FT232R USB UART I.C. Datasheet Version 1.04

© Future Technology Devices International Ltd. 2005

Page 25

8.3 USB to RS422 Converter Configuration

FT232R

100nF

3V3OUT

VCCIO

NC

RESET#

NC

+

100nF

10nF

Vcc

TXD

RXD

RTS#

CTS#

DTR#

DSR#

DCD#

RI#

CBUS0

CBUS3 -

PWREN#

CBUS2

CBUS1

USBDP

USBDM

VCC

1

2

3

4

5

OSCI

OSCO

CBUS4 -

SLEEP#

Ferrite

Bead

+

4.7uF

SHIELD

RS422 LEVEL

CONVERTER

Vcc

120R

6

9

10

4

5

2

3

SP491

11

12

7

RS422 LEVEL

CONVERTER

Vcc

120R

6

9

10

4

5

2

3

SP491

11

12

7

TXDM

TXDP

RXDP

RXDM

RTSM

RTSP

CTSP

CTSM

A

G

N

D

G

N

D

G

N

D

G

N

D

T

E

S

T

SHIELD

GND

DB9M

TXDM

TXDP

RXDP

RXDM

RTSM

RTSP

CTSP

CTSM

Vcc

Figure 17 -Example USB to RS422 Converter Configuration

Figure 17 illustrates how to connect the UART interface of the FT232R to a TTL – RS422 Level Converter I.C. to

make a USB to RS422 converter. There are many such level converter devices available – this example uses Sipex

SP491 devices which have enables on both the transmitter and receiver. Because the transmitter enable is active

high, it is connected to a CBUS pin in SLEEP# configuration. The receiver enable is active low and so is connected

to a CBUS pin PWREN# configuration. This ensures that both the transmitters and receivers are enabled when the

device is active, and disabled when the device is in USB suspend mode. If the design is USB BUS powered, it may be

necessary to use a P-Channel logic level MOSFET (controlled by PWREN#) in the VCC line of the SP491 devices to

ensure that the USB stand-by current of 500μA is met.

The SP491 is good for sending and receiving data at a rate of up to 5 Megbaud – in this case the maximum rate is

limited to 3 Megabaud by the FT232R.