DS232B Version 1.8

© Future Technology Devices Intl. Ltd. 2005

Page 18 of 25

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

Figure 10

USB <=> RS422 Converter Configuration

FT232BM

PWREN#

SLEEP#

TXD

RXD

RTS#

CTS#

DTR#

DSR#

DCD#

RI#

10

TXDEN

25

24

23

22

15

21

20

19

18

16

DB9-M

TXDM

RXDP

RTSP

CTSP

GND

VCC

5

4

12

9

3

11

2

10

14

7

6

D

R

SP491

5

4

12

9

3

11

2

10

14

7

6

D

R

SP491

VCC

120R

120R

TXDP

RXDM

RTSM

CTSM

Figure 10 illustrates how to connect the UART interface of the FT232BM to a TTL – RS422 Level Converter I.C. to

make a USB <=> 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 the SLEEP# pin. The receiver enable is active low and is connected to the PWREN# pin. 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 standby current of

500uA is met.

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

limited to 3M Baud by the FT232BM.