What is a COM port? Device, management of COM-port

The COM port, or serial port, is a bi-directional serial interface that is designed to exchange byte data. At first, this port was used to connect the terminal, and then for the modem and the mouse. Now it is used to connect an uninterruptible power supply, as well as to communicate with hardware processing of embedded systems.

Using

So, before you talk more about what a COM port is, you need to look into the past to understand its meaning. Literally 15 years ago, a method of connecting devices to a computer using a special standard connector located on the back of the system unit using a special serial RS-232 cable was used. This method has many disadvantages. Such a cable, by modern standards, provides an extremely low data transfer rate - about a hundred kilobits per second. In addition, when the physical connection of the connectors was made, it was necessary to turn off the equipment, and they themselves were attached to each other with screws providing reliability, while their dimensions were not small.

A bit of history

The COM port on computers at that time has traditionally worn number 1 or 2, as there were usually not more than two of them. You could install additional ports, if necessary. When the user made the adjustment of the software, it was necessary not to confuse and correctly install the one to which the necessary equipment was connected. Each COM-port required correct setting of the data transfer rate, as well as a number of other mysterious parameters, which were known only to a narrow circle of specialists. In order to connect the equipment successfully, all the necessary parameters needed to be learned from somewhere or experimentally picked up, since in this case there was no any automatic configuration. In addition, the connection through the COM port allowed the connection of any software with arbitrary external equipment, even completely incompatible, which caused a lot of errors during the configuration process.

Modernity

Now the connection through COM-port is completely superseded by a more modern method, which does not require special knowledge for implementation, namely via USB-port. This method is devoid of all the shortcomings mentioned earlier. However, modern compatibility standards for the connection of all kinds of GPS equipment and very heterogeneous software have been formed quite a long time around the concept of COM ports, which have become archaic at the moment.

This is due to the fact that initially virtually any equipment, including GPS, was external, and its connection to the computer was made via a serial cable connected to one of the hardware ports. From the user during the configuration required to correctly choose the port number and the speed of data translation on it. At that time, the basic standard for data transmission from the GPS receiver to the program, which is now called NMEA-0183, arose. In fact, this standard requires all developers, even the most modern hardware and software, to exchange data via COM ports. And all this in the context of the fact that on modern computers, as well as on the PDA, for a long time the main thing is the USB standard. And one more feature is that recently GPS receivers have been increasingly installed directly inside the device's case, that is, there is no connecting cable between it and the main device at all.

Virtual COM ports

The way out was invented, namely, "virtual" COM ports were developed. It turns out that the internal device of the PDA, for example, a GPS receiver, is simulated as a COM port programmatically, but not in the hardware plan. With this program, which is designed for interfacing through a similar standard, there is no difference how it is implemented. There is allowed a virtual simulation, and not necessarily the presence of hardware implementation. So it is possible to ensure the compatibility of old-model GPS programs with modern equipment.

Changes made

In this case, the management of the COM port has not changed significantly. The user in the old manner should make a complex adjustment almost by hand. However, the modern COM-port is no longer a bulky device located on the back of the system unit, but a completely different device. And here the whole point is that from a program point of view, all their implementations look faceless, that is, there is no difference between virtual and real ports. For software, the ports differ only in the numbers they are assigned by the PDA manufacturers on a completely random basis. For example, the receiver from ASUS is usually located on COM5, and PocketLOOX 560 shows the receiver on COM8. It turns out that the program that wants to receive data from the GPS receiver does not initially have any reliable information about the conditional number, under which the port prescribed for the receiver on the given PDA appears.

How does it all work?

While among all available COM ports it is possible to perform an automatic search for a suitable one, the procedure for such a poll is rather unreliable and rather cumbersome. This is due to the fact that the devices displayed in the system as COM ports can be quite diverse and not related to GPS, they can unpredictably respond to such a poll. For example, the PDA has ports that are interfaced with an internal cellular modem, with USB, with an infrared port, as well as with other elements. Referring to them a program designed to work with a specific device can lead to a completely unpredictable reaction, as well as to various malfunctions, which often causes the PDA to hang. That is why trying to open a COM port can lead to unexpected situations until Bluetooth or infrared port is turned on . And there may be more incomprehensible cases.

COM Port Operation

For COM ports, the asynchronous universal transceiver chip is used as the basis. This chip exists in several varieties: Intel 16550A, 16550, 16450, 8250. For each COM-port it contains registers of the receiver and data transmitter, as well as a number of control registers, which are accessible through BIOS, Windows and MS DOS programs. The latest versions of the chip have a set of buffers for temporary storage of transmitted and received data. Due to such an opportunity, it is possible to interrupt the operation of the central processor less often, and also to agree the speed of data transmission.

Main settings

The COM port device assumes the presence of such characteristic features:

- the base address of the port for input and output of information;

- Hardware interrupt numbers;

- the size of one block of information;

- the speed at which data is transferred;

- mode of detecting honesty;

- a way to manage information flows;

- the number of stop bits.

How can I check the COM port of my computer? What should I look for?

As mentioned earlier, this type of port is a bi-directional interface for transmitting information at the bit level in a sequential manner. A distinctive feature in comparison with a parallel port is the transfer of data bit by bit. The anatomy of the COM port is such that it does not use a serial data transfer method on the computer. For example, interfaces such as Ethernet or USB, too, use a similar principle, but it has historically happened that the serial port is usually called the RS232 standard port.

Very often it is required to open a COM port for repairing and diagnosing a computer, and it must also be checked for operability. After all, it's very simple to burn an element. Most often this is due to the fault of the user, which turns off the device incorrectly, pulling out the connector with the interface connected. The simplest way to test the functionality of the interface is to connect a mouse to it. However, it is so difficult to get a complete picture, since the manipulator only uses half the signal lines of the eight available. Only the use of a special stub and program will make it possible to conduct a performance check. For these purposes, specially developed software already exists.