FT260Q-R datasheet(42/61 Pages) FTDI

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FT260 HID-CLASS USB TO UART/I2C BRIDGE IC

Version 1.1

Document No.: FT_001272 Clearance No.: FTDI#484

8.3 USB to RS485 Converter

Figure 8.3 Application Example 3: USB to RS485 Converter

An example of using the FT260 as a USB to RS485 converter is shown in Figure 8.3. In this application, a

TTL to RS485 level converter IC is used on the serial UART interface of the FT260 to convert the TTL

levels of the FT260 to RS485 levels.

The converter device requires separate enable signals on both the transmitter and receiver. With RS485,

the transmitter is only enabled when a character is being transmitted from the UART. Setting DIO0 as

TX_ACTIVE is provided for exactly the same purpose and wired to the transmitter enable (DE) of

converter device. The pin DIO12 of FT260 can be configured as PWREN_N and wired to the receiver

enable (RE) of the converter device. With these configurations of the pins, the FT260 can be used as the

USB to RS485 converter.

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 converter device is physically positioned at

either end of the cable.

In this example, the data transmitted by the FT260 is also present on the receive path of the converter

device. This is a common feature of RS485 and requires the application software to remove the

transmitted data from the received data stream. With the FT260, it is possible to do this entirely in

hardware by modifying the example shown in Figure 8.3 by logically OR-ing the FT260 TX_ACTIVE and

the receiver output (RO) of converter device then connecting the output of the OR gate to the RXD of

FT260.

Note that the TX_ACTIVE is activated 1 bit ahead the start bit. TX_ACTIVE is de-activated at the same

time as the stop bit. This is not configurable.

FT260Q-R データシート(PDF) 42 Page - Future Technology Devices International Ltd.