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RS-232 Serial Cables

Serial communication is the process of transferring data one bit at a time, as opposed to parallel communication, where several bits are sent at the same time over multiple parallel lines. One could consider serial communication analogous to Morse code.

There are many types of serial communication (RS-422, RS-485, Ethernet, USB, etc.), but for the purpose of this white paper we will be focusing on RS-232 – a relatively simple, inexpensive and standardized means of data transfer and control that requires minimal supporting software from the host device.

On a very basic level, a serial cable only requires three signals: Transmit, Receive, and Signal Ground (see below for common abbreviations). Many devices also require additional control signals to regulate the flow of informationin in order to prevent potential data loss. All valid signals are defined as positive or negative voltage relative to the common Signal Ground.

The original Electronic Industries Association (EIA) standard RS-232-C recommended but did not require the use of DB-25 connectors in part because of the 20 different signal connections. As very few devices require more than a handful of these connections, many computer manufacturers have opted for smaller connectors. The most popular of these is the DE-9, commonly (and incorrectly) referred to as DB9. Either or both of these connectors can be used for a serial cable.

There are two general types of RS-232 serial cables: standard straight through cables (wired 1-1, 2-2, 3-3, etc.), and null modem cables. Null modem cables are not mentioned in the standard, so there is no official pinout for them. Most RS-232 applications only require seven to nine wires, though a DB25 cable with all pins connected can also be used for RS-530 or parallel printer connections if twisted pair cable is used and it is wired correctly. Below is a list of RS-232 signals and their pin assignments.

DB-25 Pin

DE-9 Pin

Name

Notes/Description

1

 

 

Shield Ground

2

3

TxD

Transmit Data (a.k.a. TD, Tx)

3

2

RxD

Receive Data (a.k.a. RD, Rx)

4

7

RTS

Request To Send

5

8

CTS

Clear To Send

6

6

DSR

Data Set Ready

7

5

SGND

Signal Ground (a.k.a. SG)

8

1

DCD

Data Carrier Detect (a.k.a. CD)

9

 

 

Reserved

10

 

 

Reserved

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, RxCLK)

18

 

LL

Local Loopback

19

 

SRTS

Secondary Request To Send

20

4

DTR

Data Terminal Ready

21

 

RL/SQ

Remote Loopback, Signal Quality Detector

22

9

RI

Ring Indicator

23

 

CH/CI

Signal Rate Selector

24

 

DA

Auxiliary Clock (a.k.a. ACLK)

25

 

 

Unassigned

 

The maximum length of a serial cable depends on a number of factors. These factors include various characteristics of the transmitters and receivers, baud rate, and the capacitance and impedance of the cable. The RS-232 standard requires that a serial port provide signals appropriate for a capacitive load of 2,500 pF (picofarads). Therefore, using a low capacitance cable will allow signal integrity to be maintained at a greater distance. Properly shielded and grounded cable is also recommended, though for some installations the potential for causing ground loops should be considered.

Our Networx brand Serial Cables are designed to work for most common applications. We can also make custom Serial cables to any configuration and length for your specific needs.