method has been used since the days of MS-DOS to transfer files from one computer to another. On UNIX systems it was used for terminal access to another machine, and on Windows operating systems it was used as a kernel-level debugger.

The serial port, quite popular at one time in IBM-compatible PCs, is now obsolete. However, it should be noted that it is still often used in industrial and highly specialized equipment, as well as on some modern computers. The serial port is actively being replaced by the USB and FireWire interface.

However, there are special standards for serial port emulation over USB and over Bluetooth. By the way, it’s interesting, but it was Bluetooth technology that the developers designed as a wireless version of the serial port. Software port emulation is still widely used today. Thus, almost all mobile phones today emulate a COM port and a modem within them in order to implement tethering (computer access to the Internet via GPRS/EGDE/3G). But directly for the physical connection to the computer, USB, Bluetooth or Wi-Fi technologies are used.

In addition, software serial port emulation is possible for guest users of VMWare and Microsoft Hyper-V virtual machines. The main purpose of this procedure is to connect the Windows kernel level debugger to the guest client.

Advantages of a COM port

The main advantage of this technology is its ease of connection.

Disadvantages of COM port

The main disadvantages of this port are its low speed, large connector sizes, and high requirements for operating system response time. Also, this standard has a high number of interrupts (one interrupt every 8 bytes).

Connectors

The most common standard connectors are 9- and 25-pin (DB-9 and DB-25, respectively), which were standardized in 1969. These are D-shaped connectors. In addition to them, others were used, but from the same family: DB-31 and round eight-pin DIN-8.

The maximum transmission speed (in the standard version) reaches 115,200 baud.

Equipment

The connector has the following contacts:

• DTR(Data Terminal Ready) - output to PC, input - to modem. Responsible for the readiness of the computer to work with the modem. The reset causes an almost complete reboot of the modem. In the case of a mouse, this wire is used to provide power.
• DSR(Data Set Ready) - input to PC, output - to modem. Responsible for modem readiness. If the line is zero, then on some operating systems it is impossible to open the port as a file.
• RxD(Receive Data) - input to PC, output - to modem. Indicates the flow of data entering the PC.
• TxD(Transmit Data) - output to PC, input - to modem. Indicates the flow of data coming from the PC.
• CTS(Clear to Send) - input to PC, output - to modem. The computer must pause the data transfer process until this wire is set to one. Used in the hardware flow control protocol to prevent overflow on the modem.
• RTS(Request to Send) - output to PC, input - to modem. The modem must pause the data transfer process until the wire is set to one. Used in hardware flow control protocol to prevent hardware/driver overflow.
• DCD(Carrier Detect) - input to PC, output - to modem. After establishing a connection with the modem on the other side, it is raised to one, reset to zero if the connection is lost. PC hardware can generate an interrupt if such an event occurs.
• R.I.(Ring Indicator) - input to PC, output - to modem. After determining the ringing signal of a telephone call, it is raised by the modem to one. The PC hardware can generate an interrupt if such an event occurs.
• S.G.(Signal Ground) - the common signal wire of the port. Important:land is not common. Typically the wire is insulated from the PC or modem case.

A null modem cable uses two crossed pairs: TXD/RXD and RTS/CTS.

UART 16550- standard port equipment. Today it is included in the SuperIO chip on the motherboard. Since the days of the IBM PC, it has been equipped with a hardware byte queue. It significantly reduces the number of interruptions that occur.

Along with the parallel port, the COM port, or serial port, is one of the traditional computer input/output ports, used in the first PCs. Although the COM port has limited use in modern computers, nevertheless, information about it may be useful to many users.

The serial port, like the parallel port, appeared long before the advent of personal computers of the IBM PC architecture. In the first personal computers, the COM port was used to connect peripheral devices. However, the scope of its application was somewhat different from the scope of the parallel port. If the parallel port was used mainly to connect printers, then the COM port (by the way, the prefix COM is just an abbreviation for the word communication) was usually used to work with telecommunications devices, such as modems. However, you can connect to the port, for example, a mouse, as well as other peripheral devices.

