RS232 Cables and Wiring

RS232 Wiring Stuff

RS232 standards are defined by EIA/TIA (Electronic Industries Alliance /Telecommunications Industry Association). RS232 defines both the physical and electrical characteristics of the interface. RS232 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:

Signal = 0 (LOW) > +3.0V

Signal = 1 (HIGH) < -3.0V

Signal voltages in the range >-3.0V to +3.0V is regarded as the 'dead area' and allows for absorption of noise.

Watch the terms DTE (Data Terminal Equipment - e.g. a terminal or PC) and DCE (Data communications Equipment - e.g. a modem) the meaning and use of certain pins may differ. All the diagrams below define the interface from the DTE perspective. An RS232 interface has a female and male connector, the male connector has got the bits sticking out!!

NOTE: The terms Data Carrier Detect (DCD) and Received Line Signal Detect (RLSD) are one and the same. We use DCD throughout 'cos we think its more common.

RS232 on DB25 (RS-232C)

Signal/pin primer

Pin No. Name Notes/Description
1 - Protective/shielded ground
2 TD Transmit Data (a.k.a TxD, Tx)
3 RD Receive Data (a.k.a RxD, Rx)
4 RTS Request To Send
5 CTS Clear To Send
6 DSR Data Set Ready
7 SGND Signal Ground
8 CD Carrier Detect (a.k.a DCD)
9 - Reserved for data set testing
10 - Reserved for data set testing
11 - Unassigned
12 SDCD Secondary Carrier Detect
13 SCTS Secondary Clear to send
14 STD Secondary Transmit Data
15 DB Transmit Clock (a.k.a TCLK, TxCLK)
16 SRD Secondary Receive Data
17 DD Receive Clock (a.k.a. RCLK)
18 LL Local Loopback
19 SRTS Secondary Request to Send
20 DTR Data Terminal Ready
21 RL/SQ Signal Quality Detector/Remote loopback
22 RI Ring Indicator (DCE raises when incoming call detected used for auto answer applications)
23 CH/CI Signal Rate selector
24 DA Auxiliary Clock (a.k.a. ACLK)
25 - Unassigned

view - looking into male connector

RS232 on DB9 (EIA/TIA 574)

Signal/pin primer

Pin No. Name Notes/Description
1 DCD Data Carrier Detect
2 RD Receive Data (a.k.a RxD, Rx)
3 TD Transmit Data (a.k.a TxD, Tx)
4 DTR Data Terminal Ready
5 SGND Ground
6 DSR Data Set Ready
7 RTS Request To Send
8 CTS Clear To Send
9 RI Ring Indicator

View - looking into male connector

RS232 on RJ45 (RS-232D)

More properly EIA/TIA - 561. Use when connecting to or from a serial port with a 8 position Modular Jack (RJ45). If you are cross-connecting from a DB9 or a DB25 use the signal names to cross connect the appropriate connections.

Signal/pin primer

Pin No. Name Notes/Description
1 DSR/RI Data set Ready/ring indicator
2 DCD Data Carrier Detect
3 DTR Data Terminal Ready
4 SGND Signal Ground
5 RD Receive Data
6 TD Transmit Data
7 CTS Clear to Send
8 RTS Request to Send

RJ45 Male Connector Pin Numbering

RS232 DB25 NULL Modem Pinout

Use when connecting two systems (e.g. PCs) via their DB25 interfaces without a modem (i.e. back-to-back). See the full signal names in the DB25 sections.

Signal/pin primer

DB25 Signal DB25 Signal
3 RD 2 TD
2 TD 3 RD
20 DTR 6,8 DSR, DCD
6,8 DSR, DCD 20 DTR
4 RTS 5 CTS
5 CTS 4 RTS<
7 SGND 7 SGND
22 RI 22 RI

NOTE: Leave all pins not specified above unconnected.

RS232 DB9 NULL Modem Pinout

Use when connecting two systems (e.g. PCs) via their DB9 interfaces without a modem (i.e. back-to-back). See the full signal names in the DB9 section.

