USB232U/485Ui
Page 19
either (a) a packet shorter than 64 bytes is received or (b) the requested data length is
reached.
The performance of the USB232U/485Ui ports is therefore not exactly equivalent to that of
legacy ports. For more information see the application note AN232-04
4.3
Testing and Troubleshooting
If the installation instructions have been correctly followed, and the serial communications
interface is being used in conjunction with a proven software driver, then it is unlikely that any
problems will be experienced in establishing communications through the serial interface.
When difficulties do occur, the following notes may be helpful in isolating the problem area.
4.3.1
Testing with the Application Software
If the port fails to install or behaviour is intermittent then the problem is probably to do with the
USB connection and the following areas should be investigated.
USB Cabling Check the device has been correctly connected using a Type B USB
cable. Check the cable is secure, and that the number of USB devices connected
to the PC does not exceed 127.
Device Installation. For detailed help regarding installation of these devices refer
to 2.2 “Installing the Interface”.
If the port successfully recognised and installed by the operating system and then the problem
is probably on the serial communication side of the interface, and the following areas should be
investigated.
Communication circuits. For a communications circuit to operate correctly, both
ends of the circuit, and the interconnecting cables must be serviceable and the
correct connections made. The vagaries of RS232 implementation often cause
problems in making the proper connections, and the manufacturers instructions for
the equipment at each end should be carefully checked.
Hardware handshaking control signals. If the communication is operating under a
software handshaking protocol (X-ON, X-OFF etc.) then only the transmitted and
received data lines need to be connected. If the application requires hardware
handshaking by the use of control lines CTS, RTS, DSR, DTR or DCD, then these
must be correctly interconnected or looped back, otherwise the program is likely to
hang.
Polarity of RS422/485 differential data lines. Not all suppliers use the same
convention for the polarity of the differential data lines. The USB485Ui lines are
labelled A and B for the transmitter and A' and B' for the receiver as defined in the
RS422/485 specification. If the other device is labelled differently, then:-
A
is normally connected to
A
or
A'
or
Y
or
High
or
+
B
is normally connected to
B
or
B'
or
Z
or
Low
or
–
If all devices are not so labelled and the polarity is not evident, or if a problem is
suspected in this area, then some experimentation may be necessary. No damage
should be done to the interface hardware by reversing polarity.
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