Document No.: FT_000329
FT232BL/BQ USB UART IC Datasheet Version 2.2
Clearance No.: FTDI# 171
8.3 USB RS485 Converter Configuration
Figure 8.3 USB-RS485 Converter Configuration
Figure 7.3 illustrates how to connect the UART interface of the FT232B to a TTL – RS485 Level Converter
I.C. to make a USB => RS485 converter. This example uses the Exar/Sipex SP481 device but there are
similar parts available from Maxim and Analog Devices amongst others. The SP481 is a RS485 device in a
compact 8 pin SOP package. It has separate enables on both the transmitter and receiver. With RS485,
the transmitter is only enabled when a character is being transmitted from the UART. The TXDEN pin on
the FT232B is provided for exactly that purpose and so the transmitter enable is wired to TXDEN. The
receiver enable is active low, so it is wired to the PWREN# pin to disable the receiver when in USB
suspend mode.
RS485 is a multi-drop network – i.e. many devices can communicate with each other over a single two
wire cable connection. The RS485 cable requires to be terminated at each end of the cable. A link is
provided to allow the cable to be terminated if the device is physically positioned at either end of the
cable.
In this example the data transmitted by the FT232B is also received by the device that is transmitting.
This is a common feature of RS485 and requires the application software to remove the transmitted data
from the received data stream. With the FT232B it is possible to do this entirely in hardware – simply
modify the schematic so that RXD of the FT232B is the logical OR of the SP481 receiver output with
TXDEN using an HC32 or similar logic gate.
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