Signal/pin primer

DB9 Signal DB9 Signal
2 RD 3 TD
3 TD 2 RD
4 DTR 6,1 DSR, DCD
6,1 DSR, DCD 4 DTR
7 RTS 8 CTS
8 CTS 7 RTS
5 SGND 5 SGND
9 RI 9 RI

NOTE: Leave all pins not specified above unconnected.

RS232 DB9 to DB25 Pinout

Use when connecting a DB9 (e.g. a PC) to a DB25 (e.g. a modem) interface. See the full signal names in the DB9 and DB25 section.

Signal/pin primer

DB9 Signal DB25
1 DCD 8
2 RD 3
3 TD 2
4 DTR 20
5 SGND 7
6 DSR 6
7 RTS 4
8 CTS 5
9 RI 22

RS232 DB9 to DB25 NULL Modem Pinout

Use when connecting two systems (e.g. PCs) when one has a DB9 interface and the other a DB25 interface without a modem (i.e. back-to-back). See the full signal names in the DB9 and DB25 sections.

Signal/pin primer

DB9 Signal DB25 Signal
2 RD 2 TD
3 TD 3 RD
4 DTR 6,8 DSR, DCD
6,1 DSR, DCD 20 DTR
7 RTS 5 CTS
8 CTS 4 RTS
5 SGND 7 SGND
9 RI 22 RI

V.35 on DB25 (RS-530-A)

The A and B below refer to each signal pair used in balanced serial interfaces. BEWARE: RS-530 is wired differently. This is the 530-A pinout spec.

Signal/pin primer

Pin No. Name Notes/Description
1 Shield -
2 Transmit Data (A) -
3 Received Data (A) -
4 RTS (A) Request To Send
5 CTS (A) Clear To Send
6 DCE Ready Data Communications Equipment Ready (modem/CSU)
7 Signal Common -
8 DCD (A) Data Carrier Detect (a.k.a CD or RLSD)
9 Receiver Signal Element Timing (B) RX Clock
10 DCD (B) Data Carrier Detect (a.k.a CD or RLSD)
11 Ext Transmit Clock (B) -
12 Tramsmit Signal Element Timing (B) TX CLOCK
13 CTS (B) Clear to Send
14 Transmitted Data (B) Secondary Transmit Data
15 Transmit Signal element Timing (A) TX CLOCK
16 Received Data (B) RX Data
17 Receiver Signal Element Timing (A) RX CLOCK
18 Local Loopback -
19 RTS (B) Request to Send
20 DTE Ready Host end
21 Remote Loopback -
22 RI Ring Indicator
23 Signal Common -
24 Ext TX Clock (A) -
25 Test Mode -

view - looking into male connector

 

Tech Info - Centronics

No one remembers Centronics (well I do) but their connector is still used to this day (is this a legacy or what!).

Centronic Printer Connector

An exotic 36 pin connector invented (I guess) by the old Centronics company who made printers (nah!!).

DB9 Male

Connector Name Function/Notes Connector Name Function/Notes
1 Data Strobe Active LOW 19 Data Strobe RETURN
2 Data Bit 1 LSB 20 Data Bit 1 RETURN
3 Data Bit 2 - 21 Data Bit 2 RETURN
4 Data Bit 3 - 22 Data Bit 3 RETURN
5 Data Bit 4 - 23 Data Bit 4 RETURN
6 Data Bit 5 - 24 Data Bit 5 RETURN
7 Data Bit 6 - 25 Data Bit 6 RETURN
8 Data Bit 7 - 26 Data Bit 7 RETURN
9 Data Bit 8 MSB 27 Data Bit 8 RETURN
10 Acknowledge Active LOW 28 Acknowledge RETURN
11 Busy Active HIGH 29 Busy RETURN
12 Paper End Active HIGH 30 Input Prime RETURN
13 Select Active HIGH 31 Input Prime Active LOW
14 Supply Ground - 32 Fault Active LOW
15 Oscillator Tx - 33 Undefined -
16 Logic Ground - 34 Undefined -
17 Chassis Ground - 35 Undefined -
18 +5V Power 36 Undefined -