RS-232 Cables, Wiring and Pinouts

September 14th, 2015
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RS-232 Cables, Wiring and Pinouts

RS-232 standards(TIA-232) are defined by TIA (Telecommunications Industry Association). RS-232 defines both the physical and electrical characteristics of the interface. RS-232 is practically identical to ITU V.24 (signal description and names) and V.28 (electrical). RS232 is an Active LOW voltage driven interface and operates at +12V to -12V where:

  1. Signal = 0 (LOW) > +3.0V (SPACE)
  2. Signal = 1 (HIGH) < -3.0V (MARK)

DTE (PC) and DCE (Modem)

In serial communications the terminal end (PC) is called the Data Terminal Equipment (DTE) and the modem end is called the Data Communications Equipment (DCE) as shown in the diagram below.


RS-232 signals have a direction (in or out) depending on whether they are with respect to a DTE or a DCE. In all the pinout diagrams below the signal direction is with respect to the DTE (PC) end.



NULL Modem Connections

When PCs are connected back-to-back each end is acting as a DTE (there is no DCE in this case) and consequently certain signals may have to be looped in the connection to satisfy any input signal requirement. This is called a NULL (no) modem configuration. For example, when the DTE raises Request to Send (RTS) it typically expects Clear to Send (CTS) from the DCE. Since there is no DCE to raise CTS, the outgoing RTS signal is looped in the NULL modem cable to the incoming CTS to satisfy the DTE’s need for this signal. This is shown in the diagram below.


DB9 and DB25 Male and Female Pin Numbering

These diagrams show the male (grey background) and female (black background) pin numbering for DB9 and DB25 sub-miniature connectors. Generally Pin 1 is marked on the front of the connector right next to the pin – though you may need a magnifying glass to read it. Some manufacturers mark each pin number on the plastic housing at the rear of the connector. The male connector has the pins sticking out!

DB25 Male and Female


DB25: View looking into male connector


DB25: View looking into female connector


DB9 Male and Female


DB9: View looking into male connector


DB9: View looking into female connector



RS232 on DB25 (RS-232C)


Pin No.NameDirNotes/Description
1Protective/shielded ground
2TDOUTTransmit Data (a.k.a TxD, Tx) (ASYNC)
3RDINReceive Data (a.k.a RxD, Rx) (ASYNC)
4RTSOUTRequest To Send (ASYNC)
5CTSINClear To Send (ASYNC)
6DSRINData Set Ready (ASYNC)
7SGNDSignal Ground
8CDINCarrier Detect (a.k.a DCD).
9Reserved for data set testing.
10Reserved for data set testing.
12SDCDINSecondary Carrier Detect. Only needed if second channel being used.
13SCTSINSecondary Clear to send. Only needed if second channel being used.
14STDOUTSecondary Transmit Data. Only needed if second channel being used.
15DBOUTTransmit Clock (a.k.a TCLK, TxCLK). Synchronous use only.
16SRDINSecondary Receive Data. Only needed if second channel being used.
17DDINReceive Clock (a.k.a. RCLK). Synchronous use only.
18LLLocal Loopback
19SRTSOUTSecondary Request to Send. Only needed if second channel being used.
20DTROUTData Terminal Ready. (ASYNC)
21RL/SQSignal Quality Detector/Remote loopback
22RIINRing Indicator. DCE (Modem) raises when incoming call detected used for auto answer applications.
23CH/CIOUTSignal Rate selector.
24DAAuxiliary Clock (a.k.a. ACLK). Secondary Channel only.


RS232 on DB9 (EIA/TIA 574)



Pin No.NameDirNotes/Description
1DCDINData Carrier Detect. Raised by DCE when modem synchronized.
2RDINReceive Data (a.k.a RxD, Rx). Arriving data from DCE.
3TDOUTTransmit Data (a.k.a TxD, Tx). Sending data from DTE.
4DTROUTData Terminal Ready. Raised by DTE when powered on. In auto-answer mode raised only when RI arrives from DCE.
6DSRINData Set Ready. Raised by DCE to indicate ready.
7RTSOUTRequest To Send. Raised by DTE when it wishes to send. Expects CTS from DCE.
8CTSINClear To Send. Raised by DCE in response to RTS from DTE.
9RIINRing Indicator. Set when incoming ring detected – used for auto-answer application. DTE raised DTR to answer.


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