COM port, main areas of application:

1. Connecting terminals
2. ~ external modems
3. ~ printers and plotters
4. ~ mice
5. Direct connection between two computers

Currently, the scope of the COM port has been significantly reduced due to the introduction of a faster and more compact, and, by the way, also serial, USB interface. External modems designed to connect to a port, as well as “COM” mice, have almost gone out of use. And it’s rare that anyone now connects two computers using a null modem cable.

However, a number of specialized devices still use the serial port. You can find it on many motherboards. The fact is that, compared to USB, a COM port has one important advantage - according to the RS-232 serial data transmission standard, it can work with devices at a distance of several tens of meters, while the range of a USB cable is usually limited to 5 meters.

The principle of operation of a serial port and its difference from a parallel one

Unlike a parallel (LPT) port, a serial port transmits data bit by bit on a single line, rather than on several lines at the same time. Sequences of bits are grouped into series of data, starting with a start bit and ending with a stop bit, as well as parity bits used for error checking. This is where another English name comes from, which has a serial port - Serial Port.

The serial port has two lines through which the data itself is transmitted - these are lines for transferring data from the terminal (PC) to the communication device and back. In addition, there are several more control lines. The Serial port is served by a special UART chip, which is capable of supporting a relatively high data transfer rate, reaching 115,000 baud (bytes/s). However, it is worth noting that the actual speed of information exchange depends on both communication devices. In addition, the functions of the UART controller include converting parallel code into serial code and vice versa.

The port uses comparatively high voltage electrical signals - up to +15 V and -15 V. The logical zero level of the serial port is +12 V, and the logical one level is -12 V. Such a large voltage drop allows us to guarantee a high degree of noise immunity of the transmitted data. On the other hand, the high voltages used in Serial port require complex circuit solutions. This circumstance also contributed to the decline in the popularity of the port.

Serial interface RS-232

Serial port operation on a PC is based on the data transfer standard for serial devices RS-232. This standard describes the process of data exchange between a telecommunications device, such as a modem, and a computer terminal. The RS-232 standard defines the electrical characteristics of signals, their purpose, duration, as well as the sizes of connectors and pinouts for them. However, RS-232 describes only the physical level of the data transfer process and does not concern the transport protocols used, which may vary depending on the communication equipment and software used.

The RS-232 standard was created in 1969, and its latest version, TIA 232, was released in 1997. RS-232 is now considered obsolete, but most operating systems still support it.

In modern computers, the Serial port connector is a 9-pin DB-9 male connector, although the RS-232 standard also describes a 25-pin DB-25 connector, which was often used on older computers. The DB-9 connector is usually located on the PC motherboard, although in older computers it may have been located on a special multicard inserted into an expansion slot.

9-pin DB-9 socket on motherboard

DB-9 connector on the cable of the device connected to the port

Unlike a parallel port, the connectors on both sides of a two-way serial cable are identical. In addition to the lines for transmitting the data itself, the port contains several service lines through which control information can be transmitted between the terminal (computer) and the telecommunications device (modem). Although theoretically only three channels are needed for a serial port to operate - data reception, data transmission and ground, practice has shown that the presence of service lines makes communication more efficient, reliable and, as a result, faster.

Purpose of the Serial port DB-9 connector lines according to RS-232 and their correspondence to the contacts of the DB-25 connector:

Configuration and interrupts

Since a computer can have several serial ports (up to 4), the system allocates two hardware interrupts for them - IRQ 3 (COM 2 and 4) and IRQ 4 (COM 1 and 3) and several BIOS interrupts. Many communication programs, as well as built-in modems, use interrupts and the address space of COM ports for their work. In this case, not real ports are usually used, but so-called virtual ports, which are emulated by the operating system itself.

As with many other motherboard components, the COM port parameters, in particular the BIOS interrupt values ​​corresponding to hardware interrupts, can be configured through the BIOS Setup interface. For this, BIOS options such as COM Port, Onboard Serial Port, Serial Port Address, etc. are used.

