Serial Wiring

NMSWhen discussing serial connections for computers you are normally referring to the RS232 interface. Leads Direct sells a wide range of RS232 compatible serial leads and cables in both 9 and 25 pin configurations, including straight through and Null Modem leads as well as USB and Firewire which are also types of serial connection.

You can view and purchase these leads by visiting our Serial Leads which has literally thousands of products each with two sizes of image.

The RS232 serial port passes data in serial format (1 bit at a time over one line). This port supports a variety of devices, including a serial mouse, serial printer or modem that requires serial data transmission. Possibly the most common serial port is a 9 pin D shape.

Connections to these connectors are most commonly made by soldering the cables to the ‘solder bucket’ terminals at the rear of the connector. Note that the same connector is often used both for chassis mounting and for cable mounting, in the latter case with the addition of a plastic or metal hood.

DB9 solder tags

Solder tags at the rear of a DB9 serial connector

Modern computers ship with both Com1 and Com2 in this style, usually assembled to the motherboard as part of the ATX format.

Older machines use a 25 pin D shaped connector for Com2, the secondary serial port.

The pins are numbered left to right across the first row, and then the same on the second row, as per the diagram below:

DB9

The pin numbers are usually stamped into the moulding at the rear of the connector making the selection of the correct pins quite an easy task.

Standard 9 Pin RS232 Serial Wiring

Pin Signal I/O Definition
1 DCD I Data carrier detect
2 RXDA I Receive data
3 TXDA O Transmit data
4 DTR O Data terminal ready
5 GND N/A Signal ground
6 DSR I Data set ready
7 RTS O Request to send
8 CTS I Clear to send
9 RI I Ring indicator
Shell N/A N/A Frame ground

 Standard 9 Pin RS232 Null Modem Wiring

Null Modem leads and cables are used to connect two serial interfaces together, typically for data transfer using two female connectors.

These are also used for interfaces between computers and other products such as barcode scanners and till control in EPOS systems, below is the standard Null Modem pinout as used in our products. Leads Direct can offer a wide range of ready made Null Modem cables as well as custom ones made to your specifications.

Pin Pin
1 n/c
2 3
3 2
4 6
5 5
6 4
7 8
8 7
9 n/c
Shell Shell or n/c

Standard 25 Pin RS232 Serial Wiring

Pin Signal I/O Description
1
GND  N/A Shield Ground
2
TXD
O
Transmit Data
3
RXD
I
Receive Data
4
RTS
O
Request to Send
5
CTS
I
Clear to Send
6
DSR
I
Data Set Ready
7
GND  N/A System Ground
8
CD
I
Carrier Detect
9
N/A
RESERVED
10
N/A
RESERVED
11
STF
O
Select Transmit Channel
12
S.CD
I
Secondary Carrier Detect
13
S.CTS
I
Secondary Clear to Send
14
S.TXD
O
Secondary Transmit Data
15
TCK
I
Transmission Signal Element Timing
16
S.RXD
I
Secondary Receive Data
17
RCK
I
Receiver Signal Element Timing
18
LL
O
Local Loop Control
19
S.RTS
O
Secondary Request to Send
20
DTR
O
Data Terminal Ready
21
RL
O
Remote Loop Control
22
RI
I
Ring Indicator
23
DSR
O
Data Signal Rate Selector
24
XCK
O
Transmit Signal Element Timing
25
TI
I
Test Indicator
Note: Shield Ground (pin 1) should not be connected to System Ground (pin 7)

 9 Pin to 25 Pin RS232 Converter Wiring

If you need to convert a DB25 cable to run on a DB9 port, the following pinout will enable you to make such a cable.  Alternatively, one can easily be made for you by the team at Leads Direct.

DB9 DB25 Function
1 8 Data carrier detect
2 3 Receive data
3 2 Transmit data
4 20 Data terminal ready
5 7 Signal ground
6 6 Data set ready
7 4 Request to send
8 5 Clear to send
9 22 Ring indicator
If you are seeking information on Serial wiring but cannot find it here, please email info@leadsdirect.co.uk and we will try to both answer your question and make sure that the information is made available through these pages for future reference

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