Copyright © 2010 Future Technology Devices International Limited

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Document Reference No.: FT_000135

USB-COM422-PLUS2 Datasheet

Version 1.2

Clearance No.: FTDI#103

8 Troubleshooting

Cables are the most common sources of trouble with external devices.

Check the following:

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USB cable is properly inserted at both ends

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Computer power is ON

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Computer is not in Sleep or Standby

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If a USB Hub is used, be sure it is set for “Self-Powered” operation

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If a USB Hub is used, be sure all cables are properly inserted

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If all the above are OK, the Yellow LED should be lit, indicating the device has been recognized by the

USB subsystem.

RS422 cables – check the following:

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Output signals (TXD+, TXD-, RTS+, RTS-) are connected to the respective inputs (RXD+, RXD-,

CTS+, CTS-) in each direction.

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Check for specific handshake requirements of your RS422 peripheral.

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If handshake signals are not used, ensure the application is set to “No Hardware Handshake”, or

equivalent.

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Test the port with a loop-back connector. Connect TXD+ to RXD+, TXD- to RXD-, RTS+ to CTS+ and

RTS- to CTS-. Use a simple terminal program to check that data is transmitted and received.

Ensure the latest device driver is in use. See www.ftdichip.com

If other devices with FTDI chips are installed in the system, check with all manufacturers of these devices

for the latest device drivers.

See the FTDI installation guides for additional details: http://ftdichip.com/Documents/InstallGuides.htm

Common Windows Device Driver Troubles:

DEVICE TIMES OUT: The default settings of the device driver assume typical data transfers of

hundreds to thousands or more bytes at a given time. Some applications, such as a GPS device,

only send data in short packets, often only a few bytes. If this is the case, it may be necessary

to adjust the driver buffer size and/or latency timer to smaller values. These values can be

adjusted through the Advanced driver options as noted in Figure 2.13. The buffer size can be

reduced to 64 bytes. The latency timer can be set as low as 2ms. A setting of 1ms will cause

unnecessary USB traffic and could adversely affect data transmission.

ERRATIC MOUSE POINTER: The device driver defaults to query an attached device to find out

whether it is a mouse or modem, consistent with native COM port operation. Some RS422

peripherals constantly send short packets of data, causing the host system to “think” a mouse or

modem has been attached. These short packets will interfere with normal mouse operation

causing the pointer to jump around the screen. If this happens, disconnect the RS422 device and

uncheck the Serial Enumerator option, also found on the Advanced driver options screen in Figure

2.13.

COM PORT IN USE: Windows keeps track of all COM port assignments. If multiple FTDIChip

products have been connected to a single system, the COM port number will increase, even if the

other devices are not attached. If the higher COM port assignments are not acceptable for the

application, known unused COM port numbers should be uninstalled according to the FTDI

installation guide: http://ftdichip.com/Documents/InstallGuides.htm.