ftdi utilities

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.

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