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7000 Bus Converter User Manual (version 1.2,Feb/2003, 7PH-006-10) --------------- 76
It is recommended to use the “trial and error” rule. The trial and error
rules are given as follows:
(1) If the length of RS-485 is about 1.2 Km, try 120Ω first
(2) Run TEST.EXE of NAP7000S
Select function_5, run continuously for at least 8 hours to make
sure communication is OK.
(3) If function_5 finds many communication errors, use an
oscilloscope to check the waveform. The waveform will tell you
whether the terminator is too small or too big. Then adjust your
terminator and run TEST.EXE again.
(4)
(5) If the correct terminators are found, run the TEST.EXE
continuously for at least 8 hours to make sure there are no
communication errors.
The function_5 of TEST.EXE, given in NAP7000S, will automatically
read “testing command” from TEST.DAT and continuously perform “send-
receive-testing”. It will continuously test and record all testing results.
Therefore this function is especially designed for RS-485 network stability
evaluation. If you run this function_5 continuously for 8 hours and find no
check errors, this means that your RS-485 network is very stable. Also
this means that your terminators match well now.
NOTE 1: The value of the terminator depends on the RS-485 wire
used. If the RS-485 path is very long, don’t use a cheaper wire. It is
recommended to select a high quality wire
such as the Belden 1583A.
NOTE 2: The terminator is different for various applications.
Therefore we can’t provide a terminator with the 7520 or the 7510. The
user must choose the correct terminator by them self. It is recommended
to use a carbon, 1/4w resistor.