Conclusion

The PC serial port is not currently a widely used means of input/output. However, since there is a large amount of equipment, primarily for telecommunications purposes, designed to work with a serial port, and also due to some of the advantages of the RS-232 serial data protocol, the serial interface should not yet be written off as a completely outdated rudiment personal computer architecture.

A modern personal computer would never have gained such enormous popularity if it only performed computing functions. The current PC is a multifunctional device, with the help of which the user can not only carry out any calculations, but also perform a lot of different things: print text, control external devices, communicate with other users using computer networks, etc. All this enormous functionality is achieved with the help of additional devices - peripherals, which are connected to a personal computer through special connectors called ports.

Personal computer ports

Port- an electronic device running directly on the PC motherboard or on additional boards installed in a personal computer. The ports have a unique connector for connecting external devices – peripherals. They are intended for data exchange between a PC and external devices (printers, modems, digital cameras, etc.). Quite often, in the literature you can find another name for ports - interfaces.

All ports can be divided into two groups:

• External- for connecting external devices (printers, scanners, plotters, video devices, modems, etc.);
• Domestic- for connecting internal devices (hard drives, expansion cards).

External ports of a personal computer

1. PS/2- port for connecting a keyboard;
2. PS/2- port for connecting a mouse;
3. Ethernet- port for connecting a local network and network devices (routers, modems, etc.);
4. USB- port for connecting external peripheral devices (printers, scanners, smartphones, etc.);
5. LPT- parallel port. Serves to connect now outdated models of printers, scanners and plotters;
6. COM- RS232 serial port. Used to connect devices such as dial-up modems and old printers. Now outdated, practically not used;
7. MIDI- a port for connecting game consoles, midi keyboards, musical instruments with the same interface. Recently, it has been practically replaced by the USB port;
8. Audio In- analog input for linear output of audio devices (tape recorders, players, etc.);
9. Audio Out- analog audio signal output (headphones, speakers, etc.);
10. Microphone- microphone output for connecting a microphone;
11. SVGA- a port for connecting video display devices: monitors, modern LED, LCD and plasma panels (this type of connector is obsolete);
12. VID Out- the port is used to output and input low-frequency video signals;
13. DVI- a port for connecting video display devices, more modern than SVGA.

Serial port (COM port)

One of the oldest ports installed in PCs for more than 20 years. You can find it quite often in literature classic name – RS232. Data exchange using it occurs in serial mode, that is, the transmission and reception lines are one-bit. Thus, information that is transmitted from a computer to a device or vice versa is divided into bits that follow each other sequentially.

The data transfer speed provided by this port is not high, and has a standardized range: 50, 100, 150, 300, 1200, 2400, 4800, 9600, 14400, 38400, 57600, 115200 Kbps.

A serial port was used to connect such “slow” devices to a PC as the first printers and plotters, dial-up modems, mice, and even to communicate between computers. No matter how slow its speed, in order to connect the devices to each other, only three wires were required - the data exchange protocol was so simple. It is clear that for full operation a larger number of conductors in the cord was required.

Today, the serial port is practically no longer used and is completely supplanted by its younger, but also faster “brother” - USB port. It should be noted, however, that some manufacturers still equip their motherboards with a COM port. However, the name itself - “serial port” is still used by software developers. For example, Bluetooth devices and cell phone ports are often presented as a “serial port.” This may be a little confusing, but this is done because they also transfer data serially, but at a higher speed.

If for some reason you may need a COM port, but your PC does not have one, then for this purpose you can use an adapter that connects to a modern USB port, which is available on all modern PCs, and on the other hand, such an adapter has serial port connector. There is, however, one limitation: if the software accessed directly the hardware of a real COM port, then it will not work with such an adapter. In this case, you need to purchase a special board that is installed inside your PC.

Structurally, the PC serial port has a male connector (with protruding pins):

Today, the 25-pin serial port connector has practically fallen out of use and has not been installed on a PC for several years. If the manufacturer provides the motherboard with a COM port, then it is a 9-pin DB9 connector.

It is an interface for connecting devices such as printers, scanners and plotters.

