Document No.: FT_000053
FT232R USB UART IC Datasheet Version 2.07
Clearance No.: FTDI# 38
7.3 USB to RS422 Converter
Vcc
Vcc
RS422 LEVEL
CONVERTER
4
10
Ferrite Bead
TXDM
5
1
TXD
RXD
9
VCC
TXDP
RXDP
2
3
2
USBDM
RTS#
CTS#
DTR#
DSR#
11
12
3
120R
USBDP
4
10nF
+
VCCIO
NC
RXDM
SP491
FT232R
GND
DB9M
5
6
7
TXDM
TXDP
RXDP
RXDM
RTSM
RTSP
CTSP
CTSM
RESET#
DCD#
Vcc
NC
SHIELD
RI#
CBUS0
CBUS1
CBUS2
OSCI
OSCO
GND
RS422 LEVEL
CONVERTER
Vcc
4
SHIELD
10
9
3V3OUT
RTSM
5
3
100nF
4.7uF
GND
+
CBUS3
PWREN#
A
G
N
D
T
E
S
T
G
N
D
RTSP
CTSP
G
N
D
G
N
D
CBUS4-
SLEEP#
100nF
GND
11
12
2
120R
Vcc
SP491
CTSM
6
7
GND
10K
Figure 7.3 USB to RS422 Converter Configuration
An example of using the FT232R as a USB to RS422 converter is shown in Figure 7.3. In this application,
two TTL to RS422 Level Converter ICs are used on the serial UART interface of the FT232R to convert the
TTL levels of the FT232R to RS422 levels. There are many suitable level converter devices available. This
example uses Sipex SP491 devices which have enables on both the transmitter and receiver. Since the
SP491 transmitter enable is active high, it is connected to a CBUS pin in SLEEP# configuration. The
SP491 receiver enable is active low and is therefore connected to a CBUS pin PWREN# configuration. This
ensures that when both the SP491 transmitters and receivers are enabled then the device is active, and
when the device is in USB suspend mode, the SP491 transmitters and receivers are disabled. If a similar
application is used, but 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 2.5mA is met.
The SP491 is specified to transmit and receive data at a rate of up to 5 Mbaud. In this example the
maximum data rate is limited to 3 Mbaud by the FT232R.
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29
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