Data Sheet

Copyright © 2013 Future Technology Devices International Limited 27
FT231X USB TO FULL HANDSHAKE UART IC
Datasheet
Version 1.2
Document No.: FT_000565 Clearance No.: FTDI# 261
7.3 USB to RS422 Converter
FT231X
1
2
3
4
5
SHIELD
Ferrite
Bead
GND
GND
VCC
GND
VCC
3V3OUT
USBDM
USBDP
VCCIO
VCC
AGND
GND
RESET#
100nF
100nF
10nF
4.7uF+
RS 422 LEVEL
CONVERTER
Vcc
ZT3491
5
3
4
76
TXDM
TXDP
RXDP
RXDM
120R
10
9
11
12
RS 422 LEVEL
CONVERTER
ZT3491
3
4
76
Vcc
Vcc
10K
2
5
120R
11
12
9
10
RTSM
RTSP
CTSP
CTSM
GND
DB9M
SHIELD
TXDM
TXDP
RXDP
RXDM
RTSM
RTSP
CTSP
CTSM
2
RXD
TXD
RTS#
CTS#
PWREN#
SLEEP#
GND
47pF
47pF
27R
27R
Figure 7.3 USB to RS422 Converter Configuration
An example of using the FT231X as a USB to RS422 converter is shown in Figure 7.3. In this application,
two 3V3-TTL to RS422 Level Converter Ics are used on the serial UART interface of the FT231X to convert
the TTL levels of the FT231X to RS422 levels. There are many suitable level converter devices available.
This example uses Zywyn ZT3491 devices which have enables on both the transmitter and receiver.
Since the ZT3491 transmitter enable is active high, it is connected to a CBUS pin in SLEEP#
configuration. The ZT3491 receiver enable is active low and is therefore connected to a CBUS pin
PWREN# configuration. This ensures that when both the ZT3491 transmitters and receivers are enabled
then the device is active, and when the device is in USB suspend mode, the ZT3491 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 ZT3491
devices to ensure that the USB standby current of 2.5mA is met.
The ZT3491 is specified to transmit and receive data at a rate of up to 16 Mbaud. In this example the
maximum data rate is limited to 3 Mbaud by the FT231X.