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Document Reference No.: FT_000120
USB to RS485 UART Serial Converter PCB Datasheet
Version 1.2
Clearance No. FTDI#80
4.1 USB-RS485-PCB Signal Descriptions
Name
GND
Terminator 1
POWER
Data +
Data –
Terminator 2
Type
GND
Input
Output
Bi-Direction
Bi-Direction
Input
Description
Device ground supply pin.
Pin 1 of 120R Terminating Resistor. Only Required if
the USB-RS485-PCB is the first or last device in a
multi-drop RS485 System, to meet RS485
Termination Requirements.
Power output. Default output is +5V when active,
external device, 250mA max. OFF during suspend
Data + Signal
Data – Signal
Pin 2 of 120R Terminating Resistor. Only Required if
the USB-RS485-PCB is the first or last device in a
multi-drop RS485 System, to meet RS485
Termination Requirements.
Table 4.1 USB-RS485-PCB Signal Descriptions
Note: In some cases when termination is enabled, unexpected zero bytes can be received at the
beginning or end of a message. To resolve this, biasing resistors can be added to the Data + and Data –
lines to ensure that the lines go to an idle state when no node is driving data onto the bus.
Typical values for these resistors would be:
750 ohms between Data + and +5V (POWER)
750 ohms between Data - and ground (GND)
The resistors would be fitted externally to the USB-RS485-PCB as the board does not have footprints for
them.
Only one set of biasing resistors should only be used on the bus. They should not be added to the USB-
RS485-PCB node if another node on the bus is already providing biasing.
Copyright ? 2011 Future Technology Devices International Limited
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