Copyright © 2010 Future Technology Devices International Limited

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Document Reference No.: FT_000204

DB9-USB-RS232 Datasheet

Version 1.2

Clearance No.: FTDI# 130

2.1.2 Replacing an RS232 DB9 MALE (DTE defined) Connector

The DB9-USB-RS232-M can be used to replace a male DB9 connector used for transmitting RS232

protocol. With the DB9-USB-RS232-M in place instead of the standard USB connector a USB bridge is

created, this allow the application to communicate with other devices via USB. Installing the DB9-USB-

RS232-M is simple. Simply replace the male DB9 connector with the DB9-USB-RS232-M connector (same

PCB footprint), install drivers and the device is ready to use.

Table 2.2 gives the pin out description of each pad of an RS232 footprint. Figure 2.2 gives a description

of the connections between the DB9-USB-RS232-M and the footprint of a male DB9 module.

Pin

Number

Pin Type of DB9-

USB-RS232 module

Pin Type at

application PCB

Description

1

Input (note1)

Input

DCD = Data Carrier Detect

2

Output

Input

RXD = Receive Data

(this is an output from the DB9-USB-

RS232-M to the application Rx input)

3

Input

Output

TXD = Transmit Data

(this is an input to the DB9-USB-RS232-M

from the application Tx output)

4

Input

Output

DTR = Data Terminal Ready

(this is an input to the DB9-USB-RS232-M

from the application DTR output)

5

Ground

Ground

GND = RS232 signal ground

6

Output

Input

DSR = Data Set Ready

(this is an output from the DB9-USB-

RS232-M to the application DSR input)

7

Input

Output

RTS = Request To Send

(this is an input to the DB9-USB-RS232-M

from the application RTS output)

8

Output

Input

CTS = Clear To Send

(this is an output from the DB9-USB-

RS232-M to the application CTS input)

9

Input (note1)

Input

RI = Ring Indicator

Shield

Ground

Case Ground

Drain = typically connected to the host PC

case

Table 2.2 – A Male RS232 DB9 footprint Pin-Out

with a null-modem wiring. The null-modem wiring allows DTE to DTE communications between the DB9-

USB-RS232-M and a standard DB9 male application board. However a DTE device is defined to have 5

inputs and 3 outputs, where a DCE (Data Communications Equipment) is defined to have 3 inputs and 5

outputs as in the case of the female DB9 application boards. This leads to a conflict in an DTE to DTE

connection application, This conflict is that there are more inputs to a connected USB-converter device than

there are outputs from the application board (and more inputs to the application board than there are outputs

of the USB converter). This conflict occurs on the DCD and RI connections and as such they cannot be

used. This issue is very common when translating RS232 to USB from a DTE perspective. Additional

information showing DTE and DCE wiring is given in Appendix B.

Figure 2.1 illustrates these connections from a PCB footprint point of view. Figure 2.2 Illustrates the

connection made to an example PCB.