Shermie's Place

RS232

 Shermie's Place
Electrical Electronic RJ11 RJ45 RS232

Abbreviations

CTS   Clear To Send [DCE --> DTE]
DCD   Data Carrier Detected (Tone from a modem) [DCE --> DTE]
DCE  Data Communications Equipment eg. modem
DSR   Data Set Ready [DCE --> DTE]
DSRS Data Signal Rate Selector [DCE --> DTE] (Not commonly used)
DTE Data Terminal Equipment eg. computer, printer
DTR Data Terminal Ready [DTE --> DCE]
FG Frame Ground (screen or chassis)
NC No Connection
RCk   Receiver (external) Clock input
RI   Ring Indicator (ringing tone detected)
RTS Ready To Send [DTE --> DCE]
RxD Received Data [DCE --> DTE]
SG  Signal Ground
SCTS  Secondary Clear To Send [DCE --> DTE]
SDCD  Secondary Data Carrier Detected (Tone from a modem) [DCE  --> DTE]
SRTS Secondary Ready To Send [DTE --> DCE]
SRxD  Secondary Received Data [DCE --> DTE]
STxD Secondary Transmitted Data [DTE --> DTE]
TxD Transmitted Data [DTE --> DTE]

Is Your Interface a DTE or a DCE?

Find out by following these steps: The point of reference for all signals is the terminal (or PC).

  1. Measure the DC voltages between (DB25) pins 2 & 7 and between pins 3 & 7. Be sure the black lead is connected to pin 7 (Signal Ground) and the red lead to whichever pin you are measuring.
     

  2. If the voltage on pin 2 (TD) is more negative than -3 Volts, then it is a DTE, otherwise it should be near zero volts.
     

  3. If the voltage on pin 3 (RD) is more negative than -3 Volts, then it is a DCE.
     

  4. If both pins 2 & 3 have a voltage of at least 3 volts, then either you are measuring incorrectly, or your device is not a standard EIA-232 device. Call technical support.
     

  5. In general, a DTE provides a voltage on TD, RTS, & DTR, whereas a DCE provides voltage on RD, CTS, DSR, & CD.
PC Com Port - EIA-574 RS-232 pin out DB-9 pin used for Asynchronous Data
 

 

9 to 25 pin cable, async data, PC AT serial cable

Description Signal 9-pin DTE 25-pin DCE Source DTE or DEC
Carrier Detect CD 1 8 from Modem
Receive Data RD 2 3 from Modem
Transmit Data TD 3 2 from Terminal/Computer
Data Terminal Ready DTR 4 20 from   Terminal/Computer
Signal Ground SG 5 7 from Modem
Data Set Ready DSR 6 6 from Modem
Request to Send RTS 7 4 from   Terminal/Computer
Clear to Send CTS 8 5 from Modem
Ring Indicator RI 9

22

 from Modem

25 pin, RS232, Async. Data

PIN SIGNAL  DESCRIPTION

1 PGND Protective Ground
2 TXD Transmit Data
3 RXD Receive Data
4 RTS Ready To Send
5 CTS Clear To Send
6 DSR Data Set Ready
7 SG Signal Ground
8 CD Carrier Detect
20 DTR Data Terminal Ready
22 RI Ring Indicator

RS232D uses RJ45 type connectors

Pin No. Signal Description
1 DCE Ready, Ring Indicator
2 Received Line Signal Detector
3 DTE Ready
4 Signal Ground
5 Received Data
6 Transmitted Data
7 Clear To Send
8 Request To Send

NULL MODEM