FT232BQ USB UART ( USB - Serial) I.C.
Figure 10
USB <=> RS422 Converter Configuration
VCC
14 SP491
FT232BM
DB9-M
FT232BQ
15
10
25
24
23
22
21
20
19
18
16
4
PWREN#
SLEEP#
TXD
10
9
TXDM
5
3
2
D
TXDP
RXDP
11
120R
R
12
RXD
RXDM
GND
6
7
RTS#
CTS#
DTR#
DSR#
DCD#
RI#
VCC
14 SP491
4
10
9
RTSM
5
3
2
D
RTSP
CTSP
11
120R
R
12
CTSM
6
7
TXDEN
Figure 10 illustrates how to connect the UART interface of the FT232BQ 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 FT232BQ.
DS232BQ Version 1.8
© Future Technology Devices Intl. Ltd. 2005
Page 19 of 25
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