Copyright © 2015 Future Technology Devices International Limited

24

FT234XD USB TO BASIC UART IC

Version 1.2

Document No.: FT_000753 Clearance No.: FTDI#324

7.2 USB to RS485 Coverter

TXD

RXD

C8

0.1uF

GND

RE

2

DE

3

DI

4

VCC

8

RO

1

6

7

GND

5

D

R

U2

ZT485EEN

GND

Data +

Data -

TXDEN

GND

VBUS

C4

10nF

C3

0.1uF

C1

4.7uF

GND

GND

FB1

600R/0.5A

VBUS

1

D-

2

D+

3

GND

4

CN1

USB

C5

47pF

C6

47pF

GND

GND

C2

0.1uF

RXD

10

USBDP

12

USBDM

1

3V3OUT

3

RESET#

2

CBUS0

6

TXD

7

CTS#

11

RTS#

8

U1

FT234XD

R1

10k

C7

10nF

GND

GND

R3

27R

R2

27R

Figure 7.2 Application Example Showing USB to RS485 Converter

An example of using the FT234XD 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 FT234XD to

convert the TTL levels of the FT234XD to RS485 levels.

This example uses the Zywyn ZT485EEN device. Equivalent devices are available from Maxim and

Analogue Devices. The ZT485EEN 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 FT234XD is provided for exactly this purpose and so the

transmitter enable is wired to CBUS which has been configured as TXDEN. 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.

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.