FT232R USB UART I.C. Datasheet Version 1.04

© Future Technology Devices International Ltd. 2005

Page 24

8.2 USB to RS485 Converter Configuration

Figure 16 - Example USB to RS485 Converter Configuration

Figure 16 illustrates how to connect the FT232R’s UART interface to a TTL – RS485 Level Converter I.C. to make a

USB to 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 signal CBUS pin option on the FT232R is provided for exactly this purpose

and so the transmitter enable is wired to CBUS2 which has been configured as TXDEN. Similarly, CBUS3 has been

configured as PWREN#. This signal is used to control the SP481’s receiver enable. The receiver enable is active low,

so it is wired to the PWREN# pin to disable the receiver when in USB suspend mode. CBUS2 = TXDEN and CBUS3 =

PWREN# are the default device configurations of these pins. See Section 10.

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 FT232R 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 FT232R it is possible to do this entirely in hardware – simply modify the schematic so that RXD

of the FT232R is the logical OR of the SP481 receiver output with TXDEN using an HC32 or similar logic gate.

FT232R

A

G

N

D

G

N

D

G

N

D

G

N

D

T

E

S

T

100nF

3V3OUT

VCCIO

NC

RESET#

NC

+

100nF

10nF

Vcc

TXD

RXD

RTS#

CTS#

DTR#

DSR#

DCD#

RI#

CBUS0

CBUS3

CBUS2

CBUS1

GPIO1

GPIO4

USBDP

USBDM

VCC

1

2

3

4

5

OSCI

OSCO

CBUS4

TXDEN#

Ferrite

Bead

+

4.7uF

SHIELD

SHIELD

GND

10

DB9M

RS485 LEVEL

CONVERTER

TXD

RXD

PWREN#

Vcc

Link

120R

5

6

7

3

4

1

2

SP481

GPIO0

Vcc