Allows you to simultaneously transmit 8 bits of data, albeit in one direction - from the computer to the periphery. In addition to this, it has 4 control bits (as with data bits, control bits are transferred from the PC to the external device), and 4 status bits (these bits can be “read” by the computer from the device).

In recent years, the LPT port has been improved, and it has become two-way, that is, it has become possible to transmit data bits through it in both directions. Today it is outdated and practically not used, although motherboard manufacturers still include it in its composition.

Enthusiasts and radio amateurs often use this port to control any non-standard devices (crafts, etc.).

USB interface

USB– this is an abbreviation of the full name of the port – universal serial bus (“universal serial bus”).

It is one of the most widely used ports on a personal computer today. And this is no coincidence - its technical characteristics and ease of use are truly impressive.

The data exchange speed for the USB 2.0 interface can reach 480 Mbit/s, and for the USB3.0 interface – up to 5 Gbit/s (!).

Moreover, all versions of this interface are compatible with each other. That is, a device using interface 2.0 can be connected to a USB3.0 port (in this case, the port will automatically reduce the speed to the desired value). Accordingly, a device using a USB 3.0 port can be connected to a USB 2.0 port. The only condition is that if normal operation requires a speed higher than the maximum speed of USB 2.0, then the normal functioning of the peripheral device will not be possible in this case.

In addition, the popularity of this port is also due to the fact that the developers included one very useful feature in it - this port can serve as a power source, for an external device connected to it. In this case, no additional unit is required to connect to the electrical network, which is very convenient.

For the USB 2.0 port version, the maximum current consumption can reach 0.5A, and for the USB3.0 version – 0.9A. It is not recommended to exceed the specified values, as this will lead to failure of the interface.

Developers of modern digital devices are constantly striving for minimization. Therefore, structurally, this port can have, in addition to a standard connector, also a mini-version for miniature devices - mini-USB. It has no fundamental differences from a standard USB port other than the design of the mini-USB connector itself.

Almost all modern devices have a USB port for connecting to a PC. Ease of installation - the connected device is recognized by the operating system almost immediately after connection, making it possible to use such a port without special “computer” knowledge. Printers, scanners, digital cameras, smartphones and tablets, external drives are just a small list of peripheral equipment that currently uses this interface. A simple principle - “plug and play” made this port truly a bestseller among all currently available personal computer interfaces.

Fire-Wire port (Other names - IEEE1394, i-Link)

This type of interface appeared relatively recently - since 1995. It is a high-speed serial bus. Data transfer rates can reach up to 400 Mbit/s in the IEEE 1394 and IEEE 1394a standard, 800 Mbit/s and 1600 Mbit/s in the IEEE1394b standard.

Initially, this interface was designed as a port for connecting internal drives (SATA type), but the licensing policy of Apple, one of the developers of this standard, required payment for each controller chip. Therefore, today only a small number of digital devices (some models of cameras and video cameras) are equipped with this type of interface. This type of port never became widespread.

The importance of this interface can hardly be overestimated; as a rule, it is what is used to connect a personal computer to a local network or to access the Internet in most cases. Almost all modern PCs, laptops and netbooks are equipped with an Ethernet port built into the motherboard. This is easy to verify if you inspect the external connectors.

To connect external devices, a special one is used, which has identical connectors at both ends. connectors – RJ-45, containing eight contacts.

The cable is symmetrical, therefore, the order in which the devices are connected does not matter - any device of your choice can be connected to any of the identical cable connectors - a PC, router, modem, etc. It is marked with the abbreviation - UTP, common name is “twisted pair”. In most cases, for both home and office use, a cable of the fifth category, UTP-5 or UTP-5E, is used.

The speed of data transmitted over an Ethernet connection depends on the technical capabilities of the port and is 10 Mbit/s, 100 Mbit/s and 1000 Mbit/s. It should be understood that this throughput is theoretical, and that in real networks it is somewhat lower due to the peculiarities of the Ethernet data transfer protocol.

Also, you should keep in mind that not all manufacturers install high-speed chips in their Ethernet controllers, since they are very expensive. This leads to the fact that in practice, the actual data transfer speed is much lower than that indicated on the packaging or in the specification. As a rule, almost all Ethernet cards are compatible with each other and from top to bottom. That is, newer models that have the ability to connect at speeds of 1000 Mbit/s (1 Gbit/s) will work without problems with older models at speeds of 10 and 100 Mbit/s.

