RS232 serial port information

An RS232 connection occurs between two devices, a “Data Terminal Equipment” (DTE), among which, for example, computers and terminals can be mentioned; and another, typically a “Data Communications Equipment” (DCE), including modems or printers. It is important to always know whether the device being connected is a DTE or a DCE, to determine the signals and their direction on the interface connector. In our production you can find a wide range of interconnection cables between the different devices.

Serial line signals

Initially, the serial line signals were assigned for the 25-pin connector, and the ability to assign a second channel was also provided. However, only one channel has always been used and, for this reason, over time, there has been an orientation towards the use of smaller connectors with a lower number of pins. The signals of the standard RS232 connection are shown in the following table:

Name Description DTE DCE DB25 DB9 DIN 8 RJ45
All connectors are seen from the front
FG Frame ground 1
TD Transmitted data Oct In 2 3 3 6
RD Received data In Oct 3 2 5 5
RTS Request to send Oct In 4 7 6 8
CTS Clear to send In Oct 5 8 2 7
DSR Data set ready In Oct 6 6
SG Signal ground 7 5 4,8 4
DCD Data carrier detected In Oct 8 1 7 2
Positive voltage 9
Negative voltage 10
Unassigned 11
SDCD Secondary DCD In Oct 12
SCTS Secondary CTS In Oct 13
STD Secondary TD Oct In 14
TC Transmit clock In Oct

15

SRD Secondary RD In Oct 16
RC Receive clock In Oct 17
Unassigned 18
SRTS Secondary RTS Oct In 19
DTR Data terminal ready Oct In 20 4 1 3
SQ Signal quality detector 21
RI Ring indicator In Oct 22  9
DRS Data rate selector 23
SCTE Serial clock transmit external Oct In 24
BUSY Busy 25

RS232 connectors

For convenience, we show a view of the connectors for the RS232 connection which the commonly used devices are equipped with.

DTE Equipment) Personal computer DB 9 male connector Pin Direction Signal Description
1 In DCD Data carrier detect
2 In RD Receive data
3 Oct TD Transmit data
4 Oct DTR Data terminal ready
5 GND Signal ground
6 In DSR Data set ready
7 Oct RTS Request to send
8 In CTS Clear to send
9 In RI Ring indicator
(DCE Equipment) DB Modem 9 female connector Pin Direction Signal Description
1 Oct DCD Data carrier detect
2 Oct TD Transmit data
3 In RD Receive data
4 In DTR Data terminal ready
5 GND Signal ground
6 Oct DSR Data set ready
7 In CTS Clear to send
8 Oct RTS Request to send
9 Oct RI Ring indicator
(DTE Equipment) ELSIST products RJ45 8pin female connector Pin Direction Signal Description
1
2 In DCD Data carrier detect
3 Oct DTR Data terminal ready
4 GND Signal ground
5 In RD Receive data
6 Oct TD Transmit data
7 In CTS Clear to send
8 Oct RTS Request to send
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