What is RS-232-C Interface standard

RS-232 - C

In the early 1960’s, a standard committee, known as Electronic Industries Association (EIA) developed a common interface standard for data communications equipment between the terminal or computer and the modem using serial binary data exchange. In the standards, the terminal or computer is officially called a DTE (Data Terminal Equipment) and the modem is officially called a DCE (Data Circuit-Terminating Equipment). This interface is also known as EIA-232 or EIA-RS-232 standards.

The information is transferred between computer and peripherals is in the form of digital or analog which is transmitted in either a serial or parallel mode. Serial transmission means sending one bit at one time and it occurs between computers and peripherals. Parallel transmission means as many bits are in the form of words being transmitted at a time and mainly used between computers and printers. Serial transmission is beneficial for long distances whereas parallel transmission is designed for short distances or when very high transmission rates are required. A modem is a device that accepts a serial stream of bits as inputs and produces a modulated carrier as output or vice versa.

RS 232 interface between DTE and DCE

The figure shows the RS-232 is defined as the interface between data terminal equipment (computer) and data circuit-terminating equipment (modem).

RS-232- C is the third revision of the original RS-232 standard.

It specifies the mechanical, electrical, functional, and procedural interface in detail.

Mechanical specification: 

RS-232-C is a 25 pin connector. The top row has 1 to 13 pins from left to right. The bottom row has pins numbered 14 to 25 (also from left to right).

Electrical specification: 

voltage levels - Binary 1: -3V to -15V Binary 0: +4V to +15V Data format: cables up to 15 meters and data rates up to 20kbps are permitted.

Functional specification:

This specification tells which circuits are connected to each of the 25-pins, and what they mean. Following figure shows 9-pins that are nearly always implemented. The remaining ones are frequently omitted.

• When the terminal or computer is powered up, it sets to logical 1 with Data Terminal Ready (pin 20).
• When the modem is powered up, it asserts (set to 1) Data set Ready (pin 6).
• When the modem detects a carrier on the telephone line, it asserts Carrier detect (pin 8).
• If the terminal wants to send data it asserts Request to send (pin 4) and if the modem is prepared to accept data i.e. Clear to Send (pin 5) is set to 1.
• Data are transmitted on the Transmit circuit (pin 2) and received on the Receive circuit (pin 3). Other circuits are provided for selecting the data rate, testing the modem, clocking the data, detecting ringing signals and sending data in the reverse direction on a secondary channel. 



RS-232-C circuit

The procedural specification:

The procedural specification is the protocol i.e. the legal sequence of events. The protocol is based on action-reaction pairs. When the terminal asserts Request to Send, the modem replies with clear to send, if it is able to accept data. If the destination machine is off then the interface problem is occurring. To solve this problem a device called a null modem is connected, which connects the transmit line of one machine to the receiving line of the other. It looks like a short cable.

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