Today, virtually all contemporary serial interfaces are EIA232-like in their signal voltages, protocols, and connectors, whether or not a modem is involved. Besides changing the name from RS232 to EIA232, some signal lines were renamed and various new ones were defined, including a shield conductor.ĭuring this 40-year-long, rapidly evolving period in electronics, manufacturers adopted simplified versions of this interface for applications that were impossible to envision in the 1960s. Over the 40+ years since this standard was developed, the Electronic Industries Association published three modifications, the most recent being the EIA232F standard introduced in 1997. It specified signal voltages, signal timing, signal function, a protocol for information exchange, and mechanical connectors. From these ideas, the RS232 standard was born. It was thought that a standard was needed first to ensure reliable communication, and second to enable the interconnection of equipment produced by different manufacturers, thereby fostering the benefits of mass production and competition. While simple in concept, the many opportunities for data error that occur when transmitting data through an analog channel require a relatively complex design. These devices were linked by telephone voice lines, and consequently required a modem at each end for signal translation. At that time, data communications was thought to mean digital data exchange between a centrally located mainframe computer and a remote computer terminal, or possibly between two terminals without a computer involved. In the early 1960s, a standards committee, today known as the Electronic Industries Association, developed a common interface standard for data communications equipment. Likely Problems when Using an EIA232 Interface (renamed the "EIA232 Standard" in the early 1990's)Ĭopyright © 1993-2015 by CAMI Research Inc., Acton, Massachusetts The "point of view" of the computer (DTE).A Tutorial with Signal Names and Definitions The names: "receive" and "transmit" should be interpreted from The meaning of the pins are the same on both Data Terminal Equipment andĭata Communications Equipment. Nowadays computers used as DTE instead of a terminal. Initially, RS232 ports were used for connecting aĭumb terminal (DTE) to a modem (DCE). On the same wire but neither would be able to receive any If you will do it, then they would both send out signals Therefore, you can't connect two devices that can send data out (DTE-DTE connection) using a straight cable. Unable to receive any signal using this pin. For example, if pin #2 outputs data, then it is You can see examples in the " SerialĪll signals of the RS232 port is are unidirectional. There are several cable layouts possible. Two DTE devices (for example, two computers or a computer and an external device). It is possible two connection types between serial ports: straight (pins 2 on both sides are connected) or null-modem (pin #2 is connected to pin #3 on another side).įirst connection type is used between DTE (Data Terminal Equipment) and DCE (Data Communications Equipment) devices (for example, a computer and modem), and the second connection type is used between How to connect RS232 ports using cablesĪ serial port is used to connect devices or computers, and therefore it should be connected to another serial port.Īn external device or modem that is connected to the serial port has an identical built-in serial
Serial Port Monitor software can work with all signal of the RS232 interface. It is possible for all the 6 signal wires, but usually in standard devices these signal pins are used as described in the table above. In other cases the modem may not handle this signal or do something else when DTR is LOW (-12V). For example, a modem can hang up the telephone line when DTR is LOW. 3V)įor example, Advanced Serial Port Monitor can automatically control the DTR signal and set it LOW (-12V) when sends data out.Ī modem (or another device) that receives thisĭTR signal on another side (if these pins are connected) may do various things. RS232 uses following voltages for signals: ITU V.24 (pins description and names) and V.28 (electrical). RS232 defines both the physical andĮlectrical layers of this interface. Telecommunications Industry Association) issued several RS232 standards. This is defined by the hardware interface and you can't change it.īut all other pins can be controlled by a software and can be used for other tasks (rarely). Only 3 pins of the 9 have a fixed meaning: send, receive and signal Note: DCD can be sometimes labeled as CD Port Pins May Have No Constant Meaning