To visually monitor the integrity of the connection, the Ethernet port has Link and Act indicators. Link indicator - lights up green when the physical connection is correct and working, i.e. the cable between the devices is connected, it is intact, the ports are working. The second Act indicator (“activity”) is usually orange and flashes while transmitting or receiving data.

Internal ports of a personal computer

As mentioned above, internal ports are designed for connecting peripherals such as hard drives, CDs and DVD-ROMs, card readers, additional COM and USB ports, etc. Internal ports are located either on the motherboard, or on additional expansion cards installed in the system bus.

A now outdated interface for connecting older models of hard drives (“hard drives”, HDD). After the creation of the SATA interface, it was called the PATA interface, or ATA for short. PATA – ParallelAdvanced Technology Attachment. This parallel data transfer interface for connecting drives was developed in mid-1986 by the now famous company WesternDigital.

Depending on the manufacturer, the motherboard may contain from one to four IDE channels. Modern manufacturers, as a rule, leave only one IDE port for compatibility, and recently it has also been excluded from the motherboard, having been completely replaced by the modern SATA interface.

The data transfer speed in the latest version of the EnhancedIDE interface can reach 150 Mbit/s. Devices are connected using an IDE cable having 40 or 80 cores for the old or new interface type, respectively.

Typically, you can connect up to two devices simultaneously to one IDE port using a single cable. In this case, using jumpers on the drives that determine the “seniority” of devices operating in pairs, the operating mode is selected - on one device - "master", and for the other "subordinate" (slave).

You can connect either the same type of device, for example, two hard drives or two DVD-ROMs, or different devices in any combination - DVD-ROM and HDD or CD-ROM and DVD-ROM. The connector for connection does not matter, you should just pay attention that the two connectors for connecting peripherals are shifted for convenience to one of the ends of the cable.

You should also keep in mind that by connecting a “fast” device designed for an 80-wire cable using an old 40-wire cable, you will greatly reduce the exchange speed. In addition, if one of the devices in the pair has an old (slow) ATA interface, then the data transfer speed in this case will be determined precisely by the speed of this device.

If there are two IDE ports and two drives inside the PC, to increase the data exchange speed, you must connect each drive to a separate IDE port.

This interface is a development of its predecessor, the IDE interface, with the only difference that, unlike its “older friend,” it is not a parallel, but a serial interface. SATA – SerialATA.

Structurally, it has only seven conductors for its operation and a much smaller area of ​​​​both the connector itself and the connecting cable.

The data transfer speed of this interface is significantly higher than the outdated IDE and, depending on the SATA version, is:

1. SATARev. 1.0 – up to 1.5 Gbit/s;
2. SATARev. 2.0 – up to 3 Gbit/s;
3. SATARev. 3.0 – up to 6 Gbit/sec.

Just like the IDE interface, the cord for connecting devices is “universal” - the connectors are the same on both sides, but unlike its “brother”, now you can connect only one device to one SATA port using one SATA cable.

But there is hardly any need to be upset about this. Manufacturers made sure that the number of ports was sufficient for a wide variety of applications, installing up to 8 SATA ports on one motherboard. The third revision SATA port connector is usually bright red.

Additional ports

Most motherboards are equipped by manufacturers with an additional number of USB ports, and sometimes with another, additional COM port.

This is done for the convenience of the user. Most modern desktop PC cases have USB connectors installed on the front panel for convenient connection of external drives. In this case, you do not need to reach to the back wall of the system unit and “get into” the USB connector, which is located on the rear panel.

This connector is on the front panel and connects to an additional USB port installed on the motherboard. Among other things, the USB interfaces located on the rear panel may simply not be enough, due to the large number of peripheral devices, in this case you can purchase additional bracket with USB connectors and connect them to additional ports.

