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	Table of Contents

RS-232-C INTERFACE

RS-232
Recommended Standard 232

This was originally a nine-wire interface standard for teletype machines from the Electronics Industry Association. Now in its third revision (RS-232-C), it's the standard for computer serial-port transfers. The RS-232 standard is probably the only computer component that's OVER 40 years old and still working. One wire is used as the ground; the rest are dedicated to detecting carrier signals, managing the timing of data transfer, oh, and sending and receiving data.

from C|NET http://www.cnet.com/Resources/Info/Glossary/Terms/rs232.html

adapted from:
ELECTRONIC WARFARE
AND RADAR SYSTEMS
ENGINEERING HANDBOOK

US Naval Air Warfare Center Weapons Division, Avionics Department TP 8347
http://ewhdbks.mugu.navy.mil/rs-232.htm

RS-232 INTERFACE

Introduction:

The RS-232 interface is the Electronic Industries Association (EIA) standard for the interchange of serial binary data between two devices. It was initially developed by the EIA to standardize the connection of computers with telephone line modems. The standard allows as many as 20 signals to be defined, but gives complete freedom to the user. Three wires are sufficient: send data, receive data, and signal ground. The remaining lines can be hardwired on or off permanently. The signal transmission is bipolar, requiring two voltages, from 5 to 25 volts, of opposite polarity.

Communication Standards:

The industry custom is to use an asynchronous word consisting of: a start bit, seven or eight data bits, an optional parity bit and one or two stop bits. The baud rate at which the word sent is device-dependent. The baud rate is usually 150 times an integer power of 2, ranging from 0 to 7 (150, 300, 600 ,...., 19,200 ). Below 150 baud, many system-unique rates are used. The standard RS-232-C connector has 25 pins, 21 pins which are used in the complete standard. Many of the modem signals are not needed when a computer terminal is connected directly to a computer, and Figure 1 illustrates how some of the "spare" pins should be linked if not needed. Figure 1 also illustrates the pin numbering used in the original DB-25 connector and that commonly used with a DB-9 connector normally used in modern computers.

[ HML note: the original IBM PC-AT computer used a DB-9 connector to replace one of the up-to-then, "more standard" DB-25 RS-232-C serial connectors. The DB-9 has since become a "de-facto standard" RS-232 connector on "IBM-compatible" and most other computers.]

Specifying compliance to RS-232 only establishes that the signal levels in two devices will be compatible and that if both devices use the suggested connector, they may be able to be connected. Compliance to RS-232 does not imply that the devices will be able to communicate or even acknowledge each other's presence.

Table 1 shows the signal names, and functions of the RS-232 serial port pinout.

Table 1. RS-232 Serial Port Pinout (with 25 pin connector)
Name	Pin	Signal Name	Function
AA	1	PG Protective Ground	This line is connected to the chassis ground of the GPIB-232CV. Since the GPIB-232CV chassis ground is not connected to earth ground, pin 1 should be connected on both serial devices.
BA	2	TxD Transmit Data	This line carries serial data from the GPIB-232CV to the serial host.
BB	3	RxD Receive Data	This line carries serial data from the serial host to the GPIB-232CV.
CA	4	RTS Request to Send	This signal line is driven by the GPIB-232CV and when asserted indicates that the GPIB-232CV is ready to accept serial data. The GPIB-232CV unasserts RTS when it is no longer ready to accept serial data because of a buffer full condition.
CB	5	CTS Clear to Send	This signal line is asserted by the serial host and sensed by the GPIB-232CV. When asserted, it indicates that the serial host is ready to accept serial data. When unasserted, it indicates that data transmission should be disabled.
AB	7	SG Signal Ground	This line establishes a reference point for all interface voltages.
CD	20	DTR Data Terminal Ready	This signal line is asserted by the GPIB-232CV to signal that it has been powered on, and is ready to operate.

Table 2 shows a complete pin description.

Table 2. RS-232C Interface Signals (25 pin numbering).
Pin	Description	Pin	Description	Pin	Description
1	Protective Ground	10	(Reserved for Data Set Testing)	19	Secondary Request to Send
2	Transmitted Data	11	Unassigned	20	Data Terminal Ready
3	Received Data	12	Sec. Rec'd. Line Sig. Detector	21	Signal Quality Detector
4	Request to Send	13	Sec. Clear to Send	22	Ring Indicator
5	Clear to Send	14	Secondary Transmitted Data	23	Data Signal Rate Selector (DTE/DCE Source)
6	Data Set Ready	15	Transmission Signal Element Timing (DCE Source)	24	Transmit Signal Element Timing (DTE Source)
7	Signal Ground (Common Return)	16	Secondary Received Data	25	Unassigned
8	Received Line Signal Detector	17	Receiver Signal Element Timing (DCE Source)	-	-
9	(Reserved for Data Set Testing)	18	Unassigned	-	-

Electrical Characteristics: The RS-232-C specifies the signaling rate between the DTE and DCE, and a digital signal is used on all interchange circuits. The RS-232 standard specifies that logic "1" is to be sent as a voltage in the range -15 to -5 V and that logic "0" is to sent as a voltage in the range +5 to +15 V. The standard specifies that voltages of at least 3 V in amplitude will always be recognized correctly at the receiver according to their polarity, so that appreciable attenuation along the line can be tolerated. The transfer rate is rated > 20 kbps and a distance of < 15m. Greater distance and data rates are possible with good design, but it is reasonable to assume that these limits apply in practice as well as in theory. The load impedance of the terminator side of the interface must be between 3000 and 7000 ohms, and not more than 2500pF.

Table 3, summarizes the functional specifications of the most important circuits.

