Seri Port Mesafe Uzatma Cihazı
- 2 (TX), 3 (RX), 4 (RTS), 5 (CTS), 6 (DSR), 8 (DCD), 20 (DTR) hatlarını kuvvetlendirerek kablo mesafesini 2 misli arttırır.
- Harici power adaptör cihazla birlikte gelmektedir.
- Giriş: 1 x DB25 dişi (DTE)
- Çıkış: 1 x DB25 erkek (DCE)
What is RS-232C?
Short for recommended standard-232C, a standard interface approved by the Electronic Industries Alliance (EIA) for connecting serial devices. In 1987, the EIA released a new version of the standard and changed the name to EIA-232-D. And in 1991, the EIA teamed up with Telecommunications Industry association (TIA) and issued a new version of the standard called EIA/TIA-232-E. Many people, however, still refer to the standard as RS-232C, or just RS-232.
Almost all modems conform to the EIA-232 standard and most personal computers have an EIA-232 port for connecting a modem or other device. In addition to modems, many display screens, mice, and serial printers are designed to connect to a EIA-232 port. In EIA-232 parlance, the device that connects to the interface is called a Data Communications Equipment (DCE) and the device to which it connects (e.g., the computer) is called a Data Terminal Equipment (DTE).
The EIA-232 standard supports two types of connectors -- a 25-pin D-type connector (DB-25) and a 9-pin D-type connector (DB-9). The type of serial communications used by PCs requires only 9 pins so either type of connector will work equally well.
Although EIA-232 is still the most common standard for serial communication, the EIA has recently defined successors to EIA-232 called RS-422 and RS-423. The new standards are backward compatible so that RS-232 devices can connect to an RS-422 port.
Originally the RS232 communication specification was devised in 1962 when the need to be able to transmit data along a variety of types of line started to grow. The telephone companies saw the need to introduce some standards. As a result the Electrical Industries Association in the U.S.A. created a standard for serial data transfer or communication known as RS232C. It defines the electrical characteristics for transmission of data between a Data Terminal Equipment (DTE) and the Data Communications Equipment (DCE). Normally the data communications equipment is the modem (modulator/demodulator) which that encodes the data into a form that can be transferred along the telephone line. A Data Terminal Equipment could be a computer.
Since it was first introduced there have been a number of updates to the RS-232 standard. These have included EIA-232 (Electronic Industries Alliance) and later EIA/TIA-232 (Telecommunications Industry Association).
The original RS-232 specification was developed in the USA and sponsored by organisations there. On a more international scale the International Telecommunications Union (ITU) developed a standard known as ITU v.24. However this is often just written V24. This standard is totally compatible with RS232, and its aim was to enable manufacturers to conform to global standards and thereby allow products that would work in all countries around the world.
The success of the RS232 standard has meant that it is now used for many more types of equipment. As a result many lines defined in the specification are rarely used. This means that care has to be taken when connecting any new equipment or defining which lines are to be used in a new design.
In RS-232, data is sent as a time-series of bits. Both synchronous and asynchronous transmissions are supported by the standard. In addition to the data circuits, the standard defines a number of control circuits used to manage the connection between the DTE and DCE. Each data or control circuit only operates in one direction, that is, signalling from a DTE to the attached DCE or the reverse. Since transmit data and receive data are separate circuits, the interface can operate in a full duplex manner, supporting concurrent data flow in both directions. The standard does not define character framing within the data stream, or character encoding.
The RS-232 standard defines the voltage levels that correspond to logical one and logical zero levels. Valid signals are plus or minus 3 to 15 volts. The range near zero volts is not a valid RS-232 level; logic one is defined as a negative voltage, the signal condition is called marking, and has the functional significance of OFF. Logic zero is positive, the signal condition is spacing, and has the function ON. The standard specifies a maximum open-circuit voltage of 25 volts; signal levels of ±5 V, ±10 V, ±12 V, and ±15 V are all commonly seen depending on the power supplies available within a device. Circuits driving an RS-232-compatible interface must be able to withstand indefinite short circuit to ground or to any voltage level up to 25 volts. The slew rate, or how fast the signal changes between levels, is also controlled.
Because the voltage levels are higher than logic levels used by integrated circuits, special intervening circuits are required to translate logic levels, and to protect circuitry internal to the device from short circuits or transients that may appear on the RS-232 interface.
RS-232 devices may be classified as Data Terminal Equipment (DTE) or Data Circuit termination Equipment (DCE); this defines at each device which wires will be sending and receiving each signal. The standard recommended but did not make mandatory the common D-subminiature 25 pin connector. In general, terminals have male connectors with DTE pin functions, and modems have female connectors with DCE pin functions. Other devices may have any combination of connector gender and pin definitions.
Presence of a 25 pin D-sub connector does not necessarily indicate an RS-232C compliant interface. For example, on the original IBM PC, a male D-sub was an RS-232C DTE port (with a non-standard current loop interface on reserved pins), but the female D-sub connector was used for a parallel Centronics printer port. Some personal computers put non-standard voltages or signals on their serial ports.
