Document No.: FT_000053
FT232R USB UART IC Datasheet Version 2.07
Clearance No.: FTDI# 38
7.2 USB to RS485 Coverter
Vcc
Vcc
RS485 LEVEL
CONVERTER
GND
DB9M
3
4
7
6
Ferrite Bead
1
TXD
RXD
TXD
RXD
VCC
2
3
4
USBDM
2
1
RTS#
CTS#
DTR#
DSR#
USBDP
10
SHIELD
10nF
+
VCCIO
NC
SP481
5
FT232R
120R
5
RESET#
Link
DCD#
NC
SHIELD
RI#
CBUS0
CBUS1
VCCIO
OSCI
OSCO
GND
GPIO0
GPIO1
Vcc
10K
CBUS2
TXDEN
3V3OUT
100nF
4.7uF
GND
+
A
G
N
D
T
E
S
T
CBUS3
G
N
D
G
N
D
G
N
D
PWREN#
CBUS4
GPO
100nF
GND
GND
Figure 7.2 Application Example Showing USB to RS485 Converter
An example of using the FT232R as a USB to RS485 converter is shown in Figure 7.2. In this application,
a TTL to RS485 level converter IC is used on the serial UART interface of the FT232R to convert the TTL
levels of the FT232R to RS485 levels.
This example uses the Sipex SP481 device. Equivalent devices are available from Maxim and Analogue
Devices. The SP481 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 FT232R 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 SP481‟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
FT232R 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 SP481 is physically positioned at either end of
the cable.
In this example the data transmitted by the FT232R is also present on the receive path of the SP481.This
is a common feature of RS485 and requires the application software to remove the transmitted data from
the received data stream. With the FT232R it is possible to do this entirely in hardware by modifying the
example shown in Figure 7.2 by logically OR‟ing the FT232R TXDEN and the SP481 receiver output and
connecting the output of the OR gate to the RXD of the FT232R.
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 © 2010 Future Technology Devices International Limited
28
FTDI [ FUTURE TECHNOLOGY DEVICES INTERNATIONAL LTD. ]