Table 3. RS-232-C Circuit Definitions

Name
Direction
to:
Function

Data Signals

Transmitted Data (BA) DCE Data generated by DTE

Received Data (BB) DTE Data Received by DTE

Timing signals

Transmitter Signal Element Timing (DA) DCE Clocking signal, transitions to ON and OFF occur at center of each signal element

Transmitter Signal Element Timing (DB) DTE Clocking signal, as above; both leads relate to signals on BA

Receiver Signal Element Timing (DD) DTE Clocking signal, as above, for circuit BB

Control Signals

Request to Send (CA) DCE DTE wishes to transmit

Clear to Send (CB) DTE DCE is ready to transmit; response to request to send

Data Set Ready (CC) DTE DCE is ready to operate

Data Terminal Ready (CD) DCE DTE is ready to operate

Ring Indicator (CE) DTE Indicates that DCE is receiving a ringing signal on the communication channel

Carrier Detect (CF) DTE Indicates that DCE is receiving a carrier signal

Signal Quality Detector (CG) DTE Asserted when there is reason to believe there is an error in the received data

Data Signal Rate Selector (CH) DCE Asserted to select the higher of two possible data rates

Data Signal Rate Selector (CI) DTE Asserted to select the higher of two possible data rates

Ground

Protective Ground (AA) N/A Attached to machine frame and possibly external grounds

Signal Ground (AB) N/A Establishes common ground reference for all circuits

Range: The RS-232-C standard specifies that the maximum length of cable between the transmitter and receiver should not exceed 100 feet, Although in practice many systems are used in which the distance between transmitter and receiver exceeds this rather low figure. The limited range of the RS-232C standard is one of its major shortcomings compared with other standards which offer greater ranges within their specifications. One reason why the range of the RS-232C standard is limited is the need to charge and discharge the capacitance of the cable connecting the transmitter and receiver.

Mechanical Characteristics: The connector for the RS-232-C is a 25 pin connector with a specific arrangement of wires. In theory, a 25 wire cable could be used to connect the Data Terminal Equipment (DTE) to the Data Communication Equipment (DCE). The DTE is a device that is acting as a data source , data sink, or both, e.g. a terminal, peripheral or computer. The DCE is a device that provides the functions required to establish, maintain, and terminate a data-transmission connecting, as well as the signal conversion, and coding required for communication between data terminal equipment and data circuit; e.g. a modem. Table 4, shows the complete summary of the RS-232-C, e.g., descriptor, sponsor, data format, etc.

Table 4. Summary of the RS-232-C

Data Format 5- to 8- bit serial

Transfer Type Asynchronous

Error Handling Optional Parity Bit

Connector 25-pin female connector on DCE; 25-pin male connector on DTE

Length 20 meters

Speed 20 kb/s

Remarks RS-232 is used in the microcomputer world for communications between two DTEs. The null-modem is included into one or both connecting devices, and/or cable and is seldom documented. As a result, establishing an RS-232 connection between two DTEs is frequently a difficult task.

Another good reference on RS-232 data busses is at: http://www.rad.com/networks/1995/rs232/pins.htm

excerpted from: http://www.rad.com/networks/1995/rs232/pins.htm

Interface Mechanical Characteristics
The connection of the DCE and the DTE is done with a pluggable connector. The female connector should be associated with the DCE. The following table lists the pin assignments defined by the standard. The type of connector to be used is not mentioned in the standard, but the DB-25 (or on IBM-AT's, a minimal DB-9) connectors are almost always used.

Pin designation for the 25-pin and 9-pin DB connector

includes equivalent CCITT V.24 identification, and signal direction

  DB-25  DB-9   Common  EIA 
  Pin #  Pin #  Name    Name   CCITT  DTE-DCE Formal Name
  -----  -----  -----   -----  -----  ------- -------------------
   1            FG      AA      101      -    Frame Ground
   2     3      TD      BA      103    ---->  Transmitted Data, TxD
   3     2      RD      BB      104    <----  Received Data, RxD
   4     7      RTS     CA      105    ---->  Request To Send
   5     8      CTS     CB      106    <----  Clear To Send
   6     6      DSR     CC      107    <----  Data Set Ready
   7     5      SG      AB      102    ----   Signal Ground, GND
   8     1      DCD     CF      109    <----  Data Carrier Detect
   9            --      --       -       -    +P
  10            --      --       -       -    -P
  11            --      --       -       -    unassigned
  12            SDCD    SCF     122    <----  Secondary Data Carrier Detect
  13            SCTS    SCB     121    <----  Secondary Clear To Send
  14            STD     SBA     118    ---->  Secondary Transmitted Data
  15            TC      DB      114    <----  Transmission Signal Element Timing
  16            SRD     SBB     119    <----  Secondary Received Data
  17            RC      DD      115    ---->  Receiver Signal Element Timing
  18            --      --       -       -    unassigned
  19            SRTS    SCA     120    ---->  Secondary Request To Send
  20     4      DTR     CD      108.2  ---->  Data Terminal Ready
  21            SQ      CG      110    <----  Signal Quality Detector
  22     9      RI      CE      125    <----  Ring Indicator
  23            --      CH/CI 111/112  ---->  Data Signal Rate Selector
  24            --      DA      113    <----  Transmitter Signal Element Timing
  25            --      --       -       -    unassigned

Diagram of the DB-25 and DB-9 connectors

male connectors , front view

    	1                         13         1        5
      _______________________________      _______________
      \  . . . . . . . . . . . . .  /      \  . . . . .  /    
       \  . . . . . . . . . . . .  /        \  . . . .  /     
        ---------------------------          -----------      
	14                      25            6     ** 9

RS-232 Signals

The number preceding each signal name correspond to the pin number defined in the standard

+P (for testing only)
-P (for testing only)