FT234XD Datasheet, PDF (24/37 Pages) Future Technology Devices International Ltd. – The FT234XD is a USB to serial UART interface with optimised packaging (3mm x 3mm 12 pin DFN) for smaller PCB designs and the following advanced features

FT234XD USB TO BASIC UART IC
Version 1.2
Document No.: FT_000753 Clearance No.: FTDI#324
7.2 USB to RS485 Coverter
VBUS
CN1
VBUS
D-
D+
GND
1
2
3
4
USB
C1 C2 R1
4.7uF 0.1uF 10k
FB1
600R/0.5A
GND
C4
10nF
C5 C6
47pF 47pF
C3
0.1uF
GND
R2
R3 27R
27R
C7
10nF
U1
3 _3V3OUT
1
12
USBDM
USBDP
2 _RESET#
TXD
RXD
RTS#
CTS#
7
10
8
11
TXD
RXD
CBUS0 ⁶
FT234XD
TXDEN
C8
0.1uF
GND
U2
1 _{RO VCC} 8
2
3
R
RE
DE
7
6
D
4 _{DI GND} 5
ZT485EEN
Data -
Data +
GND
GND GND GND
GND
GND
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.
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