All of the above also applies to other ports installed on the motherboard. For example, a COM or FireWireIEEE1394 serial port may simply not be displayed on the back panel of a personal computer, but it is still present on the motherboard. In this case, it is enough to buy the appropriate cable and take it out.

It would be technically incorrect to call these connectors ports, although the method of connecting additional cards to them is still somewhat similar to other conventional ports. The principle is the same - plug it in and turn it on. In most cases, the system will find the device itself and request (or install automatically) drivers for it.

Such buses are used to install, for example, an external graphics card, a sound card, an internal modem, a video input card, and other additional expansion cards that allow the PC to expand its functions and capabilities.

PCI and PCIe buses are incompatible with each other, so before purchasing an expansion card, you need to find out which system buses are installed on the motherboard of your PC.

PCIex 1 and PCIex 16 are modern implementations of the older PCI bus, developed in 1991. But unlike its predecessor, it is a serial bus, and in addition, all PCIe buses are connected in a star topology, while the old PCI bus was connected in parallel to each other. In addition, the new tire has the following advantages:

1. Possibility of hot replacement of boards;
2. The bandwidth has guaranteed parameters;
3. Control of data integrity during reception and transmission;
4. Controlled energy consumption.

PCI Express buses differ in the number of conductors connected to the slot, through which data is exchanged with the installed device (PCIex 1, PCIex2, PCIex 4, PCIex 8, PCIex 16, PCIex 32). The maximum data transfer speed can reach 16 Gbit/s.

The parallel port provides fairly high transfer speeds because the transfer is done one byte at a time. At the same time, when the cable is long or when data exchange is not very intensive, a serial port is more convenient.

The Serial Port transmits only 1 bit of data in one direction at a time. Data can be transferred through this port both from the computer to an external device and vice versa.

Computer serial ports usually comply with the international reference standard RS-232C (Reference Standard 232 version C), therefore any device that is also oriented to this standard can be connected to this port (for example, a mouse, modem, serial printer or serial port another computer). This interface uses 9 communication channels: one of them is used to transmit data from the computer, the other is used to receive data from a peripheral device. The remaining 7 channels are used to control the data exchange process.

The serial port consists of a UART (Universal Asynchronous Receiver/Transmitter) chip and auxiliary components. The UART chip receives bytes of data from the computer bus (in which they are transmitted in parallel), converts them into a sequence of bits, adds service bits and then performs the data transfer, and also performs the reverse steps of receiving the sequence of bits and converting the code from serial to parallel.

Modern UART chips are equipped with buffer memory and provide data transfer rates of up to 115 Kbps. New high-speed versions of the serial port - improved serial port ESP (Enhanced Serial Port) and Super ESP (Super Enhanced Serial Port) provide data transfer up to 460 Kbps.

During serial transmission, data is separated by service bits, such as a start bit and a stop bit. These bits indicate the start and end of the transmission of successive data bits. This transmission method allows synchronization between the receiving and transmitting sides, as well as equalizing the data exchange rate.

To identify and recognize errors during serial transmission, a parity bit may additionally be included in the send. The value of the parity bit is determined by the binary sum of all transmitted data bits. In Even Parity mode, the value of the parity bit is 0 if the sum of the bits is even, and 1 otherwise. Parity bits have inverse (reverse) values ​​(1 or 0, respectively) if the parity bit is odd (Odd Parity).

The standard configuration of the computer contains two serial ports. The difference between a serial port connector and a parallel one is that this connector has 9 pins, not sockets ("male" connector) (Fig. 1.3.11a). On the cable side of the connected device, the “mother” connector is used (Fig. 1.3.11b). The length of the serial port cable is limited to 18 m. The main devices connected to the serial port are old models of modems and mice.

Some computers, especially those oriented toward communications applications, may have serial ports that follow different standards (for example, RS-449A or RS-613) and have higher data transfer rates over longer distances.

