FT230XQ-R datasheet(26/44 Pages) ETC2

26 / 44 page

FT230X USB TO BASIC UART IC

Version 1.2

Document No.: FT_000566 Clearance No.: FTDI# 260

7.2 USB to RS485 Coverter

FT230X

SHIELD

Ferrite

Bead

GND

VCC

GND

VCC

3V3OUT

USBDM

USBDP

VCCIO

VCC

RESET#

100nF

10nF

4.7uF

TXD

RXD

GND

DB9M

SHIELD

PWREN#

VCCIO

10K

RS 485 LEVEL

CONVERTER

Vcc

ZT3485

Link

120R

TXDEN

27R

GND

47pF

10k

VCCIO

Figure 7.2 Application Example Showing USB to RS485 Converter

An example of using the FT230X as a USB to RS485 converter is shown in Figure 7.2. In this application,

a 3V3-TTL to RS485 level converter IC is used on the serial UART interface of the FT230X to convert the

TTL levels of the FT230X to RS485 levels.

This example uses the Zywyn ZT3485 device. Equivalent devices are available from Maxim and Analogue

Devices. The ZT3485 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 FT230X 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 ZT3485’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

FT230X 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 ZT3485 is physically positioned at either end

of the cable.

In this example the data transmitted by the FT230X is also present on the receive path of the

ZT3485.This is a common feature of RS485 and requires the application software to remove the

transmitted data from the received data stream. With the FT230X it is possible to do this entirely in

hardware by modifying the example shown in Figure 7.2 by logically OR’ing the FT230X TXDEN and the

ZT3485 receiver output and connecting the output of the OR gate to the RXD of the FT230X.

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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