ITU/CCITT-EIA-Bell RS-232 Pin Assignment Reference

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The EIA RS-232 Standard describes an electrical interconnection system for low speed serial communications. The standard stipulates the use of a subminature D connector with 25 pins (the DB 25-pin) connector. However, RS-232 compliant-transmissions can also be found using DB 9-pin and RJ45 connectors. The following table shows which signals are assigned to which pins in the various cabling systems.

DB 25-Pin DB 9-Pin
(IBM PC subset)
DIN 8-Pin
(Macintosh subset)
Circuit Name
Signal Function
1 5     FG 101 AA Ground Frame Ground
2 3 3 3 TD 103 BA To DCE Transmitted Data
3 2 5 4 RD 104 BB To DTE Received Data
4 7 1 8 RTS 105 CA To DCE Request to Send
5 8   7 CTS 106 CB To DTE Clear to Send
6 6 2   DSR 107 CC To DTE Data Set Ready
7 5 4,8 1, 2 SG 102 AB Ground Signal Ground
8 1 2 5 DCD 109 CF To DTE Data Carrier Detect
11       QM Bell 208A To DTE Equalizer Mode
12       SDCD 122 SCF To DTE Secondary Data Carrier Detect
13       SCTS 121 SCB To DTE Secondary Clear To Send
14       STD 118 SBA To DCE Secondary Transmitted Data
14       NS Bell 208 To DCE New Sync
15       TC 114 DB To DTE Transmitter Clock
16       SRD 119 SBB To DTE Secondary Received Data
16       DCT Bell 208 To DTE Divided Clock Transmitter
17       RC 115 DD To DTE Receiver Clock
18       DCR Bell 208 To DTE Divided Clock Receiver
19       SRTS 120 SCA To DCE Secondary Request To Send
20 4 1 6 DTR 108.2 CD To DCE Data Terminal Ready
21       SQ 110 CG To DTE Signal Quality Detect
22 9     RI 125 CE To DTE Ring Indicator
23         111 CH To DCE Data Rate Selector
23         112 CI To DTE Data Rate Selector
24       TC 113 DA To DCE External Transmitter Clock
25         Bell 113B To DCE Busy
(Information on printing color tables on color printers can be found here.)

Description of Table Columns

DB 25-Pin This is the name for the 25-pin subminature D connector most commonly-associated with RS-232 cabling systems. The subminature D connector consists of gold-plated (or a related compound) pins organized in two rows. The first row contains 13 pins numbered 1 through 13. The 12 pins in the second row are numbered 14 through 25 and are offset from the pins in the first row. When viewing the male DB 25-pin connector (contact pins facing you), pins 1 and 14 are on the left. The matching female connector (contact sockets facing you) has pins 1 and 14 on the right.

Until the IBM-PC was introduced, RS-232 signals were invariably provided via DB 25-pin connector systems.

DB 9-Pin When IBM introduced the IBM-PC in 1981, it offered a plug-in card containing a parallel printer port and a RS-232 serial port. However, due to mechanical size limitations of the plug-in cards that IBM used in this system (later known as ISA cards), IBM elected to deviate from the established use of a DB 25-pin connector for RS-232 use. Instead, IBM selected the nine most commonly used signals in the RS-232 standard and provided them using a DB 9-pin connector.

IBM sold translator cables which would allow traditional RS-232 devices to connect the IBM-PC. IBM apparently expected that peripherals which used RS-232 signaling would migrate to IBMs chosen nine-pin subset, but this did not occur. To this day, PC-compatible computer owners still purchase DB 9-to-DB 25 cables so that they can connect external modems and other serial devices to their computers.

The DB 9-pin contains the signals on pins 1 through 8, pin 20 and 22, which are: TD, RD, RTS, CTS, DSR, SG, DCD, DTR and RI. This is considered "full" modem control for asychronous modems. Pins above 9 are mainly for synchronous signaling systems, or in cases where a primary and secondary data channel are used.

Normally when DB 9-pin cabling are connected to full DB 25-pin cabling, SG and FG are connected together.

