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
1
2
Pin Type of DB9-
USB-RS232 module
Input (note1)
Output
Pin Type at
application PCB
Input
Input
Description
DCD = Data Carrier Detect
RXD = Receive Data
(this is an output from the DB9-USB-
RS232-M to the application Rx input)
TXD = Transmit Data
3
Input
Output
(this is an input to the DB9-USB-RS232-M
from the application Tx output)
DTR = Data Terminal Ready
4
Input
Output
(this is an input to the DB9-USB-RS232-M
from the application DTR output)
GND = RS232 signal ground
DSR = Data Set Ready
5
Ground
Ground
6
Output
Input
(this is an output from the DB9-USB-
RS232-M to the application DSR input)
RTS = Request To Send
7
Input
Output
(this is an input to the DB9-USB-RS232-M
from the application RTS output)
CTS = Clear To Send
8
Output
Input
(this is an output from the DB9-USB-
RS232-M to the application CTS input)
RI = Ring Indicator
9
Shield
Input (note1)
Ground
Input
Case Ground
Drain = typically connected to the host PC
case
Table 2.2 – A Male RS232 DB9 footprint Pin-Out
Note 1:
The DB9-USB-RS232-M is electrically configured to be a DTE (Data Terminating Equipment) device
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.
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