DS_FT231X Datasheet, PDF (26/44 Pages) List of Unclassifed Manufacturers – Future Technology Devices International Ltd FT231X

7.2 USB to RS485 Coverter
FT231X USB TO FULL HANDSHAKE UART IC
Datasheet
Version 1.2
Document No.: FT_000565 Clearance No.: FTDI# 261
1
2
3
4
5
SHIELD
Ferrite
Bead
47pF
10nF
GND
VCC
27R
27R
47pF
GND
VCC
USBDM
USBDP
TXD
FT231X
RESET#
RXD
VCCIO
3V3OUT
TXDEN
PWREN#
100nF
VCC
+
4.7uF
GND
100nF
Vcc
RS 485 LEVEL
CONVERTER
3
7
4
6
2
1
ZT3485E
5
120R
Link
VCCIO
GND
DB9M
SHIELD 10
10K
GND
Figure 7.2 Application Example Showing USB to RS485 Converter
An example of using the FT231X 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 FT231X to convert the
TTL levels of the FT231X 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 FT231X 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
FT231X 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 FT231X 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 FT231X it is possible to do this entirely in
hardware by modifying the example shown in Figure 7.2 by logically OR’ing the FT231X TXDEN and the
ZT3485 receiver output and connecting the output of the OR gate to the RXD of the FT231X.
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.
Copyright © 2013 Future Technology Devices International Limited
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