Thursday, March 26, 2009

USB to RS485 Circuit

Designing RS-485 Circuits

RS-485 in Brief
Jan Axelson
But first, a quick look at RS-485. The interface popularly known
as RS-485 is an electrical specification for multipoint systems that
use balanced lines. RS-485 is similar to RS-422, but RS-422 allows
just one driver with multiple receivers, while RS-485 supports
multiple drivers and receivers.
The specification document, TIA-485-A, defines the electrical
characteristics of the line and its drivers and receivers. There
are brief suggestions relating to terminations and wiring, but
unlike RS-232, there's no discussion of connector pinouts or
software protocols. An RS-485 network can have as many as
32 unit loads, with one unit load equivalent to an input impedance
of 12k. By using high-impedance receivers, you can have as
many as 256 nodes.
An RS-485 link can extend as far as 4000 ft., and can transfer
data at up to 10 Mbps, but not both at the same time. At 90 kbps
the maximum cable length is 4000 ft; at 1Mbps, it drops to 400 ft,
and at 10Mbps, to 50 ft. For more nodes or very long distances,
you can use repeaters that regenerate the signals and begin a
new RS-485 line


USB to RS485 Circuit

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 120X termination) allows the cable to be terminated if
the SP481 is physically positioned at either end of the cable.

FT232R Datasheet pdf

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