Model 485HF9
Page 22
10 OPEN "COM1:1200,N,8,1" AS #1
20 PRINT #1,"Loop-back test passed"
30 LB$=INPUT$((21),#1)
40 PRINT LB$
The parameters in line 10 can be changed as required to perform the test at different baud
rates etc.
If tests with the applications software or other loop-back tests indicate that the 485 Adaptor is
working properly, then the following 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 RS-232, RS-422 and RS-485
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, etc., the
program is likely to hang.
Polarity of RS-422/485 differential data lines.
Not all suppliers use the same
convention for the polarity of the differential data lines. The Model 485 Adaptor
lines are labelled A and B as defined in the RS-422/485 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 can be done to the interface hardware by reversing polarity.
Termination of Data and Control Lines.
The differential data lines must be
properly terminated as described above.
Summary of Contents for 485HF9
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