Rice. 1.3.11. Serial port: a) 9-pin computer connector;

b) adapter cable serial port-USB

1.3.2.3.13. PS/2 port

The PS/2 (6-pin) port is so named because it first appeared on IBM PS/2 series computers. Of the 6 contacts, 4 contacts are used, one of which is intended for data transmission, the second for clock frequency signals (in the range of 10-16.7 kHz), the third contact is supplied with power (+5V), and the fourth is ground. Data transfer is similar to the serial port, but one acknowledgment bit is added when transferring data to the device. Modern computers have two PS/2 ports designed to connect a mouse (green connector) and a keyboard (purple connector) (Fig. 1.3.12a), but these devices are switching to using a USB port. The cable connectors for PS/2 devices (mouse and keyboard) are shown in Fig. 1.3.12b.

Rice. 1.3.12. PS/2 port: a) computer port sockets; b) cable plugs

Serial port - concept and types. Classification and features of the "Serial Port" category 2017, 2018.

A serial port is an I/O device. As an I/O device, it is only a path for transferring data to and from the computer. There are also many other I/O devices, such as serial ports, parallel ports, disk controllers, network cards, USB universal serial bus devices, etc. Most computers have one or two serial ports. Each has a 9-pin connector (sometimes 25-pin) (Fig. 1) on the back wall of the computer system unit. Programs can send data (bytes) through a send data pin (output) and receive bytes through another data receive pin (input). All other contacts are used for control and ground.

) is somewhat more than just a connector. It converts data from parallel to serial representation and changes the electrical representation of the data. Inside a computer, data bits are transmitted in parallel (using multiple wires to transmit data simultaneously). A serial data stream is a sequence of bits over just one wire (such as the transmit and receive data wire on the serial port connector). This is what this device serves to create such a data stream from parallel to serial (inside the computer) and transmit it to the data transfer contact (and, accordingly, vice versa).

Most of the serial port's electronic components are located on a single computer chip called UART.

Contacts and wires

Older computers use 25-pin connectors, but only 9 pins are actually used today. Each of the 9 contacts is usually connected to a wire. Except for two wires for transmitting and receiving data, the rest are used for control and ground. The voltage at each of the pins and wires is measured relative to the signal ground. Therefore, the minimum number of wires for bidirectional data transmission is 3. In rare cases, two wires (without signal ground) may be enough for operation, but this can lead to low performance and sometimes errors in data transmission.

There are still a few wires left that are intended only for control (monitoring) and are not used for data transmission. All these signals could be transmitted over one line, but instead, separate wires are allocated for them. Some (or all) of these signal lines are called "modem status lines." The status lines can be in one of two states: set (on) +12 volts or cleared (off) -12 volts. One of these wires signals the computer to stop sending data through the serial port. Others in turn signal the device connected to the serial port to stop sending data to the computer. If the connected device is a modem, then the remaining lines can indicate to the modem that it needs to seize the telephone line or signal to the computer that a connection has been established or that there is a call on the telephone line (meaning someone is connecting to the computer). See the Contacts and Signals section for more complete information.

RS-232 or EIA-232, etc.

Serial port) (not to be confused with USB) usually follows the standard RS-232-C, EIA-232-D, or EIA-232-E. These are three designations for the same thing. The main RS standard (Recommended Standard) received the prefix EIA (Electronics Industries Association) and later EIA/TIA after the EIA organization was merged with TIA (Telecommunications Industries Association). The EIA-232 specification also covers synchronous data transfer, but in most cases synchronous data transfer is not supported by the chips in computers. The RS designation is obsolete but is still widely used. EIA will be used more frequently later on this site. Some documents use the full EIA/TIA designation.

Data Communication (Baud Rates)

Data (the bytes that make up letters, pictures, etc.) passes through the serial port. Data rates (such as 56k (56000) bps) are called (incorrectly) "speed". Most people incorrectly say "speed" instead of "speed factor".

It is important to know that the average data transfer speed is often less than the maximum declared. There are delays (or periods of waiting) and as a result the speed becomes slower. These delays may increase depending on the type of data transmission control. Even in the best case, there are always delays between bytes, even small ones (a few microseconds). If a device connected to a computer via a serial port cannot operate at full speed, then the average speed must be reduced.

Data transmission control

Data transmission control means the ability to limit the data flow through the serial port. For serial port this means you can stop and then resume data transfer without losing any bytes.