FT232R datasheet(28/43 Pages) FTDI

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Document No.: FT_000053

FT232R USB UART IC Datasheet Version 2.07

Clearance No.: FTDI# 38

7.2 USB to RS485 Coverter

FT232R

GND

100nF

VCC

USBDM

USBDP

VCCIO

RESET#

OSCI

OSCO

3V3OUT

TXD

RXD

RTS#

CTS#

DTR#

DSR#

DCD#

RI#

CBUS0

CBUS1

CBUS2

CBUS3

CBUS4

GND

SHIELD

GND

100nF

4.7uF +

10nF +

Ferrite Bead

Vcc

TXD

RXD

GND

DB9M

SHIELD

TXDEN

GPO

PWREN#

GPIO0

GPIO1

VCCIO

10K

RS485 LEVEL

CONVERTER

Vcc

SP481

Link

120R

Figure 7.2 Application Example Showing USB to RS485 Converter

An example of using the FT232R 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 FT232R to convert the TTL

levels of the FT232R to RS485 levels.

This example uses the 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 FT232R 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

FT232R 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 FT232R 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 FT232R it is possible to do this entirely in hardware by modifying the

example shown in Figure 7.2 by logically OR‟ing the FT232R TXDEN and the SP481 receiver output and

connecting the output of the OR gate to the RXD of the FT232R.

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.

FT232R Datasheet(PDF) 28 Page - Future Technology Devices International Ltd.

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