2-WIRE RS-485 TESTING
USING LOOPBACK OR WITHOUT LOOPBACK
PAGE 7 OF 9
10. Type some characters in the upper window for one converter. If the converter is RTS
Controlled, the Converter port with RTS high will send to the Converter with RTS low.
11. If data is shown in the lower window for one port when typed in the upper window of the
other, the Converter or port is working for Receive. The other is working for Transmit.
The characters typed should appear in the opposite window. Note that the enter key value will be
shown in brackets as <0D> and other non-printable control characters will be shown with different
values. If the characters appear odd, check that the Data A and B line pair from one converter to
the other is not interchanged. Connecting A to B and B to A will invert the data patterns causing
the computer Com port to trigger incorrectly on the data. Low or marginal port power can also
cause data errors. A external supply may be needed if using a laptop or other low power device.
Do not attempt to connect a power supply to a Port Powered device unless the converter has +12
Volt terminals. If you connect a 12 Volt DC power supply to the DTR or RTS line without a
current limiting series resistor of 180 ohms to 240 ohms, you will cause damage to the converter
circuitry.
12. Next make the opposite port active for transmit. If the converter is RTS Controlled, set RTS
for the Transmitting port high, and the RTS low for the receiving port.
13. Type characters in the window for the transmitting port, observe them in the lower window of
the receiving port.
14. If each port can transmit from one to the other and receive from the other, the converter is
most likely working fine. Any other problem may be software related.
Actual signal levels can be checked with a oscilloscope. With no termination, the levels must
exceed 200 mV at the receiver, 4 to 4.5 Volts difference between the Data A and Data B lines is
typical when the Data lines are active. With a dual trace scope in DC coupling mode, with one
channel on Data A, and the other channel on Data B, the two lines can be seen as opposite
levels, when A is high, B is low and vice versa. When the transmitter is inactive and properly
powered, 4 volts of bias voltage between Data B(+) to Data A(-) is typical for B&B RS-485
models without device connections or termination.
Note:
1. RTS Control units require RTS hardware control software to operate properly. RTS control
timing can be critical for half duplex RS-485 control, and can be erratic under windows.
2. SD Control units require a matching baud rate setting if preset for 9600 baud. Resistors
and/or capacitor values may need to be changed.
3. Many Common problems are caused by wiring custom cables for the RS-232 side, especially
DB9 to DB25 and not understanding DTE/DCE devices. Usually Rx and Tx are wrong. Refer
to our web site Support, Technical Library for DTE/DCE Illustrations.
ComTest features listed on next page.
(123 Seiten)
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