Document No.: FT_000061
FT2232H DUAL HIGH SPEED USB TO MULTIPURPOSE UART/FIFO IC
Datasheet Version 2.09
Clearance No.: FTDI#77
4.3.3 Dual Port RS485 Configuration
Figure 4.3 illustrates how the FT2232H can be configured as a dual RS485 interface.
DB9-M
RS485 Channel A
FT2232H
60
36
VCC
SP481
PWREN#
8
3
SUSPEND#
7
6
DM_A
DPA
16
17
18
19
21
22
23
24
26
4
2
1
TXD
RXD
D
R
GND
RTS#
CTS#
DTR#
DSR#
DCD#
RI#
5
120R
LINK
TXDEN
DB9-M
RS485 Channel B
VCC
SP481
8
3
7
DM_B
DP_B
GND
38
39
40
41
43
44
45
46
48
4
2
1
D
TXD
RXD
6
R
RTS#
CTS#
DTR#
DSR#
DCD#
RI#
5
120R
LINK
TXDEN
Figure 4.3 Dual RS485 Configuration
In this case both channel A and channel B are configured as RS485 operating at TTL levels. This example
uses two Sipex SP491 devices but there are similar parts available from Maxim and Analog Devices
amongst others. The SP491 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 pins on the FT2232H are provided for exactly that purpose,
and so the transmitter enables are wired to the TXDEN‟s. The receiver enable is active low, so it is wired
to the PWREN# pin to disable the receiver when in USB suspend mode.
RS485 is a multi-drop network – i.e. many devices can communicate with each other over a single two
wire cable connection. The RS485 cable requires to be terminated at each end of the cable. Links are
provided to allow the cable to be terminated if the device is physically positioned at either end of the
cable.
In this example the data transmitted by the FT2232H is also received by the device that is transmitting.
This is a common feature of RS485 and requires the application software to remove the transmitted data
from the received data stream. With the FT2232H it is possible to do this entirely in hardware – simply
modify the schematic so that RXD of the FT2232H is the logical OR of the SP481 receiver output with
TXDEN using an HC32 or similar logic gate.
Copyright © 2010 Future Technology Devices International Limited
23
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