FT231XQ-T FTDI, FT231XQ-T Datasheet - Page 26

• Manufacturer Part Number: FT231XQ-T
• Description: USB Interface IC USB to Full Serial UART IC QFN-20
• Manufacturer: FTDI

Specifications of FT231XQ-T

• Rohs: yes
• Tradename: X-Chip

7.2 USB to RS485 Coverter
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.
SHIELD
5
1
2
3
4
100nF
GND
Copyright © 2013 Future Technology Devices International Limited
10nF
Ferrite
Bead
47pF
VCC
+
GND
4.7uF
GND
VCC
47pF
GND
27R
27R
100nF
USBDM
USBDP
VCC
RESET#
VCCIO
3V3OUT
FT231X USB TO FULL HANDSHAKE UART IC
FT231X
PWREN#
TXDEN
TXD
RXD
Document No.: FT_000565 Clearance No.:
3
4
2
1
RS 485 LEVEL
CONVERTER
ZT3485E
5
Vcc
VCCIO
7
6
120R
10K
Link
GND
SHIELD
10
Datasheet
Version 1.2
DB9M
FTDI# 261
26