DS232B Version 1.8

© Future Technology Devices Intl. Ltd. 2005

Page 19 of 25

FT232BM USB UART ( USB - Serial) I.C.

Figure 11

USB <=> RS485 Converter Configuration

DB9-M

DM

GND

120R

DP

VCC

4

3

6

2

1

7

8

5

D

R

SP481

FT232BM

PWREN#

SLEEP#

TXD

RXD

RTS#

CTS#

DTR#

DSR#

DCD#

RI#

10

TXDEN

25

24

23

22

15

21

20

19

18

16

LINK

Figure 11 illustrates how to connect the UART interface of the FT232BM to a TTL – RS485 Level Converter I.C. to

make a USB => RS485 converter. This example uses the Sipex SP481 device but there are similar parts available

from Maxim and Analog Devices amongst others. The SP481 is a RS485 device in a compact 8 pin SOP package.

It has separate enables on both the transmitter and receiver. With RS485, the transmitter is only enabled when a

character is being transmitted from the UART. The TXDEN pin on the FT232BM is provided for exactly that purpose

and so the transmitter enable is wired to TXDEN. The receiver enable is active low, so it is wired to the PWREN# pin

to disable the receiver when in USB suspend mode.

RS485 is a multi-drop network – i.e. many devices can communicate with each other over a single two wire cable

connection. The RS485 cable requires to be terminated at each end of the cable. A link is provided to allow the cable

to be terminated if the device is physically positioned at either end of the cable.

In this example the data transmitted by the FT232BM is also received by the device that is transmitting. This is a

common feature of RS485 and requires the application software to remove the transmitted data from the received

data stream. With the FT232BM it is possible to do this entirely in hardware – simply modify the schematic so that

RXD of the FT232BM is the logical OR of the SP481 receiver output with TXDEN using an HC32 or similar logic gate.