FT232RN datasheet(25/40 Pages) FTDI

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FT232RN USB UART IC Datasheet

Version 1.0

Document No.: FT_001521 Clearance No.: FTDI#571

7.2 USB to RS485 Converter

Figure 7.2 Application Example Showing USB to RS485 Converter

An example of using the FT232RN as a USB to RS485 converter is shown in Figure 7.2. In this application,

a TTL to RS485 level converter IC is used on the serial UART interface of the FT232RN to convert the TTL

levels of the FT232RN to RS485 levels.

This example uses Sipex SP481 device. Equivalent devices are available from Maxim and Analogue Devices.

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 signal CBUS pin option on the FT232RN is provided for exactly this purpose

and so the transmitter enable is wired to CBUS2 which has been configured as TXDEN. Similarly, CBUS3

has been configured as PWREN#. This signal is used to control the SP481’s receiver enable. The receiver

enable is active low, so it is wired to the PWREN# pin to disable the receiver when in USB suspend mode.

CBUS2 = TXDEN and CBUS3 = PWREN# are the default device configurations of the FT232RN pins.

RS485 is a multi-drop network; so many devices can communicate with each other over a two-wire cable

interface. The RS485 cable requires to be terminated at each end of the cable. A link (which provides the

120Ω termination) allows the cable to be terminated if the SP481 is physically positioned at either end of

the cable.

In this example the data transmitted by the FT232RN is also present on the receive path of the SP481.This

is a common feature of RS485 and requires the application software to remove the transmitted data from

the received data stream. With the FT232RN it is possible to do this entirely in hardware by modifying the

example shown in Figure 7.2 by logically OR’ing the FT232RN TXDEN and the SP481 receiver output and

connecting the output of the OR gate to the RXD of the FT232RN.

Note that the TXDEN is activated 1 bit period before the start bit. TXDEN is deactivated at the same time

as the stop bit. This is not configurable.

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