Document No.: FT_000329
FT232BL/BQ USB UART IC Datasheet Version 2.2
Clearance No.: FTDI# 171
8.2 USB RS422 Converter Configuration
Figure 8.2 USB-RS422 Converter Configuration
Figure 8.2 illustrates how to connect the UART interface of the FT232B 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 Exar/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 FT232B
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