EIA RS-232C Standard

• Developed by EIA - Electronic Industries Association

• Describes functions of 25 signal & handshake lines for serial data transfer.

• Describes the electrical signal characteristics.

• Describes interface mechanical characteristics.

• Applicable for us at Data signaling rates from 0 to 20 KBits/s.

• Applicable for the interchange of data, timing and control signals each of which has a single common return (single ground) that can be connectd at the interface point. It does not apply where electrical isolation between equipment on opposite sides of the interface point is required.

• Applies to both synchronous and asynchronous serial binary data communication systems.

RS-232C TERMINOLOGY

RS-232 SERIAL DATA STANDARD [NON STANDARD!!]

Data communication equipment (Modem) - DCE

Terminals / Computer - DTE

Standard Developed by EIA (Electronic Industries Association) describes

• Function of 25 signal and handshake pins for serial data transfer
• Voltage levels, impedence, levels, rise & fall times, maximum bit rate, maximum capacitance on signal lines etc.
• Specifies
  • DTE Connector - Male .. (DB25P)
  • DCE Connector - Female .. (DB25S)
• Signal direction is specified with respect to DCE.
• Chassis ground and signal ground (Pins 1 & 8 respectively) should be connected together only at the power supply in terminal/computer (DTE).
• A logic high or mark is a voltage between -3V and -15V under load (-25V on no  load).
• A logic low or space is a voltage between +3V and +15V under Load (+25V on no load)
• Voltages + 12V are commonly used.

Introduction

Associated with a data transfer are a source, a destination, and a connection between them. Consider the problem of communication between two integrated circuits on the same printed circuit card, for example, a memory - reach cycle executed between a CPU and some on-board memory. In this case, the data travel over metallic printed circuit pathways that have been etched into coated fibre board. Typically, the data representation is binary, where a V1 (2.4 V to 5 V) is equated as logic 1 and V0 (0V to 0.4 V) is equated with logic 0. This is called TTL (for transistor-transistor logic level) signaling, and it is widely used standard for component-to-component communications inside a computer cabinet.

For internal computer data transfers, TTL-level signals are ideal for several reasons. First, their associated power requirements and heat dissipation are low. Second, TTL-level signals interface directly to integrated circuits without the need for costly line driver and receiver circuits. Finally, TTL interfaces operate at high speeds that are required for internal computer data transfers.

What about communications between distinct digital devices, such as a computer and a local terminal or two computers in the same building? It was natural to try to extend the techniques that worked so well inside the computer cabinet to data transfers between cabinets. This has been done, and today there are several commercially available terminals that support TTL-level signaling with a host computer. Unfortunately, there are several serious problems associated with TTL communications over distances of more than a few feet. First, TTL is very susceptible to externally induced noise. Second, line losses that reduce the voltage potential of transmitted signals have particularly significant impact upon TTL voltage levels in that losses of even just a volt can totally obscure the difference between a logic 0 and logic 1. TTL type communications are most often carried out via parallel data transfers. Parallel data transmission is inappropriate for distances over tens of feet for at least two important reasons : reliability and cost.

Due to these drawbacks when the data communication takes place at long distances, we need to enhance the voltage level of TTL signals so that it does not get seriously affected by line losses. The physical, electrical, and logical rules for the exchange of data between DTEs and DCEs are specified by an interface standard; the most commonly used is the EIA RS-232-C standard which is very similar to the CCITT V.24 standard used in EUROPE and other locations outside of North Ameria. The term EIA refers to the Electronics Industries Association which is a national body that represents a large percentage of manufacturers in the U.S. electronics industry. The EIA's work in the area of standards has been widely recognized and many of its standards were adopted by other standard bodies. RS-232-C is a recommended standard (RS) published by the EIA in 1969, with the number 232 referencing the identification number of one particular communications standard and the suffix C designating the latest revision to that standard.

Scope

RS-232-C standard is applicable to the interconnection of DTE and DCE employing serial binary data interchange.

