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FT230XQ-R Datasheet(PDF) 26 Page - Future Technology Devices International Ltd. |
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FT230XQ-R Datasheet(HTML) 26 Page - Future Technology Devices International Ltd. |
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26 / 44 page  Copyright © Future Technology Devices International Limited 26 FT230X USB TO BASIC UART IC Version 1.4 Document No.: FT_000566 Clearance No.: FTDI# 260 7.2 USB to RS485 Coverter FT230X 1 2 3 4 5 SHIELD Ferrite Bead GND GND VCC GND VCC 3V3OUT USBDM USBDP VCCIO VCC RESET# 100nF 100nF 10nF 4.7uF + TXD RXD GND DB9M SHIELD 10 PWREN# VCCIO 10K RS 485 LEVEL CONVERTER Vcc ZT3485 5 1 2 3 4 Link 120R 7 6 TXDEN 27R 27R GND 47pF 47pF 10k 10k 10k VCCIO Figure 7.2 Application Example Showing USB to RS485 Converter An example of using the FT230X as a USB to RS485 converter is shown in Figure 7.2. In this application, a 3V3-TTL to RS485 level converter IC is used on the serial UART interface of the FT230X to convert the TTL levels of the FT230X to RS485 levels. This example uses the Zywyn ZT3485 device. Equivalent devices are available from Maxim and Analogue Devices. The ZT3485 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 FT230X 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 ZT3485’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. 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 ZT3485 is physically positioned at either end of the cable. In this example the data transmitted by the FT230X is also present on the receive path of the ZT3485.This is a common feature of RS485 and requires the application software to remove the transmitted data from the received data stream. With the FT230X it is possible to do this entirely in hardware by modifying the example shown in Figure 7.2 by logically OR’ing the FT230X TXDEN and the ZT3485 receiver output and connecting the output of the OR gate to the RXD of the FT230X. 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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