The standard specifies 20 different signal connections. Since most devices use only a few signals, smaller connectors can be used. For example, the 9 pin DE-9 connector was used by most IBM-compatible PCs since the IBM PC AT, and has been standardized as TIA-574. More recently, modular connectors have been used. Most common are 8 pin RJ-45 connectors. Standard EIA/TIA 561 specifies a pin assignment, but the "Yost Serial Device Wiring Standard" invented by Dave Yost is common on Unix computers and newer devices from Cisco Systems. Many devices don't use either of these standards. 10 pin RJ-50 connectors can be found on some devices as well. Digital Equipment Corporation defined their own DECconnect connection system which was based on the Modified Modular Jack connector. This is a 6 pin modular jack where the key is offset from the center position. As with the Yost standard, DECconnect uses a symmetrical pin layout which enables the direct connection between two DTEs.
The interface is intended to operate over distances of up to 15 metres. This is because any modem is likely to be near the terminal. Data rates are also limited. The maximum for RS-232C is 19.2 k baud or bits per second although slower rates are often used. In theory it is possible to use any baud rate, but there area number of standard transmission speeds used.
Common Data Transmission Rates
50 75 110 150 300 600 1200 2400 4800 9600 19200 38400 76800
Note: speeds up to 19200 bits per second are normally used. Above this noise that is picked up, especially over long cable runs can introduce data errors.
RS232 signal levels
The voltage levels are one of the main items in the specification. For RS232 data signals a voltage of between -3V and -25V represents a logic 1. The logic 0 is represented by a voltage of between +3V and +25V. Control signals are in the "ON" state if their voltage is between +3V and +25V and "OFF" if they are negative, i.e. between -3V and -25V.
The data is sent serially on RS232, each bit is sent one after the next because there is only one data line in each direction. This mode of data transmission also requires that the receiver knows when the actual data bits are arriving so that it can synchronise itself to the incoming data. To achieve this a logic 0 is sent as a start bit for the synchronisation. This is followed by the data itself and there are normally seven or eight bits. The receiver obviously has to know how many data bits to expect, and there are often small dual in line switches either on the back of the equipment or inside it to set this information.
Data on RS232 is normally sent using ASCII (American Standard Code for Information Interchange). However other codes including the Murray Code or EBCDIC (Extended Binary Coded Decimal Interchange Code) can be used equally well.
After the data itself a parity bit is sent. Again this requires setting because it is optional and it can be even or odd parity. This is used to check the correctness of the received data and it can indicate whether the data has an odd or even number of logic ones. Unlike many systems these days there is no facility for error correction.
Finally a stop bit is sent. This is normally one bit long and is used to signify the end of a particular byte. Sometimes two stop bits are required and again this is an option that can often be set on the equipment.
RS232 data transmission is normally asynchronous. However transmit and receive speeds must obviously be the same. A certain degree of tolerance is allowed. Once the start bit is sent the receiver will sample the centre of each bit to see the level. Within each data word the synchronisation must not differ by more than half a bit length otherwise the incorrect data will be seen. Fortunately this is very easy to achieve with today's accurate bit or baud rate generators.
Lines and their usage
Their are four types of line defined in the RS232 specification. They are Data, Control, Timing and Ground. Not all of them are required all the time. It is possible to set up a very simple communication using very few lines. When looking at the lines and their functions it is necessary to remember that they are defined for a connection between a modem (the data set or communications equipment) and a terminal or computer (data terminal equipment) in mind. All the lines have directions, and when used in this way a one to one cable operates correctly.
The most obvious lines are the data lines. There are two of these, one for data travelling in each direction. Transmit data is carried on pin 2 and the receive data is carried on line three.
The most basic of the control circuits is Data Carrier Detected (DCD). This shows when the modem has detected a carrier on the telephone line and a connection appears to have been made. It produces a high, which is maintained until the connection is lost.
Data Terminal Ready (DTR) and Data Set Ready (DSR) are the main control circuits. They convey the main information between the terminal and modem. When the terminal is ready to start handling data it flags this on the DTR line. If the modem is also ready then it returns its signal on the DSR line. These circuits are mainly used for telephone circuits. After a connection has been made the modem will be connected to the line by using DTR. This connection will remain until the terminal is switched off line when the DTR line is dropped. The modem will detect this and release the telephone line.
Sometimes pin 20 is not assigned to DTR. Instead another signal named, Connect Data Set To Line (CDSTL) is used. This is virtually the same as DTR, but differs in that DTR merely enables the modem to be switched onto the telephone line. CDSTL commands the modem to switch, despite anything else it may be doing.
A further two circuits, Request To Send (RTS) and Clear To Send (CTS) are also used. This pair of circuits are used together. The terminal equipment will flag that it has data to send. The modem will then return the CTS signal to give the all clear after a short delay.
This signalling is used particularly when switched carriers are used. It means that the carrier is only present on the line when there is data to send. It finds its uses when one central modem is servicing several others at remote locations.
There are two types of lines that are specified in the RS-232 specification. There are the primary channels that are normally used, and operate at the normal or higher data rates. However, there is also provision for a secondary channel for providing control information. If it is used it will usually send data at a much slower rate than the primary channel.
As the secondary lines are rarely used or even implemented on equipment, manufacturers often use these connector pins for other purposes. In view of this it is worth checking that the lines are not being used for other purposes before considering using them. When the secondary system is in use, the handshaking signals operate in the same way as for the primary circuit.
The ground connections are also important. There are two. First the protective ground ensures that both equipments are at the same earth potential. This is very useful when there is a possibility that either equipment is not earthed. The signal ground is used as the return for the digital signals travelling along the data link. It is important that large currents that are not part of the signalling do not flow along this line otherwise data errors may occur.
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