• It defines the mechanical characteristics of the DTE and DCE.
• RS-232-C defines the electrical characteristics of the interchange signals and associated circuitry.
• This standard describes the protocols that are required to connect a DTE to a DCE.
• It describes the communication sequence between two DTEs when the DTEs are connected via modems and telephone line.

Mechanical Characteristics

Male/Female Connectors

The interface between the DTE and DCE is located at a pluggable connector signal interface point between the two equipment. RS-232 specifies that the DTE connector should be a male, and the DCE connector should be a female (Fig.1). A specific connector is not given, but the most commonly used connectors are the DB-25P male and the DB-25S female.

The use of an extension cable (RS-232 cable) between DTE and DCE is permitted. It will have both male and female connectors. Female part of the cable will be connected with DTE and male part will connected with DCE.

                    Fig.1 Male/Female Connectors (DTE, DCE and Cable)

The use of short cables (each less than approximately 50 feet or 15 meters) is recommended; however, longer cables are permissible, provided that the resulting load capacitance (C_L of figure 4) measured at the interface point and including the signal terminator, does not exceed 2500 picofarads.

Pin Identification

We have seen in the earlier section that a DTE has a male connector and DCE has a female connector. Each of these connectors will have total 25 pins (fig.2).

Figure - 2 : A DTE with Male Connector (25 pins)

Pin Assignments of these 25 pins are listed in Fig. 3.

Figure 3 - RS-232C Pin names and signal descriptions

Electrical Signal Characteristics

Interchange Equivalent Circuit

Fig.4 interchange equivalent circuit, shows the electrical parameters which are specified in the subsequent paragraph of this section. The equivalent circuit shown in Fig.4 is applicable to all the interchange circuits regardless of category (data, timing or control) to which they belong. The equivalent circuit is independent of whether the driver is located in the DCE and the terminator in the DTE or vice-versa.

Figure 4 : Interchange Equivalent Circuit

V_O    : is the open-circuit river voltage.

R_O    : is the driver internal dc resistance.

C_O    : is the total effective capacitance associated with the driver, measured at the interface point and including any cable to the interface point.

V₁     : is the voltage at the interface point.

C_L    : is the total effective capacitance associated with the terminator, measured at the interface point and including any cable to the interface point.

R_L    : is the terminator load dc resistance.

E_L    : is the open circuit terminator voltage (bias).

The load impedance (R_L and C_L) of the terminator side of an interchange circuit shall have a dc resistance (R_L) of not less than 3000 Ohms, measured with an applied voltage not greater than 25 volts in magnitude, or more than 7000 Ohms, measured with an applied voltage of 3 to 25 volts in magnitude.  The effective shunt capacitance (C_L) of the terminator side of an interchange circuit, measured at the interface point, shall not exceed 2500 picofarads. The reactive component of the load impedance shall not be inductive. The open circuit terminator voltage (E_L) shall not exceed 2 volts in magnitude.

Voltage Levels

The RS-232C interface specifies 25 interchange circuits or conductors that govern the data flow between the DTE and DCE. Although one can purchase a 25-conductor cable, normally a smaller number of conductors are required. For asynchronous transmission, normally 9 to 12 conductors are required, while synchronous transmission typically requires 12 to 16 conductors, with the number of conductors required a function of the operation characteristics of the modem to be connected.  Signals are transmitted through these conductors.  A signal is considered to be ON when the voltage (V1) on the interchange circuits is between +3V and +15V (figure 5). In comparison, a voltage between -3V and -15V causes the interchange circuit to be placed in the OFF condition. The voltage range from +3V to -3V is a transition region that has no effect upon the condition of the circuit. Figure 6 provides a comparison between the interchange circuit voltage, its binary state, signal condition and function.

            +15V  ...................................................................................................

                                                                                Positive range ON function

            +3 V   ....................................................................................................

                                                                                                Transition region

            -3 V   .....................................................................................................

                                                                                Negative range OFF function

            -15 V ......................................................................................................

     Figure 5 Interchange Circuit Voltage Ranges

Figure 6 : Interchange Circuit Comparison