ftdi utilities

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本文介绍如何使用FT232R芯片配置RS485接口，包括TXDEN信号的设置与验证方法。通过合理配置FT232R的CBUS引脚，实现对RS485收发器的有效控制。此外，还提供了利用MProg等工具进行设备配置的具体步骤。

验证txden信号状态的好方法是使用另一个channel，测量其txden，然后对比。

ftdi utilities such as mprog:

http://www.ftdichip.com/Support/Utilities.htm

FT_PROG is an enhanced version (and a replacement for) the previous utility used for this function called MProg as well as the VProg application

MProg is available for download by clicking here. MProg requires FTDI's D2XX drivers to run correctly.

The MProg User Guide is available for download as a .PDF by clicking here.

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.