Macintosh mini-DIN 8-pin The Apple Macintosh computer used an eight-pin mini-DIN connector for its RS-232 serial connections. It uses a subset of signals similar to that used on the IBM-PC DB 9-pin connector, Apple elected to drop RI and CTS.
RJ45 The idea of using RJ45 cables for RS-232 signals first appeared in the mid-1980s in PC-compatible computers running XENIX and similar multi-user operating systems. The need to have more than one or two serial ports per ISA slot required a higher connector density than the DB 25-pin or the DB 9-pin would provide. The earliest examples included as many as six RJ45 connectors on the edge of an ISA card.

The signals selected for the RJ45 subset of RS-232 are identical to those used by IBM in the DB 9-pin, with the exception of Ring Indicator.

The standard solid color code used in "satin-flex" telephone cabling and most pre-built DB-9 to RJ-45 or DB-25 to RJ-45 assemblies is shown in the table above.

As viewing a RJ-45 connector with the pins facing up and the connector facing away from you (cable closest), the USOC numbering of the pins is from left to right is 4 5 3 1 2 6 7 8. So for an EIA 568A cable, pins 1 and 2 are Blue and White, pins 3 and 4 are White and Orange, pins 5 and 6 White and Green, and pins 7 and 8 are White and Brown. EIA 568B exchanges the positions of Orange and Green pairs, so pins 1 and 2 are Blue and White, pins 3 and 4 are White and Green, pins 5 and 6 are White and Orange, and pins 7 and 8 are White and Brown. In all cases, pairs are positioned so that the long (or solid) White color wire of each pair appears on every other contect, starting with the left-most contact. This means that the center pair (pins 1 and 2 ) have the Blue and White colors in reverse order than occurs in all other pairs.

Common Symbolic Name These two, three or four-letter abbreviations for the various signals frequently appear on status indicators and other markings that had limited space available.

In this column of the table, these are color-coded by function. the color Red is used to indicate signals used for data, White for flow control, Orange for modem/carrier control, Purple for clocks, and Green for ground.

Note that Frame Ground is sometimes known as Protective Ground with the symbol PG.

ITU/CCITT Circuit Name The ITU (formally the CCITT) elected to use numbers as designators for each electrical circuit in the RS-232 signalling system. Some connectors have no official designation, usually because AT&T was using that contact in a similar connector for some function specific to a proprietary device that AT&T sold. A perfect example is pin 11, which is used in the Bell 208A modem to control the equalizers used to compensate for artifacts of the loop that the modem was connected to. No subsequent RS-232 device needed such a service controlled remotely, so the Bell 208A remains as the only device that used pin 11 for anything. The ITU/CCITT and the EIA have never designated another use for this pin, but they have re-used some other pins that AT&T had used previously for other purposes.

EIA Circuit Name The EIA (an American standards body) decided to create the RS-232 standard (now in revision E or RS-232-E), and in their standard, they used letter codes to identify each circuit in the RS-232 signaling system. The assignment of function to pin were copied with few changed for the ITU/CCITT standard.

Signal Direction The signals are presented from the perspective of a DTE device, in that it transmits data to a DCE device (such as a modem) on pin 2. This signal is marked "To DCE".

DTE is the Data Terminal Equipment, such as a serial terminal or a desktop computer.
DCE is the Data Communications Equipment, usually a modem or similar protocol or modulation conversion device. Some mainframe or minicomputers with serial ports have their connectors configured as DCE devices so that DTE devices may be connected without conversion cables. A NULL modem performs a similar conversion of signals.

In the table, DCE signals are color-coded Yellow, DTE signals are Blue and Ground signals are Green.

Signal Function A brief description of what the signal is used for.

Related Topics

Standard Cable Color-Coding Reference (HTML)

Data/FAX Modem Reference Index (HTML)

[Copyright 1982,2000,2001,2002,2004,2012 Frank Durda IV, All Rights Reserved.
Mirroring of any material on this site in any form is expressly prohibited.
The official web site for this material is:
Contact this address for use clearances: clearance at
Comments and queries to this address: web_reference at]

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